What are your favorite extension methods for C codeplex com extensionoverflow

Question

Let s make a list of answers where you post your excellent and favorite extension methods    The requirement is that the full code must be posted and a example and an explanation on how to use it   Based on the high interest in this topic I have setup an Open Source Project called extensionoverflow on Codeplex    Please mark your answers with an acceptance to put the code in the Codeplex project   Please post the full sourcecode and not a link   Codeplex News   24 08 2010 The Codeplex page is now here   http   extensionoverflow codeplex com   11 11 2008 XmlSerialize   XmlDeserialize is now Implemented and Unit Tested   11 11 2008 There is still room for more developers   -  Join NOW   11 11 2008 Third contributer joined ExtensionOverflow  welcome to BKristensen  11 11 2008 FormatWith is now Implemented and Unit Tested   09 11 2008 Second contributer joined ExtensionOverflow  welcome to chakrit   09 11 2008 We need more developers   -   09 11 2008 ThrowIfArgumentIsNull in now Implemented and Unit Tested on Codeplex

User · Answer

There is somethimes need to have instance of class no matter if valid but not null

public static T Safe<T>(this T obj) where T : new()
{
    if (obj == null)
    {
        obj = new T();
    }

    return obj;
}

usage will be like:

MyClass myClass = Provider.GetSomeResult();
string temp = myClass.Safe().SomeValue;

instead of:

MyClass myClass = Provider.GetSomeResult();
string temp = "some default value";
if (myClass != null)
{
        temp = myClass.SomeValue;
}

sorry if it is a duplicity, but I dont find it.

User · Answer

Ive created a extension method to select an item in a dropdown in ASP NET   Below is the code   public static class Utilities       public enum DropDownListSelectionType               ByValue          ByText            public static void SelectItem this  System Web UI WebControls DropDownList drp  string selectedValue  DropDownListSelectionType type                drp ClearSelection            System Web UI WebControls ListItem li          if  type    DropDownListSelectionType ByValue              li   drp Items FindByValue selectedValue Trim             else             li   drp Items FindByText selectedValue Trim             if  li    null              li Selected   true           This method can be called by the following lines of code to either select by text  DropDownList1 SelectItem  ABCD   Utilities DropDownListSelectionType ByText     or select by value  DropDownList1 SelectItem  11   Utilities DropDownListSelectionType ByValue     The above code doesnt select anything if it cant find the text value passed in

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Here s a to-and-from for Roman numerals   Not often used  but could be handy   Usage   if   IV  IsValidRomanNumeral            Do useful stuff with the number 4     Console WriteLine  MMMDCCCLXXXVIII  ParseRomanNumeral     Console WriteLine 3888 ToRomanNumeralString       The source       public static class RomanNumeralExtensions               private const int NumberOfRomanNumeralMaps   13           private static readonly Dictionary lt string  int gt  romanNumerals               new Dictionary lt string  int gt  NumberOfRomanNumeralMaps                                   M   1000                        CM   900                        D   500                        CD   400                        C   100                        XC   90                        L   50                        XL   40                        X   10                        IX   9                        V   5                        IV   4                        I   1                           private static readonly Regex validRomanNumeral   new Regex                  i     MDCLXVI    M 0 3    C DM    D C 0 3                       X LC    L XX 0 2   L    I VX    V  II 0 2    V                     RegexOptions Compiled            public static bool IsValidRomanNumeral this string value                        return validRomanNumeral IsMatch value                      public static int ParseRomanNumeral this string value                        if  value    null                                throw new ArgumentNullException  value                               value   value ToUpperInvariant   Trim                 var length   value Length               if   length    0      value IsValidRomanNumeral                                  throw new ArgumentException  Empty or invalid Roman numeral string     value                               var total   0              var i   length               while  i  gt  0                                var digit   romanNumerals value --i  ToString                      if  i  gt  0                                        var previousDigit   romanNumerals value i - 1  ToString                          if  previousDigit  lt  digit                                                digit -  previousDigit                          i--                                                           total    digit                             return total                     public static string ToRomanNumeralString this int value                        const int MinValue   1              const int MaxValue   3999               if   value  lt  MinValue      value  gt  MaxValue                                 throw new ArgumentOutOfRangeException  value   value   Argument out of Roman numeral range                                const int MaxRomanNumeralLength   15              var sb   new StringBuilder MaxRomanNumeralLength                foreach  var pair in romanNumerals                                while  value   pair Value  gt  0                                        sb Append pair Key                       value -  pair Value                                               return sb ToString

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Values ordered true false    True false values separated by a capital letter    Only two values allowed    ---------------------------    Limited  but could be useful public enum BooleanFormat       OneZero      YN      YesNo      TF      TrueFalse      PassFail      YepNope    public static class BooleanExtension            lt summary gt          Converts the boolean value of this instance to the specified string value            lt  summary gt      private static string ToString this bool value  string passValue  string failValue                return value   passValue   failValue                  lt summary gt          Converts the boolean value of this instance to a string            lt  summary gt           lt param name  booleanFormat  gt A BooleanFormat value           Example  BooleanFormat PassFail would return  Pass  if true and  Fail  if false  lt  param gt           lt returns gt Boolean formatted string lt  returns gt      public static string ToString this bool value  BooleanFormat booleanFormat                string booleanFormatString   Enum GetName booleanFormat GetType    booleanFormat           return ParseBooleanString value  booleanFormatString                       Parses boolean format strings  not optimized     private static string ParseBooleanString bool value  string booleanFormatString                StringBuilder trueString   new StringBuilder            StringBuilder falseString   new StringBuilder             int charCount   booleanFormatString Length           bool isTrueString   true           for  int i   0  i    charCount  i                          if  char IsUpper booleanFormatString i    amp  amp  i    0                  isTrueString   false               if  isTrueString                  trueString Append booleanFormatString i                else                 falseString Append booleanFormatString i                       return  value    true   trueString ToString     falseString ToString

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And one more   public enum ParseFailBehavior      ReturnNull     ReturnDefault     ThrowException    public static T  ParseNullableEnum lt T gt  this string theValue  ParseFailBehavior desiredBehavior   ParseFailBehavior ReturnNull  where T struct      T output     T  result   Enum TryParse lt T gt  theValue  out output            T  output         desiredBehavior    ParseFailBehavior ReturnDefault             T  default T             null      if result    null  amp  amp  desiredBehavior    ParseFailBehavior ThrowException        throw new ArgumentException  Parse Failed for value  0  of enum type  1             FormatWith theValue  typeof T  Name               This version requires  NET 4 0  in 3 5 you have no TryParse and no optional parameters  you re stuck with Enum Parse   which you have to try-catch  It s still totally doable in 3 5  and much more useful as Enum Parse   is oogly and your only other option    public static T  ParseNummableEnum lt T gt  this string theValue        return theValue ParseNullableEnum lt T gt  ParseFailBehavior ReturnNull      public static T  ParseNullableEnum lt T gt  this string theValue       ParseFailBehavior desiredBehavior  where T struct       try               return  T   Enum Parse typeof  T   theValue             catch  Exception                if desiredBehavior    ParseFailBehavior ThrowException  throw             return desiredBehavior    ParseFailBehavior ReturnDefault    T  default T    null      Usage     returns null if OptionOne isn t an enum constant var myEnum    OptionOne  ParseNullableEnum lt OptionEnum gt         guarantees a return value IF the enum has a  zero  constant value  generally a good practice  var myEnum    OptionTwo  ParseNullableEnum lt OptionEnum gt  ParseFailBehavior ReturnDefault  Value

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I have implemented a package of extension methods  available at http   foop codeplex com   and some of my daily used ones are      the most beloved extension method for me is Pipe   lt    variable Pipe x   gt  this SomeFunction x   Pipe y   gt                 return this SomeOtherFunction y        gt   var d   28 December 2009      some extension methods for creating DateTime DateTime justDatePart   d JustDate    TimeSpan justTimePart   d JustTime    var nextTime   d Add 5 Hours      using StreamReader reader   new StreamReader  lines-of-data-file-for-example                     for reading streams line by line and usable in LINQ     var query   from line in reader Lines                     where line Contains  today                  select new   Parts   PartsOf line   Time    now       500 Sleep     XmlSerialize and XmlDeserialize  IsNull and IsNotNull  IfTrue  IfFalse and Iff  true IfTrue      gt  Console WriteLine  it is true then      IfNull and IfNotNull

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I have an extension method for logging exceptions   public static void Log this Exception obj        your logging logic here     And it is used like this   try         Your stuff here   catch Exception ex        ex Log         sorry for posting twice  the 2nd one is better designed  -

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I use these in my Silverlight projects   public static void Show this UIElement element        element Visibility   Visibility Visible     public static void Hide this UIElement element        element Visibility   Visibility Collapsed

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These extension methods invoke an event asynchronously  They were inspired by this StackOverflow answer        lt summary gt      Invoke an event asynchronously  Each subscriber to the event will be invoked on a separate thread       lt  summary gt       lt param name  someEvent  gt The event to be invoked asynchronously  lt  param gt       lt param name  sender  gt The sender of the event  lt  param gt       lt param name  args  gt The args of the event  lt  param gt       lt typeparam name  TEventArgs  gt The type of  lt see cref  EventArgs   gt  to be used with the event  lt  typeparam gt  public static void InvokeAsync lt TEventArgs gt  this EventHandler lt TEventArgs gt  someEvent  object sender  TEventArgs args      where TEventArgs   EventArgs       if  someEvent    null                return             var eventListeners   someEvent GetInvocationList         AsyncCallback endAsyncCallback   delegate IAsyncResult iar                var ar   iar as AsyncResult          if  ar    null                        return                     var invokedMethod   ar AsyncDelegate as EventHandler lt TEventArgs gt           if  invokedMethod    null                        invokedMethod EndInvoke iar                         foreach  EventHandler lt TEventArgs gt  methodToInvoke in eventListeners                methodToInvoke BeginInvoke sender  args  endAsyncCallback  null                 lt summary gt      Invoke an event asynchronously  Each subscriber to the event will be invoked on a separate thread       lt  summary gt       lt param name  someEvent  gt The event to be invoked asynchronously  lt  param gt       lt param name  sender  gt The sender of the event  lt  param gt       lt param name  args  gt The args of the event  lt  param gt  public static void InvokeAsync this EventHandler someEvent  object sender  EventArgs args        if  someEvent    null                return             var eventListeners   someEvent GetInvocationList         AsyncCallback endAsyncCallback   delegate IAsyncResult iar                var ar   iar as AsyncResult          if  ar    null                        return                     var invokedMethod   ar AsyncDelegate as EventHandler          if  invokedMethod    null                        invokedMethod EndInvoke iar                         foreach  EventHandler methodToInvoke in eventListeners                methodToInvoke BeginInvoke sender  args  endAsyncCallback  null             To use   public class Foo       public event EventHandler lt EventArgs gt  Bar       public void OnBar                 Bar InvokeAsync this  EventArgs Empty             Notice the added benefit that you don t have to check for null on the event before invoking it  e g    EventHandler lt EventArgs gt  handler   Bar  if  handler    null           Invoke the event     To test   void Main         EventHandler lt EventArgs gt  handler1       delegate object sender  EventArgs args                   Simulate performing work in handler1         Thread Sleep 100           Console WriteLine  Handled 1                EventHandler lt EventArgs gt  handler2       delegate object sender  EventArgs args                   Simulate performing work in handler2         Thread Sleep 50           Console WriteLine  Handled 2                EventHandler lt EventArgs gt  handler3       delegate object sender  EventArgs args                   Simulate performing work in handler3         Thread Sleep 25           Console WriteLine  Handled 3                var foo   new Foo        foo Bar    handler1      foo Bar    handler2      foo Bar    handler3      foo OnBar         Console WriteLine  Start executing important stuff            Simulate performing some important stuff here  where we don t want to        wait around for the event handlers to finish executing     Thread Sleep 1000        Console WriteLine  Finished executing important stuff        Invoking the event will  usually  yield this output      Start executing important stuff   Handled 3   Handled 2   Handled 1   Finished executing important stuff   If the event were invoked synchronously  it would always yield this output - and delay execution of the  important  stuff      Handled 1   Handled 2   Handled 3   Start executing important stuff   Finished executing important stuff

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Here s a bitmap extension which can convert bitmaps to grayscale   public static Bitmap GrayScale this Bitmap bitmap        Bitmap newBitmap   new Bitmap bitmap Width  bitmap Height       Graphics g   Graphics FromImage newBitmap          the grayscale ColorMatrix     ColorMatrix colorMatrix   new ColorMatrix new float                   new float     3f   3f   3f  0  0               new float     59f   59f   59f  0  0               new float     11f   11f   11f  0  0               new float    0  0  0  1  0               new float    0  0  0  0  1               ImageAttributes attributes   new ImageAttributes        attributes SetColorMatrix colorMatrix       g DrawImage bitmap  new Rectangle 0  0  bitmap Width  bitmap Height   0  0  bitmap Width  bitmap Height  GraphicsUnit Pixel  attributes       g Dispose        return newBitmap      Sample usage   Bitmap grayscaled   bitmap GrayScale

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Convert any string to type Int32     Calls the underlying int TryParse method to convert a string    representation of a number to its 32-bit signed integer equivalent     Returns Zero if conversion fails   public static int ToInt32 this string s        int retInt      int TryParse s  out retInt       return retInt      SAMPLE USE  string s    999   int i   s ToInt32

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ForEach for IEnumerables  public static class FrameworkExtensions          a map function     public static void ForEach lt T gt  this IEnumerable lt T gt   enum  Action lt T gt  mapFunction                foreach  var item in  enum  mapFunction item             Naive example   var buttons   GetListOfButtons   as IEnumerable lt Button gt       click all buttons buttons ForEach b   gt  b Click       Cool example      no need to type the same assignment 3 times  just    new   up an array and use foreach   lambda    everything is properly inferred by csc  -  new   itemA  itemB  itemC        ForEach item   gt            item Number   1          item Str    Hello World              Note   This is not like Select because Select expects your function to return something as for transforming into another list   ForEach simply allows you to execute something for each of the items without any transformations data manipulation   I made this so I can program in a more functional style and I was surprised that List has a ForEach while IEnumerable does not   Put this in the codeplex project

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static string Format  this string str                        params Expression lt Func lt string object gt  gt    args        var parameters   args ToDictionary                           e  gt string Format     0     e Parameters 0  Name                            e  gt e Compile   e Parameters 0  Name         var sb   new StringBuilder str       foreach var kv in parameters                sb Replace  kv Key                     kv Value    null   kv Value ToString                     return sb ToString        With the above extension you can write this   var str     foo   bar   baz   Format foo  gt foo  bar  gt 2  baz  gt new object       and you ll get  foo 2 System Object

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Below is an extension method that adapts Rick Strahl s code  and the comments too  to stop you having to guess or read the byte order mark of a byte array or text file each time you convert it to a string   The snippet allows you to simply do   byte   buffer   File ReadAllBytes   C  file txt    string content   buffer GetString      If you find any bugs please add to the comments  Feel free to include it in the Codeplex project   public static class Extensions            lt summary gt          Converts a byte array to a string  using its byte order mark to convert it to the right encoding          Original article  http   www west-wind com WebLog posts 197245 aspx          lt  summary gt           lt param name  buffer  gt An array of bytes to convert lt  param gt           lt returns gt The byte as a string  lt  returns gt      public static string GetString this byte   buffer                if  buffer    null    buffer Length    0              return                 Ansi as default         Encoding encoding   Encoding Default                                 EF BB BF    UTF-8              FF FE UTF-16    little endian              FE FF UTF-16    big endian              FF FE 00 00 UTF-32  little endian              00 00 FE FF UTF-32  big-endian                       if  buffer 0     0xef  amp  amp  buffer 1     0xbb  amp  amp  buffer 2     0xbf              encoding   Encoding UTF8          else if  buffer 0     0xfe  amp  amp  buffer 1     0xff              encoding   Encoding Unicode          else if  buffer 0     0xfe  amp  amp  buffer 1     0xff              encoding   Encoding BigEndianUnicode     utf-16be         else if  buffer 0     0  amp  amp  buffer 1     0  amp  amp  buffer 2     0xfe  amp  amp  buffer 3     0xff              encoding   Encoding UTF32          else if  buffer 0     0x2b  amp  amp  buffer 1     0x2f  amp  amp  buffer 2     0x76              encoding   Encoding UTF7           using  MemoryStream stream   new MemoryStream                          stream Write buffer  0  buffer Length               stream Seek 0  SeekOrigin Begin               using  StreamReader reader   new StreamReader stream  encoding                                 return reader ReadToEnd

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Compare the equality of two objects without  necessarily  overriding Equals or implementing IEquatable lt     Why would you want to do this  When you really want to know if two objects are equal  but you re too lazy to override Equals object  or implement IEquatable lt T gt   Or  more realistically  if you have a terribly complex class and implementing Equals by hand would be extremely tedious  error prone  and not fun to maintain  It also helps if you don t care too much about performance   I am currently using IsEqualTo because of the second reason - I have a class with many properties whose types are other user-defined classes  each of which has many other properties whose types are other user-defined classes  ad infinitum  Throw in a bunch of collections in many of these classes  and implementing Equals object  truly becomes a nightmare   Usage   if  myTerriblyComplexObject IsEqualTo myOtherTerriblyComplexObject            Do something terribly interesting      In order to determine equality  I make numerous comparisons  I make every attempt to do the  right  one in the  right  order  The comparisons  in order are    Use the static Equals object  object  method  If it returns true  return true  It will return true if the references are the same  It will also return true if thisObject overrides Equals object   If thisObject is null  return false  No further comparisons can be made if it is null  If thisObject has overridden Equals object   return false  Since it overrides Equals  it must mean that Equals was executed at step  1 and returned false  If someone has bothered to override Equals  we should respect that and return what Equals returns  If thisObject inherits from IEquatable lt T gt   where otherObject can be assigned to T  get the Equals T  method using reflection  Invoke that method and return its return value  If both objects are IEnumerable  return whether contain the same items  in the same order  using IsEqualTo to compare the items  If the objects have different types  return false  Since we know now that thisObject does not have an Equals method  there isn t any way to realistically evaluate two object of different types to be true  If the objects are a value type  primitive or struct  or a string  return false  We have already failed the Equals object  test - enough said  For each property of thisObject  test its value with IsEqualTo  If any return false  return false  If all return true  return true    String comparisons could be better  but easy to implement  Also  I m not 100  sure I m handling structs right   Without further ado  here is the extension method        lt summary gt      Provides extension methods to determine if objects are equal       lt  summary gt  public static class EqualsEx            lt summary gt          The  lt see cref  Type   gt  of  lt see cref  string   gt            lt  summary gt      private static readonly Type StringType   typeof string             lt summary gt          The  lt see cref  Type   gt  of  lt see cref  object   gt            lt  summary gt      private static readonly Type ObjectType   typeof object             lt summary gt          The  lt see cref  Type   gt  of  lt see cref  IEquatable T    gt            lt  summary gt      private static readonly Type EquatableType   typeof IEquatable lt  gt              lt summary gt          Determines whether  lt paramref name  thisObject   gt  is equal to  lt paramref name  otherObject   gt            lt  summary gt           lt param name  thisObject  gt          This object           lt  param gt           lt param name  otherObject  gt          The other object           lt  param gt           lt returns gt          True  if they are equal  otherwise false           lt  returns gt      public static bool IsEqualTo this object thisObject  object otherObject                if  Equals thisObject  otherObject                            Always check Equals first  If the object has overridden Equals  use it  This will also capture the case where both are the same reference              return true                     if  thisObject    null                           Because Equals object  object  returns true if both are null  if either is null  return false              return false                     var thisObjectType   thisObject GetType            var equalsMethod   thisObjectType GetMethod  Equals   BindingFlags Public   BindingFlags Instance  null  new     ObjectType    null           if  equalsMethod DeclaringType    thisObjectType                           thisObject overrides Equals  and we have already failed the Equals test  so return false              return false                     var otherObjectType   otherObject    null   null   otherObject GetType                If thisObject inherits from IEquatable lt  gt   and otherObject can be passed into its Equals method  use it          var equatableTypes   thisObjectType GetInterfaces   Where                                              Get interfaces of thisObjectType that                i   gt  i IsGenericType                                                                                  are generic                 amp  amp  i GetGenericTypeDefinition      EquatableType                                                      and are IEquatable of some type                 amp  amp   otherObjectType     null    i GetGenericArguments   0  IsAssignableFrom otherObjectType           and otherObjectType can be assigned to the IEquatable s type           if  equatableTypes Any                             If we found any interfaces that meed our criteria  invoke the Equals method for each interface                 If any return true  return true  If all return false  return false              return equatableTypes                  Select equatableType   gt  equatableType GetMethod  Equals   BindingFlags Public   BindingFlags Instance                    Any equatableEqualsMethod   gt   bool equatableEqualsMethod Invoke thisObject  new     otherObject                         if  thisObjectType    StringType  amp  amp  thisObject is IEnumerable  amp  amp  otherObject is IEnumerable                           If both are IEnumerable  check their items              var thisEnumerable     IEnumerable thisObject  Cast lt object gt                 var otherEnumerable     IEnumerable otherObject  Cast lt object gt                  return thisEnumerable SequenceEqual otherEnumerable  IsEqualToComparer Instance                      if  thisObjectType    otherObjectType                           If they have different types  they cannot be equal              return false                     if  thisObjectType IsValueType    thisObjectType    StringType                           If it is a value type  we have already determined that they are not equal  so return false              return false                        Recurse into each public property  if any are not equal  return false  If all are true  return true          return   from propertyInfo in thisObjectType GetProperties                    let thisPropertyValue   propertyInfo GetValue thisObject  null                   let otherPropertyValue   propertyInfo GetValue otherObject  null                   where  thisPropertyValue IsEqualTo otherPropertyValue                   select thisPropertyValue  Any                    lt summary gt          A  lt see cref  IEqualityComparer T    gt  to be used when comparing sequences of collections           lt  summary gt      private class IsEqualToComparer   IEqualityComparer lt object gt                     lt summary gt              The singleton instance of  lt see cref  IsEqualToComparer   gt                lt  summary gt          public static readonly IsEqualToComparer Instance                lt summary gt              Initializes static members of the  lt see cref  EqualsEx IsEqualToComparer   gt  class               lt  summary gt          static IsEqualToComparer                         Instance   new IsEqualToComparer                            lt summary gt              Prevents a default instance of the  lt see cref  EqualsEx IsEqualToComparer   gt  class from being created               lt  summary gt          private IsEqualToComparer                                     lt summary gt              Determines whether the specified objects are equal               lt  summary gt               lt param name  x  gt              The first object to compare               lt  param gt               lt param name  y  gt              The second object to compare               lt  param gt               lt returns gt              true if the specified objects are equal  otherwise  false               lt  returns gt          bool IEqualityComparer lt object gt  Equals object x  object y                        return x IsEqualTo y                           lt summary gt              Not implemented - throws an  lt see cref  NotImplementedException   gt                lt  summary gt               lt param name  obj  gt              The  lt see cref  object   gt  for which a hash code is to be returned               lt  param gt               lt returns gt              A hash code for the specified object               lt  returns gt          int IEqualityComparer lt object gt  GetHashCode object obj                        throw new NotImplementedException

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Values ordered true false    True false values separated by a capital letter    Only two values allowed    ---------------------------    Limited  but could be useful public enum BooleanFormat       OneZero      YN      YesNo      TF      TrueFalse      PassFail      YepNope    public static class BooleanExtension            lt summary gt          Converts the boolean value of this instance to the specified string value            lt  summary gt      private static string ToString this bool value  string passValue  string failValue                return value   passValue   failValue                  lt summary gt          Converts the boolean value of this instance to a string            lt  summary gt           lt param name  booleanFormat  gt A BooleanFormat value           Example  BooleanFormat PassFail would return  Pass  if true and  Fail  if false  lt  param gt           lt returns gt Boolean formatted string lt  returns gt      public static string ToString this bool value  BooleanFormat booleanFormat                string booleanFormatString   Enum GetName booleanFormat GetType    booleanFormat           return ParseBooleanString value  booleanFormatString                       Parses boolean format strings  not optimized     private static string ParseBooleanString bool value  string booleanFormatString                StringBuilder trueString   new StringBuilder            StringBuilder falseString   new StringBuilder             int charCount   booleanFormatString Length           bool isTrueString   true           for  int i   0  i    charCount  i                          if  char IsUpper booleanFormatString i    amp  amp  i    0                  isTrueString   false               if  isTrueString                  trueString Append booleanFormatString i                else                 falseString Append booleanFormatString i                       return  value    true   trueString ToString     falseString ToString

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This one is for MVC it adds the ability to generate a  lt label   gt  tag to the Html variable that is available in every ViewPage  Hopefully it will be of use to others trying to develop similar extensions   Use    lt    Html Label  LabelId    ForId    Text    gt    Output    lt label id  LabelId  for  ForId  gt Text lt  label gt    Code   public static class HtmlHelperExtensions       public static string Label this HtmlHelper Html  string  for  string text                return Html Label null   for  text              public static string Label this HtmlHelper Html  string  for  string text  object htmlAttributes                return Html Label null   for  text  htmlAttributes              public static string Label this HtmlHelper Html  string  for  string text  IDictionary lt string  object gt  htmlAttributes                return Html Label null   for  text  htmlAttributes              public static string Label this HtmlHelper Html  string id  string  for  string text                return Html Label id   for  text  null              public static string Label this HtmlHelper Html  string id  string  for  string text  object htmlAttributes                return Html Label id   for  text  new RouteValueDictionary htmlAttributes               public static string Label this HtmlHelper Html  string id  string  for  string text  IDictionary lt string  object gt  htmlAttributes                TagBuilder tag   new TagBuilder  label             tag MergeAttributes htmlAttributes            if   string IsNullOrEmpty id               tag MergeAttribute  id   Html AttributeEncode id             tag MergeAttribute  for   Html AttributeEncode  for             tag SetInnerText Html Encode text             return tag ToString TagRenderMode Normal

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Some Date functions   public static bool IsFuture this DateTime date  DateTime from        return date Date  gt  from Date     public static bool IsFuture this DateTime date        return date IsFuture DateTime Now      public static bool IsPast this DateTime date  DateTime from        return date Date  lt  from Date     public static bool IsPast this DateTime date        return date IsPast DateTime Now

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I use the following extensions to extend all collections  maybe someone find these useful         lt summary gt      Collection Helper      lt  summary gt       lt remarks gt      Use IEnumerable by default  but when altering or getting item at index use IList       lt  remarks gt  public static class CollectionHelper         region Alter            lt summary gt          Swap item to another place          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  IndexA  gt Index a lt  param gt           lt param name  IndexB  gt Index b lt  param gt           lt returns gt New collection lt  returns gt      public static IList lt T gt  Swap lt T gt  this IList lt T gt   this  Int32 IndexA  Int32 IndexB                T Temp    this IndexA            this IndexA     this IndexB            this IndexB    Temp          return  this                  lt summary gt          Swap item to the left          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index lt  param gt           lt returns gt New collection lt  returns gt      public static IList lt T gt  SwapLeft lt T gt  this IList lt T gt   this  Int32 Index                return  this Swap Index  Index - 1                   lt summary gt          Swap item to the right          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index lt  param gt           lt returns gt New collection lt  returns gt      public static IList lt T gt  SwapRight lt T gt  this IList lt T gt   this  Int32 Index                return  this Swap Index  Index   1               endregion Alter        region Action            lt summary gt          Execute action at specified index          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index lt  param gt           lt param name  ActionAt  gt Action to execute lt  param gt           lt returns gt New collection lt  returns gt      public static IList lt T gt  ActionAt lt T gt  this IList lt T gt   this  Int32 Index  Action lt T gt  ActionAt                ActionAt  this Index            return  this              endregion Action        region Randomize            lt summary gt          Take random items          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Count  gt Number of items to take lt  param gt           lt returns gt New collection lt  returns gt      public static IEnumerable lt T gt  TakeRandom lt T gt  this IEnumerable lt T gt   this  Int32 Count                return  this Shuffle   Take Count                   lt summary gt          Take random item          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt returns gt Item lt  returns gt      public static T TakeRandom lt T gt  this IEnumerable lt T gt   this                return  this TakeRandom 1  Single                    lt summary gt          Shuffle list          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt returns gt New collection lt  returns gt      public static IEnumerable lt T gt  Shuffle lt T gt  this IEnumerable lt T gt   this                return  this OrderBy Item   gt  Guid NewGuid                 endregion Randomize        region Navigate            lt summary gt          Get next item in collection and give first item  when last item is selected           lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index in collection lt  param gt           lt returns gt Next item lt  returns gt      public static T Next lt T gt  this IList lt T gt   this  ref Int32 Index                Index     Index  gt   0  amp  amp  Index  lt   this Count   Index   0          return  this Index                   lt summary gt          Get previous item in collection and give last item  when first item is selected           lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index in collection lt  param gt           lt returns gt Previous item lt  returns gt      public static T Previous lt T gt  this IList lt T gt   this  ref Int32 Index                Index   --Index  gt   0  amp  amp  Index  lt   this Count   Index    this Count - 1          return  this Index               endregion Navigate        region Clone            lt summary gt                    lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt returns gt Cloned collection lt  returns gt      public static IEnumerable lt T gt  Clone lt T gt  this IEnumerable lt T gt   this  where T   ICloneable               return  this Select Item   gt   T Item Clone                 endregion Clone        region String            lt summary gt          Joins multiple string with Separator          lt  summary gt           lt param name  this  gt Collection lt  param gt           lt param name  Separator  gt Separator lt  param gt           lt returns gt Joined string lt  returns gt      public static String Join this IEnumerable lt String gt   this  String Separator                     return String Join Separator   this               endregion String

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Some DataSet DataRow extensions to make working with db results a little simpler  Just use  Field  fieldname   on the DataRow and it will cast it if it can  optional default can be included   Also  HasRows   on the DataSet so you don t need to check for the existence of a table and rows   Example   using  DataSet ds   yourcall         if  ds HasRows             foreach  DataRow dr in ds Tables 0  Rows                 int id   dr Field lt int gt   ID            string name   dr Field lt string gt   Name            string Action   dr Field lt string gt   Action    N A                   Code   using System  using System Data   public static class DataSetExtensions       public static T Field lt T gt  this DataRow row  string columnName  T defaultValue                try                       return row Field lt T gt  columnName                     catch                       return defaultValue                       public static T Field lt T gt  this DataRow row  string columnName                if  row columnName     null              throw new NullReferenceException columnName     does not exist in DataRow             string value   row columnName  ToString             if  typeof T        GetType                          return  T Convert ChangeType value  typeof T                      else if  typeof T     0 GetType                          return  T Convert ChangeType int Parse value   typeof T                      else if  typeof T     false GetType                          return  T Convert ChangeType bool Parse value   typeof T                      else if  typeof T     DateTime Now GetType                          return  T Convert ChangeType DateTime Parse value   typeof T                      else if  typeof T     new byte   GetType                          return  T Convert ChangeType byte Parse value   typeof T                      else if  typeof T     new float   GetType                          return  T Convert ChangeType float Parse value   typeof T                      else                       throw new ArgumentException string Format  Cannot cast   0   to   1      value  typeof T  ToString                           public static bool HasRows this DataSet dataSet                 return  dataSet Tables Count  gt  0  amp  amp  dataSet Tables 0  Rows Count  gt  0

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Would be great to have Unix TimeStamp and ISO 8601 formatted date and times  heavily used in websites and rest services   I use it in my Facebook Library  You can find the source http   github com prabirshrestha FacebookSharp blob master src FacebookSharp Core FacebookUtils DateUtils cs  private static readonly DateTime EPOCH   DateTime SpecifyKind new DateTime 1970  1  1  0  0  0  0  DateTimeKind Utc    public static DateTime FromUnixTimestamp long timestamp        return EPOCH AddSeconds timestamp      public static long ToUnixTimestamp DateTime date        TimeSpan diff   date ToUniversalTime   - EPOCH      return  long diff TotalSeconds     public static DateTime FromIso8601FormattedDateTime string iso8601DateTime       return DateTime ParseExact iso8601DateTime   o   System Globalization CultureInfo InvariantCulture      public static string ToIso8601FormattedDateTime DateTime dateTime        return dateTime ToString  o        Feel free to use in the codeplex project

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Function to compare Files Directories through the OS File System Info  This is useful to compare shares with local files     Usage   DirectoryInfo dir   new DirectoryInfo   C  test myShareDir    Console WriteLine dir IsSameFileAs     myMachineName myShareDir      FileInfo file   new FileInfo   C  test myShareDir file txt    Console WriteLine file IsSameFileAs     myMachineName myShareDir file txt       Code   public static class FileExtensions       struct BY HANDLE FILE INFORMATION               public uint FileAttributes          public System Runtime InteropServices ComTypes FILETIME CreationTime          public System Runtime InteropServices ComTypes FILETIME LastAccessTime          public System Runtime InteropServices ComTypes FILETIME LastWriteTime          public uint VolumeSerialNumber          public uint FileSizeHigh          public uint FileSizeLow          public uint NumberOfLinks          public uint FileIndexHigh          public uint FileIndexLow                       CreateFile constants            const uint FILE SHARE READ   0x00000001      const uint OPEN EXISTING   3      const uint GENERIC READ    0x80000000       const uint FILE FLAG BACKUP SEMANTICS   0x02000000         DllImport  kernel32 dll   SetLastError   true       static extern IntPtr CreateFile          string lpFileName          uint dwDesiredAccess          uint dwShareMode          IntPtr lpSecurityAttributes          uint dwCreationDisposition          uint dwFlagsAndAttributes          IntPtr hTemplateFile         DllImport  kernel32 dll   SetLastError   true       static extern bool GetFileInformationByHandle IntPtr hFile  out BY HANDLE FILE INFORMATION lpFileInformation        public static bool IsSameFileAs this FileSystemInfo file  string path                BY HANDLE FILE INFORMATION fileInfo1  fileInfo2          IntPtr ptr1   CreateFile file FullName  GENERIC READ  FILE SHARE READ  IntPtr Zero  OPEN EXISTING  FILE FLAG BACKUP SEMANTICS  IntPtr Zero           if   int ptr1    -1                        System ComponentModel Win32Exception e   new System ComponentModel Win32Exception Marshal GetLastWin32Error                 throw e                    IntPtr ptr2   CreateFile path  GENERIC READ  FILE SHARE READ  IntPtr Zero  OPEN EXISTING  FILE FLAG BACKUP SEMANTICS  IntPtr Zero           if   int ptr2    -1                        System ComponentModel Win32Exception e   new System ComponentModel Win32Exception Marshal GetLastWin32Error                 throw e                    GetFileInformationByHandle ptr1  out fileInfo1           GetFileInformationByHandle ptr2  out fileInfo2            return   fileInfo1 FileIndexHigh    fileInfo2 FileIndexHigh   amp  amp               fileInfo1 FileIndexLow    fileInfo2 FileIndexLow

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Wildcard string comparison   public static bool MatchesWildcard this string text  string pattern        int it   0      while  text CharAt it     0  amp  amp             pattern CharAt it                        if  pattern CharAt it     text CharAt it   amp  amp  pattern CharAt it                      return false          it               int cp   0      int mp   0      int ip   it       while  text CharAt it     0                if  pattern CharAt ip                                if  pattern CharAt   ip     0                  return true              mp   ip              cp   it   1                    else if  pattern CharAt ip     text CharAt it     pattern CharAt ip                                ip                it                      else                       ip   mp              it   cp                         while  pattern CharAt ip                        ip              return pattern CharAt ip     0     public static char CharAt this string s  int index        if  index  lt  s Length          return s index       return   0       It s a direct translation of the C code from this article  hence the CharAt method that returns 0 for the end of the string  if  fileName MatchesWildcard    cs          Console WriteLine   0  is a C  source file   fileName

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Some Date functions   public static bool IsFuture this DateTime date  DateTime from        return date Date  gt  from Date     public static bool IsFuture this DateTime date        return date IsFuture DateTime Now      public static bool IsPast this DateTime date  DateTime from        return date Date  lt  from Date     public static bool IsPast this DateTime date        return date IsPast DateTime Now

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Comes in useful for unit testing   public static IList lt T gt  Clone lt T gt  this IList lt T gt  list  where T   ICloneable       var ret   new List lt T gt  list Count       foreach  var item in list          ret Add  T item Clone             done     return ret      A series of these like TWith2Sugars  alternate shortened syntax   public static long  ToNullableInt64 this string val        long ret      return Int64 TryParse val  out ret    ret   new long         And finally this - is there something already in the BCL that does the following   public static void Split lt T gt  this T   array       Func lt T bool gt  determinator       IList lt T gt  onTrue       IList lt T gt  onFalse        if  onTrue    null          onTrue   new List lt T gt         else         onTrue Clear         if  onFalse    null          onFalse   new List lt T gt         else         onFalse Clear         if  determinator    null          return       foreach  var item in array                if  determinator item               onTrue Add item           else             onFalse Add item

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Comes in useful for unit testing   public static IList lt T gt  Clone lt T gt  this IList lt T gt  list  where T   ICloneable       var ret   new List lt T gt  list Count       foreach  var item in list          ret Add  T item Clone             done     return ret      A series of these like TWith2Sugars  alternate shortened syntax   public static long  ToNullableInt64 this string val        long ret      return Int64 TryParse val  out ret    ret   new long         And finally this - is there something already in the BCL that does the following   public static void Split lt T gt  this T   array       Func lt T bool gt  determinator       IList lt T gt  onTrue       IList lt T gt  onFalse        if  onTrue    null          onTrue   new List lt T gt         else         onTrue Clear         if  onFalse    null          onFalse   new List lt T gt         else         onFalse Clear         if  determinator    null          return       foreach  var item in array                if  determinator item               onTrue Add item           else             onFalse Add item

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string Format shortcut   public static class StringExtensions          Enable quick and more natural string Format calls     public static string F this string s  params object   args                return string Format s  args             Example   var s    The co-ordinate is   0    1    F point X  point Y     For quick copy-and-paste go here   Don t you find it more natural to type  some string  F  param   instead of string Format  some string    param      For a more readable name  try one of these suggestion   s    Hello  0  world  1    Fmt  Stack    Overflow    s    Hello  0  world  1    FormatBy  Stack    Overflow    s    Hello  0  world  1    FormatWith  Stack    Overflow    s    Hello  0  world  1    Display  Stack    Overflow    s    Hello  0  world  1    With  Stack    Overflow

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I have various extension methods in my MiscUtil project  full source is available there - I m not going to repeat it here   My favourites  some of which involve other classes  such as ranges    Date and time stuff - mostly for unit tests  Not sure I d use them in production     var birthday   19 June 1976   var workingDay   7 Hours     30 Minutes      Ranges and stepping - massive thanks to Marc Gravell for his operator stuff to make this possible   var evenNaturals   2 To int MaxValue  Step 2   var daysSinceBirth   birthday To DateTime Today  Step 1 Days       Comparisons   var myComparer   ProjectionComparer Create Person p   gt  p Name   var next   myComparer ThenBy p   gt  p Age   var reversed   myComparer Reverse      Argument checking   x ThrowIfNull  x      LINQ to XML applied to anonymous types  or other types with appropriate properties        lt Name gt Jon lt  Name gt  lt Age gt 32 lt  Age gt  new   Name  Jon   Age 32  ToXElements       Name  Jon  Age  32   as XAttributes  obviously  new   Name  Jon   Age 32  ToXAttributes     Push LINQ - would take too long to explain here  but search for it

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The extension method   public static void AddRange lt T  S gt  this ICollection lt T gt  list  params S   values      where S   T       foreach  S value in values          list Add value       The method applies for all types and lets you add a range of items to a list as parameters   Example   var list   new List lt Int32 gt     list AddRange 5  4  8  4  2

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How about      public static bool IsWinXPOrHigher this OperatingSystem OS      return  OS Platform    PlatformID Win32NT       amp  amp    OS Version Major  gt  5       OS Version Major    5   amp  amp   OS Version Minor  gt   1        public static bool IsWinVistaOrHigher this OperatingSystem OS      return  OS Platform    PlatformID Win32NT       amp  amp   OS Version Major  gt   6      public static bool IsWin7OrHigher this OperatingSystem OS      return  OS Platform    PlatformID Win32NT       amp  amp    OS Version Major  gt  6       OS Version Major    6   amp  amp   OS Version Minor  gt   1        public static bool IsWin8OrHigher this OperatingSystem OS      return  OS Platform    PlatformID Win32NT       amp  amp    OS Version Major  gt  6       OS Version Major    6   amp  amp   OS Version Minor  gt   2         Usage   if  Environment OSVersion IsWinXPOrHigher           do stuff    if  Environment OSVersion IsWinVistaOrHigher           do stuff    if  Environment OSVersion IsWin7OrHigher           do stuff    if  Environment OSVersion IsWin8OrHigher           do stuff

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How about      public static bool IsWinXPOrHigher this OperatingSystem OS      return  OS Platform    PlatformID Win32NT       amp  amp    OS Version Major  gt  5       OS Version Major    5   amp  amp   OS Version Minor  gt   1        public static bool IsWinVistaOrHigher this OperatingSystem OS      return  OS Platform    PlatformID Win32NT       amp  amp   OS Version Major  gt   6      public static bool IsWin7OrHigher this OperatingSystem OS      return  OS Platform    PlatformID Win32NT       amp  amp    OS Version Major  gt  6       OS Version Major    6   amp  amp   OS Version Minor  gt   1        public static bool IsWin8OrHigher this OperatingSystem OS      return  OS Platform    PlatformID Win32NT       amp  amp    OS Version Major  gt  6       OS Version Major    6   amp  amp   OS Version Minor  gt   2         Usage   if  Environment OSVersion IsWinXPOrHigher           do stuff    if  Environment OSVersion IsWinVistaOrHigher           do stuff    if  Environment OSVersion IsWin7OrHigher           do stuff    if  Environment OSVersion IsWin8OrHigher           do stuff

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HTH  These are some of my main ones   using System  using System Collections Generic  using System Globalization  using System Linq  using System Text   namespace Insert Your Namespace Here Helpers       public static class Extensions               public static bool IsNullOrEmpty lt T gt  this IEnumerable lt T gt  iEnumerable                           Cheers to Joel Mueller for the bugfix  Was  Count    now it s  Any               return iEnumerable    null                        iEnumerable Any                       public static IList lt T gt  ToListIfNotNullOrEmpty lt T gt  this IList lt T gt  iList                        return iList IsNullOrEmpty     null   iList                     public static PagedList lt T gt  ToPagedListIfNotNullOrEmpty lt T gt  this PagedList lt T gt  pagedList                        return pagedList IsNullOrEmpty     null   pagedList                     public static string ToPluralString this int value                        return value    1   string Empty    s                      public static string ToReadableTime this DateTime value                        TimeSpan span   DateTime Now Subtract value               const string plural    s                 if  span Days  gt  7                                return value ToShortDateString                               switch  span Days                                case 0                      switch  span Hours                                                case 0                              if  span Minutes    0                                                                return span Seconds  lt   0                                               now                                               string Format   0  second 1  ago                                                              span Seconds                                                             span Seconds    1   plural   string Empty                                                             return string Format   0  minute 1  ago                                                    span Minutes                                                   span Minutes    1   plural   string Empty                           default                              return string Format   0  hour 1  ago                                                    span Hours                                                   span Hours    1   plural   string Empty                                         default                      return string Format   0  day 1  ago                                            span Days                                           span Days    1   plural   string Empty                                    public static string ToShortGuidString this Guid value                        return Convert ToBase64String value ToByteArray                     Replace                            Replace       -                    Substring 0  22                      public static Guid FromShortGuidString this string value                        return new Guid Convert FromBase64String value Replace                                                                     Replace  -                                     public static string ToStringMaximumLength this string value  int maximumLength                        return ToStringMaximumLength value  maximumLength                             public static string ToStringMaximumLength this string value  int maximumLength  string postFixText                        if  string IsNullOrEmpty postFixText                                 throw new ArgumentNullException  postFixText                               return value Length  gt  maximumLength                          string Format CultureInfo InvariantCulture                                           0  1                                           value Substring 0  maximumLength - postFixText Length                                          postFixText                                                      value                     public static string SlugDecode this string value                        return value Replace                               public static string SlugEncode this string value                        return value Replace

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Smalltalk style if else in c    Feel free to put this on codeplex under whatever license you are using  using System  namespace SmalltalkBooleanExtensionMethods        public static class BooleanExtension               public static T ifTrue lt T gt   this bool aBoolean  Func lt T gt  method                    if  aBoolean              return  T method            else             return default T                  public static void ifTrue  this bool aBoolean  Action method                        if  aBoolean                  method                        public static T ifFalse lt T gt   this bool aBoolean  Func lt T gt  method                        if   aBoolean                  return  T method                else                 return default T                      public static void ifFalse  this bool aBoolean  Action method                        if   aBoolean                  method                        public static T ifTrueifFalse lt T gt   this Boolean aBoolean  Func lt T gt  methodA  Func lt T gt  methodB                        if  aBoolean                  return  T methodA                else                 return  T methodB                       public static void ifTrueifFalse  this Boolean aBoolean  Action methodA  Action methodB                        if  aBoolean                  methodA                else                 methodB                           You probably already have a timesRepeat method but its in there   using System   namespace SmalltalkBooleanExtensionMethods       public static class IntExtension               public static int timesRepeat lt T gt  this int x  Func lt T gt  method                        for  int i   x  i  gt  0  i--                                method                               return x                     public static int timesRepeat this int x  Action method                        for  int i   x  i  gt  0  i--                                method                               return x                      Nunit Tests  using System  using SmalltalkBooleanExtensionMethods  using NUnit Framework   namespace SmalltalkBooleanExtensionMethodsTest        TestFixture      public class SBEMTest               int i          bool itWorks            SetUp          public void Init                          i   0              itWorks   false                      Test            public void TestifTrue                          itWorks    true ifTrue      gt  true                Assert IsTrue itWorks                      Test            public void TestifFalse                         itWorks    false ifFalse      gt  true                Assert IsTrue itWorks                       Test            public void TestifTrueifFalse                         itWorks   false ifTrueifFalse      gt  false       gt  true               Assert IsTrue itWorks               itWorks   false              itWorks   true ifTrueifFalse      gt  true       gt  false               Assert IsTrue itWorks                       Test            public void TestTimesRepeat                          5  timesRepeat      gt  i   i   1               Assert AreEqual i  5                       Test            public void TestVoidMethodIfTrue                          true ifTrue      gt  SetItWorksBooleanToTrue                 Assert IsTrue itWorks                       Test            public void TestVoidMethodIfFalse                          false ifFalse      gt  SetItWorksBooleanToTrue                 Assert IsTrue itWorks                      public void TestVoidMethodIfTrueIfFalse                         true ifTrueifFalse      gt  SetItWorksBooleanToTrue         gt  SetItWorksBooleanToFalse                 false ifTrueifFalse      gt  SetItWorksBooleanToFalse         gt  SetItWorksBooleanToTrue                 Assert IsTrue itWorks                       public void TestVoidMethodTimesRepeat                          5  timesRepeat      gt  AddOneToi                 Assert AreEqual i  5                      public void SetItWorksBooleanToTrue                         itWorks   true                     public void SetItWorksBooleanToFalse                         itWorks   false                     public void AddOneToi                         i   i   1

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For adding multiple elements to a collection that doesn t have AddRange  e g   collection Add item1  item2  itemN    static void Add lt T gt  this ICollection lt T gt  coll  params T   items     foreach  var item in items  coll Add item       The following is like string Format   but with custom string representation of arguments  e g     0   1   2   Format lt Custom gt  c  gt c Name  string  new object   new Custom    results in  string  System Object  Custom1Name   static string Format lt T gt    this string format                           Func lt T object gt  select                           params object   args     for int i 0  i  lt  args Length    i        var x   args i  as T        if  x    null  args i    select x            return string Format format  args

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Some tools to IEnumerable  ToString Format   ToString Function  and Join Separator    For example   var names   new      Wagner    Francine    Arthur    Bernardo      names ToString  Name   0  n       Name  Wagner    Name  Francine    Name  Arthur    Name  Bernardo  names ToString name   gt  name Length  gt  6   String Format   0     name    String Empty      Francine Bernardo  names Join   -        Wagner - Francine - Arthur - Bernardo   Extensions   public static string ToString lt T gt  this IEnumerable lt T gt  self  string format        return self ToString i   gt  String Format format  i       public static string ToString lt T gt  this IEnumerable lt T gt  self  Func lt T  object gt  function        var result   new StringBuilder         foreach  var item in self  result Append function item         return result ToString       public static string Join lt T gt  this IEnumerable lt T gt  self  string separator        return String Join separator  values  self ToArray

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In  NET  there is a IndexOf and a LastIndexOf methods that return the index of the first and the last occurrence of a match in a String object  I have an extension method to get the index of the nth occurrence   public static partial class StringExtensions        public static int NthIndexOf this String str  String match  int occurrence            int i   1          int index   0           while  i  lt   occurrence  amp  amp                 index   str IndexOf match  index   1       -1                 if  i    occurrence                       Occurrence match found                  return index                            i                          Match not found         return -1

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I ve written like a quad zillion extension methods  so here are a few ones I find particulary usefull  Feel free to implement   public static class ControlExtenders            lt summary gt          Advanced version of find control           lt  summary gt           lt typeparam name  T  gt Type of control to find  lt  typeparam gt           lt param name  id  gt Control id to find  lt  param gt           lt returns gt Control of given type  lt  returns gt           lt remarks gt          If the control with the given id is not found         a new control instance of the given type is returned           lt  remarks gt      public static T FindControl lt T gt  this Control control  string id  where T   Control                  User normal FindControl method to get the control         Control  control   control FindControl id               If control was found and is of the correct type we return it         if   control    null  amp  amp   control is T                           Return new control             return  T  control                        Create new control instance          control    T Activator CreateInstance typeof T                Add control to source control so the            next it is found and the value can be            passed on itd  remember to hide it and            set an ID so it can be found next time         if     control is ExtenderControlBase                          control Visible   false                     control ID   id          control Controls Add  control               Use reflection to create a new instance of the control         return  T  control           public static class GenericListExtenders            lt summary gt          Sorts a generic list by items properties           lt  summary gt           lt typeparam name  T  gt Type of collection  lt  typeparam gt           lt param name  list  gt Generic list  lt  param gt           lt param name  fieldName  gt Field to sort data on  lt  param gt           lt param name  sortDirection  gt Sort direction  lt  param gt           lt remarks gt          Use this method when a dinamyc sort field is requiered  If the          sorting field is known manual sorting might improve performance           lt  remarks gt      public static void SortObjects lt T gt  this List lt T gt  list  string fieldName  SortDirection sortDirection                PropertyInfo propInfo   typeof T  GetProperty fieldName           if  propInfo    null                        Comparison lt T gt  compare   delegate T a  T b                                bool asc   sortDirection    SortDirection Ascending                  object valueA   asc   propInfo GetValue a  null    propInfo GetValue b  null                   object valueB   asc   propInfo GetValue b  null    propInfo GetValue a  null                   return valueA is IComparable     IComparable valueA  CompareTo valueB    0                             list Sort compare                             lt summary gt          Creates a pagged collection from generic list           lt  summary gt           lt typeparam name  T  gt Type of collection  lt  typeparam gt           lt param name  list  gt Generic list  lt  param gt           lt param name  sortField  gt Field to sort data on  lt  param gt           lt param name  sortDirection  gt Sort direction  lt  param gt           lt param name  from  gt Page from item index  lt  param gt           lt param name  to  gt Page to item index  lt  param gt           lt param name  copy  gt Creates a copy and returns a new list instead of changing the current one  lt  param gt           lt returns gt Pagged list collection  lt  returns gt      public static List lt T gt  Page lt T gt  this List lt T gt  list  string sortField  bool sortDirection  int from  int to  bool copy                List lt T gt   pageList   new List lt T gt                 Copy list         if  copy                        T    arrList   new T list Count               list CopyTo  arrList                pageList   new List lt T gt   arrList                     else                        pageList   list                        Make sure there are enough items in the list         if  from  gt   pageList Count                        int diff   Math Abs from - to               from    pageList Count - diff                    if  to  gt   pageList Count                        to    pageList Count                        Sort items         if   string IsNullOrEmpty sortField                         SortDirection sortDir   SortDirection Descending              if   sortDirection  sortDir   SortDirection Ascending               pageList SortObjects sortField  sortDir                         Calculate max number of items per page         int count   to - from          if  from   count  gt   pageList Count  count -   from   count  -  pageList Count              Get max number of items per page         T   pagged   new T count            pageList CopyTo from  pagged  0  count               Return pagged items         return new List lt T gt  pagged                   lt summary gt          Shuffle s list items           lt  summary gt           lt typeparam name  T  gt List type  lt  typeparam gt           lt param name  list  gt Generic list  lt  param gt      public static void Shuffle lt T gt  this List lt T gt  list                Random rng   new Random            for  int i   list Count - 1  i  gt  0  i--                        int swapIndex   rng Next i   1               if  swapIndex    i                                T tmp   list swapIndex                   list swapIndex    list i                   list i    tmp                                          lt summary gt          Converts generic List to DataTable           lt  summary gt           lt typeparam name  T  gt Type  lt  typeparam gt           lt param name  list  gt Generic list  lt  param gt           lt param name  columns  gt Name of the columns to copy to the DataTable  lt  param gt           lt returns gt DataTable  lt  returns gt      public static DataTable ToDataTable lt T gt  this List lt T gt  list  string   columns                List lt string gt   columns   new List lt string gt  columns           DataTable dt   new DataTable             foreach  PropertyInfo info in typeof T  GetProperties                          if   columns Contains info Name     columns    null                                dt Columns Add new DataColumn info Name  info PropertyType                                    foreach  T t in list                        DataRow row   dt NewRow                foreach  PropertyInfo info in typeof T  GetProperties                                  if   columns Contains info Name     columns    null                                        row info Name    info GetValue t  null                                               dt Rows Add row                     return dt           public static class DateTimeExtenders            lt summary gt          Returns number of month from a string representation           lt  summary gt           lt returns gt Number of month  lt  returns gt      public static int MonthToNumber this DateTime datetime  string month                month   month ToLower            for  int i   1  i  lt   12  i                          DateTime  dt   DateTime Parse  1     i     2000                string  month   CultureInfo InvariantCulture DateTimeFormat GetMonthName i  ToLower                if   month    month                                return i                                  return 0                  lt summary gt          Returns month name from month number           lt  summary gt           lt returns gt Name of month  lt  returns gt      public static string MonthToName this DateTime datetime  int month                for  int i   1  i  lt   12  i                          if  i    month                                return CultureInfo InvariantCulture DateTimeFormat GetMonthName i                                   return              public static class ObjectExtender       public static object CloneBinary lt T gt  this T originalObject                using  var stream   new System IO MemoryStream                          BinaryFormatter binaryFormatter   new BinaryFormatter                binaryFormatter Serialize stream  originalObject               stream Position   0              return  T binaryFormatter Deserialize stream                        public static object CloneObject this object obj                using  MemoryStream memStream   new MemoryStream                          BinaryFormatter binaryFormatter   new BinaryFormatter null  new StreamingContext StreamingContextStates Clone                binaryFormatter Serialize memStream  obj               memStream Position   0              return binaryFormatter Deserialize memStream                      public static class StringExtenders            lt summary gt          Returns string as unit           lt  summary gt           lt param name  value  gt Value  lt  param gt           lt returns gt Unit lt  returns gt      public static Unit ToUnit this string value                   Return empty unit         if  string IsNullOrEmpty value               return Unit Empty              Trim value         value   value Trim                Return pixel unit         if  value EndsWith  px                             Set unit type             string  int   value Replace  px                        Try parsing to int             double  val   0              if   double TryParse  int  out  val                                    Invalid value                 return Unit Empty                                Return unit             return new Unit  val  UnitType Pixel                         Return percent unit         if  value EndsWith                                Set unit type             string  int   value Replace                           Try parsing to int             double  val   0              if   double TryParse  int  out  val                                    Invalid value                 return Unit Empty                                Return unit             return new Unit  val  UnitType Percentage                         No match found         return new Unit                    lt summary gt          Returns alternative string if current string is null or empty           lt  summary gt           lt param name  str  gt  lt  param gt           lt param name  alternative  gt  lt  param gt           lt returns gt  lt  returns gt      public static string Alternative this string str  string alternative                if  string IsNullOrEmpty str   return alternative          return str                  lt summary gt          Removes all HTML tags from string           lt  summary gt           lt param name  html  gt String containing HTML tags  lt  param gt           lt returns gt String with no HTML tags  lt  returns gt      public static string StripHTML this string html                string nohtml   Regex Replace html    lt     n    gt                 nohtml   nohtml Replace   r n       Replace   n       Replace   amp nbsp        Trim            return nohtml            The first one is my favourite as it enables me to replace   Control c   this FindControl  tbName    if  c    null           Do something with c     customer Name     TextBox c  Text      With this   TextBox c   this FindControl lt TextBox gt   tbName    customer Name   c Text    Settings default string values   string str       if  string IsNullOrEmpty str         str    I m empty        Becomes   str   str Alternative  I m empty

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This one is for MVC it adds the ability to generate a  lt label   gt  tag to the Html variable that is available in every ViewPage  Hopefully it will be of use to others trying to develop similar extensions   Use    lt    Html Label  LabelId    ForId    Text    gt    Output    lt label id  LabelId  for  ForId  gt Text lt  label gt    Code   public static class HtmlHelperExtensions       public static string Label this HtmlHelper Html  string  for  string text                return Html Label null   for  text              public static string Label this HtmlHelper Html  string  for  string text  object htmlAttributes                return Html Label null   for  text  htmlAttributes              public static string Label this HtmlHelper Html  string  for  string text  IDictionary lt string  object gt  htmlAttributes                return Html Label null   for  text  htmlAttributes              public static string Label this HtmlHelper Html  string id  string  for  string text                return Html Label id   for  text  null              public static string Label this HtmlHelper Html  string id  string  for  string text  object htmlAttributes                return Html Label id   for  text  new RouteValueDictionary htmlAttributes               public static string Label this HtmlHelper Html  string id  string  for  string text  IDictionary lt string  object gt  htmlAttributes                TagBuilder tag   new TagBuilder  label             tag MergeAttributes htmlAttributes            if   string IsNullOrEmpty id               tag MergeAttribute  id   Html AttributeEncode id             tag MergeAttribute  for   Html AttributeEncode  for             tag SetInnerText Html Encode text             return tag ToString TagRenderMode Normal

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Checks for an empty collection  and sends the value set in the default constructor for the desired field public static TResult MinGuarded lt T  TResult gt  this IEnumerable lt T gt  items  Func lt T  TResult gt  expression  where T   new         if items IsEmpty              return  new List lt T gt    new T      Min expression             return items Min expression         Checks for an empty collection  and sends the value set in the default constructor for the desired field public static TResult MaxGuarded lt T  TResult gt  this IEnumerable lt T gt  items  Func lt T  TResult gt  expression  where T   new         if items IsEmpty              return  new List lt T gt    new T      Max expression             return items Max expression       I am not sure if there is a better way to do this  but this extension is very helpful whenever I want to have control over the default values of fields in my object  For instance  if I want to control the value of a DateTime and want to be set as per my business logic  then I can do so in the default constructor  Otherwise  it comes out to be DateTime MinDate

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I just went through all 4 pages of this so far  and I was rather surprised that I didn t see this way to shorten a check for InvokeRequired   using System  using System Windows Forms        lt summary gt      Extension methods acting on Control objects       lt  summary gt  internal static class ControlExtensionMethods            lt summary gt          Invokes the given action on the given control s UI thread  if invocation is needed           lt  summary gt           lt param name  control  gt Control on whose UI thread to possibly invoke  lt  param gt           lt param name  action  gt Action to be invoked on the given control  lt  param gt      public static void MaybeInvoke this Control control  Action action                if  control    null  amp  amp  control InvokeRequired                        control Invoke action                     else                       action                              lt summary gt          Maybe Invoke a Func that returns a value           lt  summary gt           lt typeparam name  T  gt Return type of func  lt  typeparam gt           lt param name  control  gt Control on which to maybe invoke  lt  param gt           lt param name  func  gt Function returning a value  to invoke  lt  param gt           lt returns gt The result of the call to func  lt  returns gt      public static T MaybeInvoke lt T gt  this Control control  Func lt T gt  func                if  control    null  amp  amp  control InvokeRequired                        return  T  control Invoke func                      else                       return func                        Usage   myForm MaybeInvoke      gt  this Text    Hello world        Sometimes the control might be null  but that s okay  var dialogResult   this Parent MaybeInvoke      gt  MessageBox Show this   Yes or no     Choice   MessageBoxButtons YesNo

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static string Format  this string str                        params Expression lt Func lt string object gt  gt    args        var parameters   args ToDictionary                           e  gt string Format     0     e Parameters 0  Name                            e  gt e Compile   e Parameters 0  Name         var sb   new StringBuilder str       foreach var kv in parameters                sb Replace  kv Key                     kv Value    null   kv Value ToString                     return sb ToString        With the above extension you can write this   var str     foo   bar   baz   Format foo  gt foo  bar  gt 2  baz  gt new object       and you ll get  foo 2 System Object

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Convert a double to string formatted using the specified culture   public static class ExtensionMethods      public static string ToCurrency this double value  string cultureName          CultureInfo currentCulture   new CultureInfo cultureName       return  string Format currentCulture    0 C    value            Example   double test   154 20  string testString   test ToCurrency  en-US        154 20

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I didn t want to add anything that was already said  so here are some that I use that haven t been mentioned   Sorry if this is too lengthy    public static class MyExtensions       public static bool IsInteger this string input                int temp           return int TryParse input  out temp              public static bool IsDecimal this string input                decimal temp           return decimal TryParse input  out temp              public static int ToInteger this string input  int defaultValue                int temp           return  int TryParse input  out temp     temp   defaultValue             public static decimal ToDecimal this string input  decimal defaultValue                decimal temp           return  decimal TryParse input  out temp     temp   defaultValue             public static DateTime ToFirstOfTheMonth this DateTime input                return input Date AddDays -1   input Day   1                 Intentionally returns 0 if the target date is before the input date      public static int MonthsUntil this DateTime input  DateTime targetDate                input   input ToFirstOfTheMonth             targetDate   targetDate ToFirstOfTheMonth             int result   0           while  input  lt  targetDate                    input   input AddMonths 1               result                       return result                Used for backwards compatibility in a system built before my time      public static DataTable ToDataTable this IEnumerable input                   too much code to show here right now

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Here is another ThrowIfNull implementation    ThreadStatic  private static string lastMethodName   null    ThreadStatic  private static int lastParamIndex   0    MethodImpl MethodImplOptions NoInlining   public static void ThrowIfNull lt T gt  this T parameter        var currentStackFrame   new StackFrame 1       var props   currentStackFrame GetMethod   GetParameters         if   String IsNullOrEmpty lastMethodName             if  currentStackFrame GetMethod   Name    lastMethodName                lastParamIndex   0            else if  lastParamIndex  gt   props Length - 1                lastParamIndex   0            else               lastParamIndex                    else           lastParamIndex   0             if   typeof T  IsValueType            for  int i   lastParamIndex  i  amp lt  props Length  i                  if  props i  ParameterType IsValueType                    lastParamIndex                  else                   break                                     if  parameter    null            string paramName   props lastParamIndex  Name          throw new ArgumentNullException paramName              lastMethodName   currentStackFrame GetMethod   Name      It s not as efficient as the other impementations  but has cleaner usage   public void Foo         Bar 1  2   Hello    World      no exception     Bar 1  2   Hello   null     exception     Bar 1  2  null   World      exception    public void Bar int x  int y  string someString1  string someString2          will also work with comments removed       x ThrowIfNull          y ThrowIfNull        someString1 ThrowIfNull        someString2 ThrowIfNull           Do something incredibly useful here      Changing the parameters to int  will also work   -bill

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We have a deployment tool to deploy between environments  Since the files could be marked as modified but not actually different I came up with this        lt summary gt      Compares the files to see if they are different       First checks file size     Then modified if the file is larger than the specified size     Then compares the bytes      lt  summary gt       lt param name  file1  gt The source file lt  param gt       lt param name  file2  gt The destination file lt  param gt       lt param name  mb  gt Skip the smart check if the file is larger than this many megabytes  Default is 10  lt  param gt       lt returns gt  lt  returns gt  public static bool IsDifferentThan this FileInfo file1  FileInfo file2  int mb   10      var ret   false        different size is a different file   if file1 Length    file2 Length  return true        if the file times are different and the file is bigger than 10mb flag it for updating   if file1 LastWriteTimeUtc  gt  file2 LastWriteTimeUtc  amp  amp  file1 Length  gt    mb 1024  1024   return true     var f1   File ReadAllBytes file1 FullName     var f2   File ReadAllBytes file2 FullName         loop through backwards because if they are different      it is more likely that the last few bytes will be different      than the first few   for var i   file1 Length - 1  i  gt  0  i--          if f1 i     f2 i               ret   true        break               return ret

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NullPartial HTML helper for ASP MVC   When passed a null Model  HTML Partial and HTML RenderPartial will provide the View s model  if this partial is strongly typed and the View has a different type it will throw an exception rather than passing a null reference  These helpers let you specify two different partials so you can keep your null tests out of the view   You have permission to include this on the Codeplex page  public static class nullpartials               public static MvcHtmlString NullPartial this HtmlHelper helper  string Partial  string NullPartial  object Model                        if  Model    null                  return helper Partial NullPartial               else                 return helper Partial Partial  Model                      public static MvcHtmlString NullPartial this HtmlHelper helper  string Partial  string NullPartial  object Model  ViewDataDictionary viewdata                        if  Model    null                  return helper Partial NullPartial  viewdata               else                 return helper Partial Partial  Model  viewdata                      public static void RenderNullPartial this HtmlHelper helper  string Partial  string NullPartial  object Model                        if  Model    null                                helper RenderPartial NullPartial                   return                            else                               helper RenderPartial Partial  Model                   return                                   public static void RenderNullPartial this HtmlHelper helper  string Partial  string NullPartial  object Model  ViewDataDictionary viewdata                        if  Model    null                                helper RenderPartial NullPartial  viewdata                   return                            else                               helper RenderPartial Partial  Model  viewdata                   return

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By all means put this in the codeplex project   Serializing   Deserializing objects to XML        lt summary gt Serializes an object of type T in to an xml string lt  summary gt       lt typeparam name  T  gt Any class type lt  typeparam gt       lt param name  obj  gt Object to serialize lt  param gt       lt returns gt A string that represents Xml  empty otherwise lt  returns gt  public static string XmlSerialize lt T gt  this T obj  where T   class  new         if  obj    null  throw new ArgumentNullException  obj         var serializer   new XmlSerializer typeof T        using  var writer   new StringWriter                  serializer Serialize writer  obj           return writer ToString                  lt summary gt Deserializes an xml string in to an object of Type T lt  summary gt       lt typeparam name  T  gt Any class type lt  typeparam gt       lt param name  xml  gt Xml as string to deserialize from lt  param gt       lt returns gt A new object of type T is successful  null if failed lt  returns gt  public static T XmlDeserialize lt T gt  this string xml  where T   class  new         if  xml    null  throw new ArgumentNullException  xml         var serializer   new XmlSerializer typeof T        using  var reader   new StringReader xml                 try   return  T serializer Deserialize reader             catch   return null       Could not be deserialized to this type

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If you need for check your string for Is All char is 0     static public bool   IsAllZero             this string input                        if string IsNullOrEmpty input                   return true              foreach  char ch in input                                if ch     0                       return false                            return true

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The extension method   public static void AddRange lt T  S gt  this ICollection lt T gt  list  params S   values      where S   T       foreach  S value in values          list Add value       The method applies for all types and lets you add a range of items to a list as parameters   Example   var list   new List lt Int32 gt     list AddRange 5  4  8  4  2

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public static bool In lt T gt  this T source  params T   list      if null  source  throw new ArgumentNullException  source      return list Contains source       Allows me to replace   if reallyLongIntegerVariableName    1         reallyLongIntegerVariableName    6         reallyLongIntegerVariableName    9         reallyLongIntegerVariableName    11         do something        and  if reallyLongStringVariableName     string1          reallyLongStringVariableName     string2          reallyLongStringVariableName     string3          do something        and  if reallyLongMethodParameterName    SomeEnum Value1         reallyLongMethodParameterName    SomeEnum Value2         reallyLongMethodParameterName    SomeEnum Value3         reallyLongMethodParameterName    SomeEnum Value4         do something         With   if reallyLongIntegerVariableName In 1 6 9 11              do something        and  if reallyLongStringVariableName In  string1   string2   string3               do something        and  if reallyLongMethodParameterName In SomeEnum Value1  SomeEnum Value2  SomeEnum Value3  SomeEnum Value4         do something

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For raising events concisely   public static void Raise this EventHandler handler  object sender  EventArgs e        if  handler    null                handler sender  e            public static void Raise lt T gt  this EventHandler lt T gt  handler  object sender  T e  where T   EventArgs       if  handler    null                handler sender  e             Usage   public event EventHandler Bar   public void Foo         Bar Raise this  EventArgs Empty       There s a bit of discussion about potential thread-safety issues here  Since  NET 4  the above form is thread-safe  but requires rearranging and some locks if using an older version

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DateTimeExtensions  Examples   DateTime firstDayOfMonth   DateTime Now First    DateTime lastdayOfMonth   DateTime Now Last    DateTime lastFridayInMonth   DateTime Now Last DayOfWeek Friday   DateTime nextFriday   DateTime Now Next DayOfWeek Friday   DateTime lunchTime   DateTime Now SetTime 11  30   DateTime noonOnFriday   DateTime Now Next DayOfWeek Friday  Noon    DateTime secondMondayOfMonth   DateTime Now First DayOfWeek Monday  Next DayOfWeek Monday  Midnight

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Simple but nicer than  Enumerable Range   IMHO        lt summary gt      Replace  Enumerable Range n   with  n Range          lt  summary gt       lt param name  n  gt iterations lt  param gt       lt returns gt 0  n-1 lt  returns gt  public static IEnumerable lt int gt  Range this int n        for  int i   0  i  lt  n  i            yield return i

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http   www codeplex com linqext

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Here s another control-extension i ve been using though i don t know if it s posted here before   public static class ControlExtensions       public static void DoubleBuffer this Control control                    http   stackoverflow com questions 76993 how-to-double-buffer-net-controls-on-a-form 77233 77233            Taxes  Remote Desktop Connection and painting  http   blogs msdn com oldnewthing archive 2006 01 03 508694 aspx          if  System Windows Forms SystemInformation TerminalServerSession  return          System Reflection PropertyInfo dbProp   typeof System Windows Forms Control  GetProperty  DoubleBuffered   System Reflection BindingFlags NonPublic   System Reflection BindingFlags Instance           dbProp SetValue control  true  null             usage   this someControl DoubleBuffer

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For adding multiple elements to a collection that doesn t have AddRange  e g   collection Add item1  item2  itemN    static void Add lt T gt  this ICollection lt T gt  coll  params T   items     foreach  var item in items  coll Add item       The following is like string Format   but with custom string representation of arguments  e g     0   1   2   Format lt Custom gt  c  gt c Name  string  new object   new Custom    results in  string  System Object  Custom1Name   static string Format lt T gt    this string format                           Func lt T object gt  select                           params object   args     for int i 0  i  lt  args Length    i        var x   args i  as T        if  x    null  args i    select x            return string Format format  args

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Easily serialize objects into XML   public static string ToXml lt T gt  this T obj  where T   class       XmlSerializer s   new XmlSerializer obj GetType         using  StringWriter writer   new StringWriter                  s Serialize writer  obj           return writer ToString               lt root gt  lt child gt foo lt  child lt  root gt   ToXml lt MyCustomType gt

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My most used extension is one which can format byte arrays        lt summary gt      Returns a string representation of a byte array       lt  summary gt       lt param name  bytearray  gt The byte array to represent  lt  param gt       lt param name  subdivision  gt The number of elements per group      or 0 to not restrict it  The default is 0  lt  param gt       lt param name  subsubdivision  gt The number of elements per line      or 0 to not restrict it  The default is 0  lt  param gt       lt param name  divider  gt The string dividing the individual bytes  The default is      lt  param gt       lt param name  subdivider  gt The string dividing the groups  The default is       lt  param gt       lt param name  subsubdivider  gt The string dividing the lines  The default is   r n   lt  param gt       lt param name  uppercase  gt Whether the representation is in uppercase hexadecimal      The default is  lt see langword  true   gt   lt  param gt       lt param name  prebyte  gt The string to put before each byte  The default is an empty string  lt  param gt       lt param name  postbyte  gt The string to put after each byte  The default is an empty string  lt  param gt       lt returns gt The string representation  lt  returns gt       lt exception cref  ArgumentNullException  gt       lt paramref name  bytearray   gt  is  lt see langword  null   gt        lt  exception gt  public static string ToArrayString this byte   bytearray      int subdivision   0      int subsubdivision   0      string divider            string subdivider             string subsubdivider     r n       bool uppercase   true      string prebyte           string postbyte              region Contract     if  bytearray    null          throw new ArgumentNullException  bytearray         endregion      StringBuilder sb   new StringBuilder          bytearray Length    2   divider Length   prebyte Length   postbyte Length             subdivision  gt  0    bytearray Length   subdivision    subdivider Length   0             subsubdivision  gt  0    bytearray Length   subsubdivision    subsubdivider Length   0        int groupElements    subdivision  gt  0   subdivision - 1   -1       int lineElements    subsubdivision  gt  0   subsubdivision - 1   -1       for  long i   0  i  lt  bytearray LongLength - 1  i                  sb Append prebyte           sb Append String Format CultureInfo InvariantCulture   uppercase     0 X2       0 x2     bytearray i             sb Append postbyte            if  lineElements    0                        sb Append subsubdivider               groupElements   subdivision              lineElements   subsubdivision                    else if  groupElements    0                        sb Append subdivider               groupElements   subdivision                    else             sb Append divider            lineElements--          groupElements--            sb Append prebyte       sb Append String Format CultureInfo InvariantCulture   uppercase     0 X2       0 x2     bytearray bytearray LongLength - 1         sb Append postbyte        return sb ToString        By default ToArrayString   just prints the byte array as a long string of individual bytes  However  ToArrayString 4  16  groups the bytes in groups of four  with 16 bytes on a line  just as it is in your favorite hex editor  And the following nicely formats the byte array for usage in C  code   byte   bytearray   new byte           Console Write bytearray ToArrayString 4  16                   r n   true   0x       It was written by me  so you may put it on Codeplex

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I use this extension method usually with anonymous types to get a dictionary ala ruby  public static Dictionary lt string  object gt  ToDictionary this object o        var dictionary   new Dictionary lt string  object gt          foreach  var propertyInfo in o GetType   GetProperties                  if  propertyInfo GetIndexParameters   Length    0                        dictionary Add propertyInfo Name  propertyInfo GetValue o  null                         return dictionary      You can use it   var dummy   new   color     000000   width    100    id    myid     Dictionary lt string  object gt  dict   dummy ToDictionary      And with an extended method as   public static void ForEach lt T gt  this IEnumerable lt T gt  source  Action lt T gt  action        foreach  T item in source                action item             You can do it  dummy ToDictionary   ForEach  p    gt  Console Write   0    1      p Key  p Value      Output   color   000000  width  100   id  myid

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ForEach for IEnumerables  public static class FrameworkExtensions          a map function     public static void ForEach lt T gt  this IEnumerable lt T gt   enum  Action lt T gt  mapFunction                foreach  var item in  enum  mapFunction item             Naive example   var buttons   GetListOfButtons   as IEnumerable lt Button gt       click all buttons buttons ForEach b   gt  b Click       Cool example      no need to type the same assignment 3 times  just    new   up an array and use foreach   lambda    everything is properly inferred by csc  -  new   itemA  itemB  itemC        ForEach item   gt            item Number   1          item Str    Hello World              Note   This is not like Select because Select expects your function to return something as for transforming into another list   ForEach simply allows you to execute something for each of the items without any transformations data manipulation   I made this so I can program in a more functional style and I was surprised that List has a ForEach while IEnumerable does not   Put this in the codeplex project

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Hate this kind of code   CloneableClass cc1   new CloneableClass     CloneableClass cc2   null  CloneableClass cc3   null   cc3    CloneableClass  cc1 Clone        this is ok cc3   cc2 Clone        this throws null ref exception    code to handle both cases cc3   cc1    null    CloneableClass  cc1 Clone      null    It s a bit clunky  so I replace it with this extension  which I call CloneOrNull -   public static T CloneOrNull lt T gt   this T self  where T   class  ICloneable       if  self    null  return null      return  T  self Clone         Usage is like    CloneableClass cc1   new CloneableClass     CloneableClass cc2   null  CloneableClass cc3   null   cc3   cc1 CloneOrNull        clone of cc1 cc3   cc2 CloneOrNull        null    look mom  no casts    Please feel free to use this anywhere

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One of my favorites is an IsLike   extension on String   IsLike   matches VB s Like operator  and is handy when you don t want to write a full-on regex to solve your problem   Usage would be something like this    abc  IsLike  a        true  Abc  IsLike   A-Z  a-z  a-z        true  abc123  IsLike             true  hat  IsLike   at       true  joe  IsLike    aeiou         true   joe  IsLike   at       false  joe  IsLike   A-Z  a-z  a-z        false   Here s the code  public static class StringEntentions            lt summary gt          Indicates whether the current string matches the supplied wildcard pattern   Behaves the same         as VB s  Like  Operator           lt  summary gt           lt param name  s  gt The string instance where the extension method is called lt  param gt           lt param name  wildcardPattern  gt The wildcard pattern to match   Syntax matches VB s Like operator  lt  param gt           lt returns gt true if the string matches the supplied pattern  false otherwise  lt  returns gt           lt remarks gt See http   msdn microsoft com en-us library swf8kaxw v VS 100  aspx lt  remarks gt      public static bool IsLike this string s  string wildcardPattern            if  s    null    String IsNullOrEmpty wildcardPattern   return false             turn into regex pattern  and match the whole string with            var regexPattern         Regex Escape wildcardPattern                     add support for              and             regexPattern   regexPattern Replace                                                   Replace                                                 Replace                                                 Replace                                                 Replace                                                  Replace           d             var result   false          try               result   Regex IsMatch s  regexPattern                     catch  ArgumentException ex                throw new ArgumentException String Format  Invalid pattern   0    wildcardPattern   ex                     return result

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A couple of useful extensions if you work with Fiscal Years       lt summary gt      Returns the fiscal year for the passed in date      lt  summary gt       lt param name  value  gt the date lt  param gt       lt returns gt the fiscal year lt  returns gt  public static int FiscalYear this DateTime value      int ret   value Year    if  value Month  gt   7  ret      return ret          lt summary gt      Returns the fiscal year for the passed in date      lt  summary gt       lt param name  value  gt the date lt  param gt       lt returns gt the fiscal year lt  returns gt  public static string FiscalYearString this DateTime value      int fy   FiscalYear value     return   0   1   Format fy - 1  fy

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Some handy string helpers   Usage   I hate unwanted spaces trailing or leading strings and since string can take on a null value  it can be tricky  so i use this   public bool IsGroup   get   return  this GroupName IsNullOrTrimEmpty          Here is another extention method that i use for a new validation framework i m trialing  You can see the regex extensions within that help clean otherwise messy regex   public static bool IsRequiredWithLengthLessThanOrEqualNoSpecial this String str  int length        return  str IsNullOrTrimEmpty    amp  amp          str RegexMatch                  -  r n            amp         w   1  0     RegexReplace     0     length ToString                 RegexOptions Multiline     str      Source   public static class StringHelpers            lt summary gt          Same as String IsNullOrEmpty except that         it captures the Empty state for whitespace         strings by Trimming first           lt  summary gt      public static bool IsNullOrTrimEmpty this String helper                if  helper    null              return true          else             return String Empty    helper Trim               public static int TrimLength this String helper                return helper Trim   Length                  lt summary gt          Returns the matched string from the regex pattern  The         groupName is for named group match values in the form    lt name gt group            lt  summary gt      public static string RegexMatch this String helper  string pattern  RegexOptions options  string groupName                if  groupName IsNullOrTrimEmpty                return Regex Match helper  pattern  options  Value          else             return Regex Match helper  pattern  options  Groups groupName  Value             public static string RegexMatch this String helper  string pattern                return RegexMatch helper  pattern  RegexOptions None  null              public static string RegexMatch this String helper  string pattern  RegexOptions options                return RegexMatch helper  pattern  options  null              public static string RegexMatch this String helper  string pattern  string groupName                return RegexMatch helper  pattern  RegexOptions None  groupName                   lt summary gt          Returns true if there is a match from the regex pattern          lt  summary gt      public static bool IsRegexMatch this String helper  string pattern  RegexOptions options                return helper RegexMatch pattern  options  Length  gt  0             public static bool IsRegexMatch this String helper  string pattern                return helper IsRegexMatch pattern  RegexOptions None                   lt summary gt          Returns a string where matching patterns are replaced by the replacement string           lt  summary gt           lt param name  pattern  gt The regex pattern for matching the items to be replaced lt  param gt           lt param name  replacement  gt The string to replace matching items lt  param gt           lt returns gt  lt  returns gt      public static string RegexReplace this String helper  string pattern  string replacement  RegexOptions options                return Regex Replace helper  pattern  replacement  options              public static string RegexReplace this String helper  string pattern  string replacement                return Regex Replace helper  pattern  replacement  RegexOptions None             I like to do a lot of regex so i consider these easier than adding the using statement and the extra code to handle named groups

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Here is another ThrowIfNull implementation    ThreadStatic  private static string lastMethodName   null    ThreadStatic  private static int lastParamIndex   0    MethodImpl MethodImplOptions NoInlining   public static void ThrowIfNull lt T gt  this T parameter        var currentStackFrame   new StackFrame 1       var props   currentStackFrame GetMethod   GetParameters         if   String IsNullOrEmpty lastMethodName             if  currentStackFrame GetMethod   Name    lastMethodName                lastParamIndex   0            else if  lastParamIndex  gt   props Length - 1                lastParamIndex   0            else               lastParamIndex                    else           lastParamIndex   0             if   typeof T  IsValueType            for  int i   lastParamIndex  i  amp lt  props Length  i                  if  props i  ParameterType IsValueType                    lastParamIndex                  else                   break                                     if  parameter    null            string paramName   props lastParamIndex  Name          throw new ArgumentNullException paramName              lastMethodName   currentStackFrame GetMethod   Name      It s not as efficient as the other impementations  but has cleaner usage   public void Foo         Bar 1  2   Hello    World      no exception     Bar 1  2   Hello   null     exception     Bar 1  2  null   World      exception    public void Bar int x  int y  string someString1  string someString2          will also work with comments removed       x ThrowIfNull          y ThrowIfNull        someString1 ThrowIfNull        someString2 ThrowIfNull           Do something incredibly useful here      Changing the parameters to int  will also work   -bill

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Here s a bitmap extension which can convert bitmaps to grayscale   public static Bitmap GrayScale this Bitmap bitmap        Bitmap newBitmap   new Bitmap bitmap Width  bitmap Height       Graphics g   Graphics FromImage newBitmap          the grayscale ColorMatrix     ColorMatrix colorMatrix   new ColorMatrix new float                   new float     3f   3f   3f  0  0               new float     59f   59f   59f  0  0               new float     11f   11f   11f  0  0               new float    0  0  0  1  0               new float    0  0  0  0  1               ImageAttributes attributes   new ImageAttributes        attributes SetColorMatrix colorMatrix       g DrawImage bitmap  new Rectangle 0  0  bitmap Width  bitmap Height   0  0  bitmap Width  bitmap Height  GraphicsUnit Pixel  attributes       g Dispose        return newBitmap      Sample usage   Bitmap grayscaled   bitmap GrayScale

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Wildcard string comparison   public static bool MatchesWildcard this string text  string pattern        int it   0      while  text CharAt it     0  amp  amp             pattern CharAt it                        if  pattern CharAt it     text CharAt it   amp  amp  pattern CharAt it                      return false          it               int cp   0      int mp   0      int ip   it       while  text CharAt it     0                if  pattern CharAt ip                                if  pattern CharAt   ip     0                  return true              mp   ip              cp   it   1                    else if  pattern CharAt ip     text CharAt it     pattern CharAt ip                                ip                it                      else                       ip   mp              it   cp                         while  pattern CharAt ip                        ip              return pattern CharAt ip     0     public static char CharAt this string s  int index        if  index  lt  s Length          return s index       return   0       It s a direct translation of the C code from this article  hence the CharAt method that returns 0 for the end of the string  if  fileName MatchesWildcard    cs          Console WriteLine   0  is a C  source file   fileName

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I got tired of tedious null-checking while pulling values from MySqlDataReader  so   public static DateTime  GetNullableDateTime this MySqlDataReader dr  string fieldName        DateTime  nullDate   null      return dr IsDBNull dr GetOrdinal fieldName     nullDate   dr GetDateTime fieldName      public static string GetNullableString this MySqlDataReader dr  string fieldName        return dr IsDBNull dr GetOrdinal fieldName     String Empty   dr GetString fieldName      public static char  GetNullableChar this MySqlDataReader dr  string fieldName        char  nullChar   null      return dr IsDBNull dr GetOrdinal fieldName     nullChar   dr GetChar fieldName       Of course this could be used with any SqlDataReader     Both hangy and Joe had some good comments on how to do this  and I have since had an opportunity to implement something similar in a different context  so here is another version   public static int  GetNullableInt32 this IDataRecord dr  int ordinal        int  nullInt   null      return dr IsDBNull ordinal    nullInt   dr GetInt32 ordinal      public static int  GetNullableInt32 this IDataRecord dr  string fieldname        int ordinal   dr GetOrdinal fieldname       return dr GetNullableInt32 ordinal      public static bool  GetNullableBoolean this IDataRecord dr  int ordinal        bool  nullBool   null      return dr IsDBNull ordinal    nullBool   dr GetBoolean ordinal      public static bool  GetNullableBoolean this IDataRecord dr  string fieldname        int ordinal   dr GetOrdinal fieldname       return dr GetNullableBoolean ordinal

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In ASP NET I always get fed up using FindControl and then having to cast and check if the value is null before referencing  So  I added a TryParse   method to Control that mirrors the similar ones in the framework for Int32 etc   public static bool TryParse lt T gt  this Control control  string id  out T result       where T   Control       result   control FindControl id  as T      return result    null      So now you can do this in ASP NET web-form pages   Label lbl  if  Page TryParse  Label1   out lbl         lbl Text    Safely set text

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Wraps a string every n chars   public static string WrapAt this string str  int WrapPos        if  string IsNullOrEmpty str           throw new ArgumentNullException  str    Cannot wrap a null string        str   str Replace   r       Replace   n             if  str Length  lt   WrapPos          return str       for  int i   str Length  i  gt   0  i--          if  i   WrapPos    0  amp  amp  i  gt  0  amp  amp  i    str Length              str   str Insert i    r n        return str

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I use this extension method usually with anonymous types to get a dictionary ala ruby  public static Dictionary lt string  object gt  ToDictionary this object o        var dictionary   new Dictionary lt string  object gt          foreach  var propertyInfo in o GetType   GetProperties                  if  propertyInfo GetIndexParameters   Length    0                        dictionary Add propertyInfo Name  propertyInfo GetValue o  null                         return dictionary      You can use it   var dummy   new   color     000000   width    100    id    myid     Dictionary lt string  object gt  dict   dummy ToDictionary      And with an extended method as   public static void ForEach lt T gt  this IEnumerable lt T gt  source  Action lt T gt  action        foreach  T item in source                action item             You can do it  dummy ToDictionary   ForEach  p    gt  Console Write   0    1      p Key  p Value      Output   color   000000  width  100   id  myid

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Several times I found myself wanting something like  I think  Groovy s  Safe navigation    From http   groovy codehaus org Statements      If you are walking a complex object   graph and don t want to have   NullPointerExceptions thrown you can   use the    operator rather than   to   perform your navigation       def foo   null def bar     foo  something  myMethod   assert bar      null   So  do you think is a good idea adding an extension method for it  Something like   obj SafelyNavigate x   gt  x SomeProperty MaybeAMethod   AnotherProperty     I think it would be nice even if it can also bring some trouble   If you think it s a good idea    What would you think it should happen for value types    return default  throw   disable it by generic constraint   Swallowing NullReferenceException to implement it would be too risky   What do you propose    Walking the expression tree executing every call or member access seems difficult and kind of overkill  if at all possible  doesn t it     Maybe it s just a bad idea  D  but I see it like something that can be useful if done right  If there s nothing like it and you think it holds some value  I may give it a shot and edit the answer afterwards

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NullPartial HTML helper for ASP MVC   When passed a null Model  HTML Partial and HTML RenderPartial will provide the View s model  if this partial is strongly typed and the View has a different type it will throw an exception rather than passing a null reference  These helpers let you specify two different partials so you can keep your null tests out of the view   You have permission to include this on the Codeplex page  public static class nullpartials               public static MvcHtmlString NullPartial this HtmlHelper helper  string Partial  string NullPartial  object Model                        if  Model    null                  return helper Partial NullPartial               else                 return helper Partial Partial  Model                      public static MvcHtmlString NullPartial this HtmlHelper helper  string Partial  string NullPartial  object Model  ViewDataDictionary viewdata                        if  Model    null                  return helper Partial NullPartial  viewdata               else                 return helper Partial Partial  Model  viewdata                      public static void RenderNullPartial this HtmlHelper helper  string Partial  string NullPartial  object Model                        if  Model    null                                helper RenderPartial NullPartial                   return                            else                               helper RenderPartial Partial  Model                   return                                   public static void RenderNullPartial this HtmlHelper helper  string Partial  string NullPartial  object Model  ViewDataDictionary viewdata                        if  Model    null                                helper RenderPartial NullPartial  viewdata                   return                            else                               helper RenderPartial Partial  Model  viewdata                   return

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In  NET  there is a IndexOf and a LastIndexOf methods that return the index of the first and the last occurrence of a match in a String object  I have an extension method to get the index of the nth occurrence   public static partial class StringExtensions        public static int NthIndexOf this String str  String match  int occurrence            int i   1          int index   0           while  i  lt   occurrence  amp  amp                 index   str IndexOf match  index   1       -1                 if  i    occurrence                       Occurrence match found                  return index                            i                          Match not found         return -1

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Find more examples here  www extensionmethod net

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Reduces the length of a string to toLength and adds an additional string to the end of the shortened string to denote that the string was shortened  Default       public static string Shorten this string str  int toLength  string cutOffReplacement                 if  string IsNullOrEmpty str     str Length  lt   toLength          return str      else         return str Remove toLength    cutOffReplacement

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FindControl with built-in casting   public static T FindControl lt T gt  this Control control  string id  where T   Control       return  T control FindControl id       It s nothing amazing  but I feel it makes for cleaner code        With extension method container FindControl lt TextBox gt   myTextBox   SelectedValue    Hello world        Without extension method   TextBox container FindControl  myTextBox    SelectedValue    Hello world      This can be put this in the codeplex project  if so desired

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Please mark your answers with an acceptance to put the code in the Codeplex project    Why  All the Stuff on this site under CC-by-sa-2 5  so just put your Extension overflow Project under the same license and you can freely use it   Anyway  here is a String Reverse function  based on this question        lt summary gt      Reverse a String      lt  summary gt       lt param name  input  gt The string to Reverse lt  param gt       lt returns gt The reversed String lt  returns gt  public static string Reverse this string input        char   array   input ToCharArray        Array Reverse array       return new string array

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I m disappointed that the  NET Framework prefers that files and directories be represented as strings rather than objects  and that the FileInfo and DirectoryInfo types aren t as powerful as I d wish   So  I started to write fluent extension methods as I needed them  e g    public static FileInfo SetExtension this FileInfo fileInfo  string extension        return new FileInfo Path ChangeExtension fileInfo FullName  extension       public static FileInfo SetDirectory this FileInfo fileInfo  string directory        return new FileInfo Path Combine directory  fileInfo Name        Yes  you can put this in the codeplex

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DateTimeExtensions  Examples   DateTime firstDayOfMonth   DateTime Now First    DateTime lastdayOfMonth   DateTime Now Last    DateTime lastFridayInMonth   DateTime Now Last DayOfWeek Friday   DateTime nextFriday   DateTime Now Next DayOfWeek Friday   DateTime lunchTime   DateTime Now SetTime 11  30   DateTime noonOnFriday   DateTime Now Next DayOfWeek Friday  Noon    DateTime secondMondayOfMonth   DateTime Now First DayOfWeek Monday  Next DayOfWeek Monday  Midnight

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This file contains extension methods for generic List lt  gt  class to operate on sorted lists     Duplicate values are OK     O ln n   is still much faster then the O n  of LINQ s searches filters  static partial class SortedList          Return the index of the first element with the key greater then provided         If there s no such element within the provided range  it returns iAfterLast      public static int sortedFirstGreaterIndex lt tElt  tKey gt   this IList lt tElt gt  list  Func lt tElt  tKey  int gt  comparer  tKey key  int iFirst  int iAfterLast                 if  iFirst  lt  0    iAfterLast  lt  0    iFirst  gt  list Count    iAfterLast  gt  list Count               throw new IndexOutOfRangeException            if  iFirst  gt  iAfterLast               throw new ArgumentException            if  iFirst    iAfterLast               return iAfterLast           int low   iFirst  high   iAfterLast             The code below is inspired by the following article             http   en wikipedia org wiki Binary search Single comparison per iteration         while  low  lt  high                         int mid     high   low     2                  mid  might be  iFirst  in case  iFirst 1    iAfterLast                   mid  will never be  iAfterLast               if  comparer  list  mid    key    lt   0        lt    since we gonna find the first  greater  element                 low   mid   1              else                 high   mid                    return low                Return the index of the first element with the key greater then the provided key         If there s no such element  returns list Count      public static int sortedFirstGreaterIndex lt tElt  tKey gt   this IList lt tElt gt  list  Func lt tElt  tKey  int gt  comparer  tKey key                 return list sortedFirstGreaterIndex  comparer  key  0  list Count                  Add an element to the sorted array         This could be an expensive operation if frequently adding elements that sort firstly         This is cheap operation when adding elements that sort near the tail of the list      public static int sortedAdd lt tElt gt   this List lt tElt gt  list  Func lt tElt  tElt  int gt  comparer  tElt elt                 if  list Count    0    comparer  list  list Count - 1    elt    lt   0                            either the list is empty  or the item is greater then all elements already in the collection              list Add  elt                return list Count - 1                    int ind   list sortedFirstGreaterIndex  comparer  elt            list Insert  ind  elt            return ind                Find first exactly equal element  return -1 if not found      public static int sortedFindFirstIndex lt tElt  tKey gt   this List lt tElt gt  list  Func lt tElt  tKey  int gt  comparer  tKey elt                 int low   0  high   list Count - 1           while  low  lt  high                         int mid     high   low     2              if  comparer  list  mid    elt    lt  0                   low   mid   1              else                 high   mid     this includes the case when we ve found an element exactly matching the key                   if  high  gt   0  amp  amp  0    comparer  list  high    elt                 return high          return -1                Return the IEnumerable that returns array elements in the reverse order      public static IEnumerable lt tElt gt  sortedReverse lt tElt gt   this List lt tElt gt  list                 for  int i list Count - 1  i  gt   0  i--               yield return list  i

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public static class DictionaryExtensions       public static Nullable lt TValue gt  GetValueOrNull lt TKey  TValue gt  this Dictionary lt TKey  TValue gt  dictionary  TKey key          where TValue   struct               TValue result          if  dictionary TryGetValue key  out result               return result          else             return null            Free to use  just mention my name  Janko R  bisch  in the code

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I miss the Visual Basic s With statement when moving to C   so here it goes   public static void With lt T gt  this T obj  Action lt T gt  act    act obj       And here s how to use it in C    someVeryVeryLonggggVariableName With x   gt        x Int   123      x Str    Hello       x Str2     World          Saves a lot of typing   Compare this to   someVeryVeryLonggggVariableName Int   123  someVeryVeryLonggggVariableName Str    Hello   someVeryVeryLonggggVariableName Str2     World      put in codeplex project

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public static class EnumerableExtensions        Pure      public static U MapReduce lt T  U gt  this IEnumerable lt T gt  enumerable  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways enumerable    null           CodeContract RequiresAlways enumerable Skip 1  Any             CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null           return enumerable AsParallel   Select map  Aggregate reduce              Pure      public static U MapReduce lt T  U gt  this IList lt T gt  list  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways list    null           CodeContract RequiresAlways list Count  gt   2           CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null           U result   map list 0            for  int i   1  i  lt  list Count  i                          result   reduce result map list i                       return result               Parallel version  creates garbage      Pure      public static U MapReduce lt T  U gt  this IList lt T gt  list  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways list    null           CodeContract RequiresAlways list Skip 1  Any             CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null            U   mapped   new U list Count           Parallel For 0  mapped Length  i   gt                                mapped i    map list i                            U result   mapped 0           for  int i   1  i  lt  list Count  i                          result   reduce result  mapped i                      return result

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I thought I d seen this somewhere before  but couldn t find it suggested anywhere here   MS has a TryGetValue function on the IDictionary interface  but it returns a bool and gives the value in an out parameter  so here s a simpler  cleaner implementation   public static TVal GetValueOrDefault lt TKey  TVal gt  this IDictionary lt TKey  TVal gt  d  TKey key      if  d ContainsKey key       return d key     return default TVal

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I use this a lot with nullable numbers  I helps catch those division by 0  NaN  Infinity     public static bool IsNullOrDefault lt T gt  this T  o       where T   struct           return o    null    o Value Equals default T

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Turn this   DbCommand command   connection CreateCommand    command CommandText    SELECT  param    DbParameter param   command CreateParameter    param ParameterName     param   param Value    Hello World    command Parameters Add param         into this   DbCommand command   connection CreateCommand  SELECT  0     Hello World          using this extension method   using System  using System Data Common  using System Globalization  using System Reflection   namespace DbExtensions       public static class Db          static readonly Func lt DbConnection  DbProviderFactory gt  getDbProviderFactory        static readonly Func lt DbCommandBuilder  int  string gt  getParameterName        static readonly Func lt DbCommandBuilder  int  string gt  getParameterPlaceholder         static Db               getDbProviderFactory    Func lt DbConnection  DbProviderFactory gt  Delegate CreateDelegate typeof Func lt DbConnection  DbProviderFactory gt    typeof DbConnection  GetProperty  DbProviderFactory   BindingFlags Instance   BindingFlags NonPublic  GetGetMethod true             getParameterName    Func lt DbCommandBuilder  int  string gt  Delegate CreateDelegate typeof Func lt DbCommandBuilder  int  string gt    typeof DbCommandBuilder  GetMethod  GetParameterName   BindingFlags Instance   BindingFlags NonPublic  Type DefaultBinder  new Type     typeof Int32     null             getParameterPlaceholder    Func lt DbCommandBuilder  int  string gt  Delegate CreateDelegate typeof Func lt DbCommandBuilder  int  string gt    typeof DbCommandBuilder  GetMethod  GetParameterPlaceholder   BindingFlags Instance   BindingFlags NonPublic  Type DefaultBinder  new Type     typeof Int32     null                   public static DbProviderFactory GetProviderFactory this DbConnection connection             return getDbProviderFactory connection                  public static DbCommand CreateCommand this DbConnection connection  string commandText  params object   parameters              if  connection    null  throw new ArgumentNullException  connection              return CreateCommandImpl GetProviderFactory connection  CreateCommandBuilder    connection CreateCommand    commandText  parameters                  private static DbCommand CreateCommandImpl DbCommandBuilder commandBuilder  DbCommand command  string commandText  params object   parameters              if  commandBuilder    null  throw new ArgumentNullException  commandBuilder             if  command    null  throw new ArgumentNullException  command             if  commandText    null  throw new ArgumentNullException  commandText              if  parameters    null    parameters Length    0                command CommandText   commandText              return command                       object   paramPlaceholders   new object parameters Length             for  int i   0  i  lt  paramPlaceholders Length  i                   DbParameter dbParam   command CreateParameter                dbParam ParameterName   getParameterName commandBuilder  i               dbParam Value   parameters i     DBNull Value              command Parameters Add dbParam                paramPlaceholders i    getParameterPlaceholder commandBuilder  i                        command CommandText   String Format CultureInfo InvariantCulture  commandText  paramPlaceholders             return command                   More ADO NET extension methods  DbExtensions

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Here s another pair I ve found endless use for   public static T ObjectWithMin lt T  TResult gt  this IEnumerable lt T gt  sequence  Func lt T  TResult gt  predicate      where T   class     where TResult   IComparable       if   sequence Any    return null         get the first object with its predicate value     var seed   sequence Select x   gt  new  Object   x  Value   predicate x    FirstOrDefault          compare against all others  replacing the accumulator with the lesser value       tie goes to first object found     return         sequence Select x   gt  new  Object   x  Value   predicate x                 Aggregate seed  acc  x    gt  acc Value CompareTo x Value   lt   0   acc   x  Object     public static T ObjectWithMax lt T  TResult gt  this IEnumerable lt T gt  sequence  Func lt T  TResult gt  predicate      where T   class     where TResult   IComparable       if   sequence Any    return null         get the first object with its predicate value     var seed   sequence Select x   gt  new  Object   x  Value   predicate x    FirstOrDefault          compare against all others  replacing the accumulator with the greater value       tie goes to last object found     return         sequence Select x   gt  new  Object   x  Value   predicate x                 Aggregate seed   acc  x    gt  acc Value CompareTo x Value   gt  0   acc   x  Object      Usage   var myObject   myList ObjectWithMin x  gt x PropA     These methods basically replace usages like  var myObject   myList OrderBy x  gt x PropA  FirstOrDefault      O nlog n   and unstable   and  var myObject   myList Where x  gt x PropA    myList Min x  gt x PropA   FirstOrDefault      O N 2  but stable   and  var minValue   myList Min x  gt x PropA   var myObject   myList Where x  gt x PropA    minValue  FirstOrDefault      not a one-liner  and though linear and stable it s slower  evaluates the enumerable twice

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Here s a fun one from our codebase at work  Walk an expensive lazy-eval enumerable on a job thread and push the results back through an observable   public static IObservable lt T gt  ToAsyncObservable lt T gt  this IEnumerable lt T gt   this        return Observable Create lt T gt  observer   gt                var task   new Task      gt                        try                                this Run observer OnNext                   observer OnCompleted                              catch  Exception e                                observer OnError e                                      task Start             return      gt                   Silly sample   new DirectoryInfo   c  program files        EnumerateFiles      SearchOption AllDirectories       ToAsyncObservable        BufferWithTime TimeSpan FromSeconds 0 5        ObserveOnDispatcher        Subscribe          l   gt  Console WriteLine   0  received   l Count                gt  Console WriteLine  Done       for            Thread Sleep 10       Dispatcher PushFrame new DispatcherFrame         Obviously this extension will be useless to you if you aren t using the brilliant Reactive Extensions   UPDATE thanks to Richard in the comments  this extension method is unnecessary  RX already has an extension method  ToObservable  that takes an IScheduler  Use that instead

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When using a dictionary where the key is a string  return existing key using a case-insensitive search  Our use case for this was for file paths        lt summary gt      Gets the key using  lt paramref name  caseInsensitiveKey   gt  from  lt paramref name  dictionary   gt        lt  summary gt       lt typeparam name  T  gt The dictionary value  lt  typeparam gt       lt param name  dictionary  gt The dictionary  lt  param gt       lt param name  caseInsensitiveKey  gt The case insensitive key  lt  param gt       lt returns gt      An existing key  or  lt see cref  string Empty   gt  if not found       lt  returns gt  public static string GetKeyIgnoringCase lt T gt  this IDictionary lt string  T gt  dictionary  string caseInsensitiveKey        if  string IsNullOrEmpty caseInsensitiveKey   return string Empty      foreach  string key in dictionary Keys                if  key Equals caseInsensitiveKey  StringComparison InvariantCultureIgnoreCase                         return key                      return string Empty

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I have an extension method for logging exceptions   public static void Log this Exception obj        your logging logic here     And it is used like this   try         Your stuff here   catch Exception ex        ex Log         sorry for posting twice  the 2nd one is better designed  -

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ASP NET HTML Encode - short and sweet   public static string ToHtmlEncodedString this string s        if  String IsNullOrEmpty s           return s      return HttpUtility HtmlEncode s

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GetMemberName allows to get the string with the name of a member with compile time safety   public static string GetMemberName lt T  TResult gt       this T anyObject       Expression lt Func lt T  TResult gt  gt  expression        return   MemberExpression expression Body  Member Name      Usage    blah  GetMemberName x   gt  x Length      returns  Length      It comes together with a non-extension static method if you don t have a instance   public static string GetMemberName lt T  TReturn gt       Expression lt Func lt T  TReturn gt  gt  expression      where T   class       return   MemberExpression expression Body  Member Name      But the call doesn t look as pretty of course   ReflectionUtility GetMemberName  string  s   gt  s Length      returns  Length      You can put it on Codeplex if you want

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I m disappointed that the  NET Framework prefers that files and directories be represented as strings rather than objects  and that the FileInfo and DirectoryInfo types aren t as powerful as I d wish   So  I started to write fluent extension methods as I needed them  e g    public static FileInfo SetExtension this FileInfo fileInfo  string extension        return new FileInfo Path ChangeExtension fileInfo FullName  extension       public static FileInfo SetDirectory this FileInfo fileInfo  string directory        return new FileInfo Path Combine directory  fileInfo Name        Yes  you can put this in the codeplex

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I like these NUnit Assert extensions  http   svn caffeine-it com openrasta trunk src Rasta Testing AssertExtensions cs

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Reduces the length of a string to toLength and adds an additional string to the end of the shortened string to denote that the string was shortened  Default       public static string Shorten this string str  int toLength  string cutOffReplacement                 if  string IsNullOrEmpty str     str Length  lt   toLength          return str      else         return str Remove toLength    cutOffReplacement

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Turn this   DbCommand command   connection CreateCommand    command CommandText    SELECT  param    DbParameter param   command CreateParameter    param ParameterName     param   param Value    Hello World    command Parameters Add param         into this   DbCommand command   connection CreateCommand  SELECT  0     Hello World          using this extension method   using System  using System Data Common  using System Globalization  using System Reflection   namespace DbExtensions       public static class Db          static readonly Func lt DbConnection  DbProviderFactory gt  getDbProviderFactory        static readonly Func lt DbCommandBuilder  int  string gt  getParameterName        static readonly Func lt DbCommandBuilder  int  string gt  getParameterPlaceholder         static Db               getDbProviderFactory    Func lt DbConnection  DbProviderFactory gt  Delegate CreateDelegate typeof Func lt DbConnection  DbProviderFactory gt    typeof DbConnection  GetProperty  DbProviderFactory   BindingFlags Instance   BindingFlags NonPublic  GetGetMethod true             getParameterName    Func lt DbCommandBuilder  int  string gt  Delegate CreateDelegate typeof Func lt DbCommandBuilder  int  string gt    typeof DbCommandBuilder  GetMethod  GetParameterName   BindingFlags Instance   BindingFlags NonPublic  Type DefaultBinder  new Type     typeof Int32     null             getParameterPlaceholder    Func lt DbCommandBuilder  int  string gt  Delegate CreateDelegate typeof Func lt DbCommandBuilder  int  string gt    typeof DbCommandBuilder  GetMethod  GetParameterPlaceholder   BindingFlags Instance   BindingFlags NonPublic  Type DefaultBinder  new Type     typeof Int32     null                   public static DbProviderFactory GetProviderFactory this DbConnection connection             return getDbProviderFactory connection                  public static DbCommand CreateCommand this DbConnection connection  string commandText  params object   parameters              if  connection    null  throw new ArgumentNullException  connection              return CreateCommandImpl GetProviderFactory connection  CreateCommandBuilder    connection CreateCommand    commandText  parameters                  private static DbCommand CreateCommandImpl DbCommandBuilder commandBuilder  DbCommand command  string commandText  params object   parameters              if  commandBuilder    null  throw new ArgumentNullException  commandBuilder             if  command    null  throw new ArgumentNullException  command             if  commandText    null  throw new ArgumentNullException  commandText              if  parameters    null    parameters Length    0                command CommandText   commandText              return command                       object   paramPlaceholders   new object parameters Length             for  int i   0  i  lt  paramPlaceholders Length  i                   DbParameter dbParam   command CreateParameter                dbParam ParameterName   getParameterName commandBuilder  i               dbParam Value   parameters i     DBNull Value              command Parameters Add dbParam                paramPlaceholders i    getParameterPlaceholder commandBuilder  i                        command CommandText   String Format CultureInfo InvariantCulture  commandText  paramPlaceholders             return command                   More ADO NET extension methods  DbExtensions

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I got tired of tedious null-checking while pulling values from MySqlDataReader  so   public static DateTime  GetNullableDateTime this MySqlDataReader dr  string fieldName        DateTime  nullDate   null      return dr IsDBNull dr GetOrdinal fieldName     nullDate   dr GetDateTime fieldName      public static string GetNullableString this MySqlDataReader dr  string fieldName        return dr IsDBNull dr GetOrdinal fieldName     String Empty   dr GetString fieldName      public static char  GetNullableChar this MySqlDataReader dr  string fieldName        char  nullChar   null      return dr IsDBNull dr GetOrdinal fieldName     nullChar   dr GetChar fieldName       Of course this could be used with any SqlDataReader     Both hangy and Joe had some good comments on how to do this  and I have since had an opportunity to implement something similar in a different context  so here is another version   public static int  GetNullableInt32 this IDataRecord dr  int ordinal        int  nullInt   null      return dr IsDBNull ordinal    nullInt   dr GetInt32 ordinal      public static int  GetNullableInt32 this IDataRecord dr  string fieldname        int ordinal   dr GetOrdinal fieldname       return dr GetNullableInt32 ordinal      public static bool  GetNullableBoolean this IDataRecord dr  int ordinal        bool  nullBool   null      return dr IsDBNull ordinal    nullBool   dr GetBoolean ordinal      public static bool  GetNullableBoolean this IDataRecord dr  string fieldname        int ordinal   dr GetOrdinal fieldname       return dr GetNullableBoolean ordinal

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Aww why not  Here s an extension to IList  can t be IEnumerable because i use list specific features  for insertion sort   internal static class SortingHelpers            lt summary gt          Performs an insertion sort on this list           lt  summary gt           lt typeparam name  T  gt The type of the list supplied  lt  typeparam gt           lt param name  list  gt the list to sort  lt  param gt           lt param name  comparison  gt the method for comparison of two elements  lt  param gt           lt returns gt  lt  returns gt      public static void InsertionSort lt T gt  this IList lt T gt  list  Comparison lt T gt  comparison                for  int i   2  i  lt  list Count  i                          for  int j   i  j  gt  1  amp  amp  comparison list j   list j - 1    lt  0  j--                                T tempItem   list j                   list RemoveAt j                   list Insert j - 1  tempItem                                     An example   List lt int gt  list1     3  5  1  2  9  4  6    list1 InsertionSort  a b    gt  a - b     list is now in order of 1 2 3 4 5 6 9

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For raising events concisely   public static void Raise this EventHandler handler  object sender  EventArgs e        if  handler    null                handler sender  e            public static void Raise lt T gt  this EventHandler lt T gt  handler  object sender  T e  where T   EventArgs       if  handler    null                handler sender  e             Usage   public event EventHandler Bar   public void Foo         Bar Raise this  EventArgs Empty       There s a bit of discussion about potential thread-safety issues here  Since  NET 4  the above form is thread-safe  but requires rearranging and some locks if using an older version

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Some of my best method extensions  I have a lot      public static T ToEnum lt T gt  this string str  where T   struct       return  T Enum Parse typeof T   str        DayOfWeek sunday     Sunday  ToEnum lt DayOfWeek gt      public static string ToString lt T gt  this IEnumerable lt T gt  collection  string separator        return ToString collection  t   gt  t ToString    separator      public static string ToString lt T gt  this IEnumerable lt T gt  collection  Func lt T  string gt  stringElement  string separator        StringBuilder sb   new StringBuilder        foreach  var item in collection                sb Append stringElement item            sb Append separator             return sb ToString 0  Math Max 0  sb Length - separator Length        quita el ultimo separador      new    1 2 3  ToString i  gt i 2         -- gt   2  4  6    Also  the next ones are meant to be able to continue in the same line in almost any situation  not declaring new variables and then removing state   public static R Map lt T  R gt  this T t  Func lt T  R gt  func        return func t      ExpensiveFindWally   Map wally  gt wally FirstName         wally LastName   public static R TryCC lt T  R gt  this T t  Func lt T  R gt  func      where T   class     where R   class       if  t    null  return null      return func t      public static R  TryCS lt T  R gt  this T t  Func lt T  R gt  func      where T   class     where R   struct       if  t    null  return null      return func t      public static R  TryCS lt T  R gt  this T t  Func lt T  R  gt  func      where T   class     where R   struct       if  t    null  return null      return func t      public static R TrySC lt T  R gt  this T  t  Func lt T  R gt  func      where T   struct     where R   class       if  t    null  return null      return func t Value      public static R  TrySS lt T  R gt  this T  t  Func lt T  R gt  func      where T   struct     where R   struct       if  t    null  return null      return func t Value      public static R  TrySS lt T  R gt  this T  t  Func lt T  R  gt  func      where T   struct     where R   struct       if  t    null  return null      return func t Value        int  bossNameLength    Departament Boss TryCC b  gt b Name  TryCS s  gt s Length     public static T ThrowIfNullS lt T gt  this T  t  string mensaje      where T   struct       if  t    null          throw new NullReferenceException mensaje       return t Value     public static T ThrowIfNullC lt T gt  this T t  string mensaje      where T   class       if  t    null          throw new NullReferenceException mensaje       return t     public static T Do lt T gt  this T t  Action lt T gt  action        action t       return t       Button b   new Button Content    Click   Do b  gt Canvas SetColumn b 2     public static T TryDo lt T gt  this T t  Action lt T gt  action  where T   class       if  t    null          action t       return t     public static T  TryDoS lt T gt  this T  t  Action lt T gt  action  where T   struct       if  t    null          action t Value       return t      Hope it doesn t look like coming from Mars

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These extension methods are pretty useful for me when parsing form input before putting into the database  public static int  ToInt this string input         int val      if  int TryParse input  out val           return val      return null     public static DateTime  ToDate this string input        DateTime val      if  DateTime TryParse input  out val           return val      return null     public static decimal  ToDecimal this string input        decimal val      if  decimal TryParse input  out val           return val      return null

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A convenient way to deal with sizes    public static class Extensions       public static int K this int value            return value   1024            public static int M this int value            return value   1024   1024           public class Program       public void Main             WSHttpContextBinding serviceMultipleTokenBinding   new WSHttpContextBinding                 MaxBufferPoolSize   2 M       instead of 2097152             MaxReceivedMessageSize   64 K       instead of 65536

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Didn t check the whole thread  so it may already be here  but   public static class FluentOrderingExtensions     public class FluentOrderer lt T gt    IEnumerable lt T gt                internal List lt Comparison lt T gt  gt  Comparers   new List lt Comparison lt T gt  gt              internal IEnumerable lt T gt  Source           public FluentOrderer IEnumerable lt T gt  source                        Source   source                      region Implementation of IEnumerable          public IEnumerator lt T gt  GetEnumerator                         var workingArray   Source ToArray                Array Sort workingArray  IterativeComparison                foreach var element in workingArray  yield return element                     private int IterativeComparison T a  T b                        foreach  var comparer in Comparers                                var result   comparer a b                   if result    0  return result                            return 0                     IEnumerator IEnumerable GetEnumerator                         return GetEnumerator                        endregion            public static FluentOrderer lt T gt  OrderFluentlyBy lt T TResult gt  this IEnumerable lt T gt  source  Func lt T TResult gt  predicate           where TResult   IComparable lt TResult gt                var result   new FluentOrderer lt T gt  source           result Comparers Add  a b   gt predicate a  CompareTo predicate b             return result             public static FluentOrderer lt T gt  OrderFluentlyByDescending lt T TResult gt  this IEnumerable lt T gt  source  Func lt T TResult gt  predicate           where TResult   IComparable lt TResult gt                var result   new FluentOrderer lt T gt  source           result Comparers Add  a b   gt predicate a  CompareTo predicate b     -1           return result             public static FluentOrderer lt T gt  ThenBy lt T  TResult gt  this FluentOrderer lt T gt  source  Func lt T  TResult gt  predicate          where TResult   IComparable lt TResult gt                source Comparers Add  a  b    gt  predicate a  CompareTo predicate b             return source             public static FluentOrderer lt T gt  ThenByDescending lt T  TResult gt  this FluentOrderer lt T gt  source  Func lt T  TResult gt  predicate          where TResult   IComparable lt TResult gt                source Comparers Add  a  b    gt  predicate a  CompareTo predicate b     -1           return source            Usage   var myFluentlyOrderedList   GetABunchOfComplexObjects        OrderFluentlyBy x  gt x PropertyA       ThenByDescending x  gt x PropertyB       ThenBy x  gt x SomeMethod         ThenBy x  gt SomeOtherMethodAppliedTo x        ToList          assuming of course that all the predicates return types that are IComparable to themselves  It would work better with a stable sort like a MergeSort instead of  NET s built-in QuickSort  but it provides you with readable multi-field ordering ability similar to SQL  as close as a method chain can get  anyway   You can extend this to accomodate members that aren t IComparable  by defining overloads that take a comparison lambda instead of creating it based on a predicate   EDIT  A little explanation  since the commenter got some upticks  this set of methods improves upon the basic OrderBy   functionality by allowing you to sort based on multiple fields in descending order of importance  A real-world example would be sorting a list of invoices by customer  then by invoice number  or invoice date   Other methods of getting the data in this order either wouldn t work  OrderBy   uses an unstable sort  so it cannot be chained  or would be inefficient and not look like it does what you re trying to do

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Similar to the string As and Is above  but global to all objects   It s quite simple  but I use these a lot to alleviate parens explosion with boxing   public static class ExtensionMethods Object        DebuggerStepThrough        public static bool Is lt T gt  this object item  where T   class               return item is T              DebuggerStepThrough        public static bool IsNot lt T gt  this object item  where T   class               return   item Is lt T gt                  DebuggerStepThrough        public static T As lt T gt  this object item  where T   class               return item as T            I am happy for this code to be used at codeplex  indeed it already is

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Convert a double to string formatted using the specified culture   public static class ExtensionMethods      public static string ToCurrency this double value  string cultureName          CultureInfo currentCulture   new CultureInfo cultureName       return  string Format currentCulture    0 C    value            Example   double test   154 20  string testString   test ToCurrency  en-US        154 20

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Function to compare Files Directories through the OS File System Info  This is useful to compare shares with local files     Usage   DirectoryInfo dir   new DirectoryInfo   C  test myShareDir    Console WriteLine dir IsSameFileAs     myMachineName myShareDir      FileInfo file   new FileInfo   C  test myShareDir file txt    Console WriteLine file IsSameFileAs     myMachineName myShareDir file txt       Code   public static class FileExtensions       struct BY HANDLE FILE INFORMATION               public uint FileAttributes          public System Runtime InteropServices ComTypes FILETIME CreationTime          public System Runtime InteropServices ComTypes FILETIME LastAccessTime          public System Runtime InteropServices ComTypes FILETIME LastWriteTime          public uint VolumeSerialNumber          public uint FileSizeHigh          public uint FileSizeLow          public uint NumberOfLinks          public uint FileIndexHigh          public uint FileIndexLow                       CreateFile constants            const uint FILE SHARE READ   0x00000001      const uint OPEN EXISTING   3      const uint GENERIC READ    0x80000000       const uint FILE FLAG BACKUP SEMANTICS   0x02000000         DllImport  kernel32 dll   SetLastError   true       static extern IntPtr CreateFile          string lpFileName          uint dwDesiredAccess          uint dwShareMode          IntPtr lpSecurityAttributes          uint dwCreationDisposition          uint dwFlagsAndAttributes          IntPtr hTemplateFile         DllImport  kernel32 dll   SetLastError   true       static extern bool GetFileInformationByHandle IntPtr hFile  out BY HANDLE FILE INFORMATION lpFileInformation        public static bool IsSameFileAs this FileSystemInfo file  string path                BY HANDLE FILE INFORMATION fileInfo1  fileInfo2          IntPtr ptr1   CreateFile file FullName  GENERIC READ  FILE SHARE READ  IntPtr Zero  OPEN EXISTING  FILE FLAG BACKUP SEMANTICS  IntPtr Zero           if   int ptr1    -1                        System ComponentModel Win32Exception e   new System ComponentModel Win32Exception Marshal GetLastWin32Error                 throw e                    IntPtr ptr2   CreateFile path  GENERIC READ  FILE SHARE READ  IntPtr Zero  OPEN EXISTING  FILE FLAG BACKUP SEMANTICS  IntPtr Zero           if   int ptr2    -1                        System ComponentModel Win32Exception e   new System ComponentModel Win32Exception Marshal GetLastWin32Error                 throw e                    GetFileInformationByHandle ptr1  out fileInfo1           GetFileInformationByHandle ptr2  out fileInfo2            return   fileInfo1 FileIndexHigh    fileInfo2 FileIndexHigh   amp  amp               fileInfo1 FileIndexLow    fileInfo2 FileIndexLow

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Simple but nicer than  Enumerable Range   IMHO        lt summary gt      Replace  Enumerable Range n   with  n Range          lt  summary gt       lt param name  n  gt iterations lt  param gt       lt returns gt 0  n-1 lt  returns gt  public static IEnumerable lt int gt  Range this int n        for  int i   0  i  lt  n  i            yield return i

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Here s a fun one from our codebase at work  Walk an expensive lazy-eval enumerable on a job thread and push the results back through an observable   public static IObservable lt T gt  ToAsyncObservable lt T gt  this IEnumerable lt T gt   this        return Observable Create lt T gt  observer   gt                var task   new Task      gt                        try                                this Run observer OnNext                   observer OnCompleted                              catch  Exception e                                observer OnError e                                      task Start             return      gt                   Silly sample   new DirectoryInfo   c  program files        EnumerateFiles      SearchOption AllDirectories       ToAsyncObservable        BufferWithTime TimeSpan FromSeconds 0 5        ObserveOnDispatcher        Subscribe          l   gt  Console WriteLine   0  received   l Count                gt  Console WriteLine  Done       for            Thread Sleep 10       Dispatcher PushFrame new DispatcherFrame         Obviously this extension will be useless to you if you aren t using the brilliant Reactive Extensions   UPDATE thanks to Richard in the comments  this extension method is unnecessary  RX already has an extension method  ToObservable  that takes an IScheduler  Use that instead

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I find myself doing this  over and over  again     public static bool EqualsIgnoreCase this string a  string b        return string Equals a  b  StringComparison OrdinalIgnoreCase          followed by StartsWithIgnoreCase  EndsWithIgnoreCase and ContainsIgnoreCase

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I have various extension methods in my MiscUtil project  full source is available there - I m not going to repeat it here   My favourites  some of which involve other classes  such as ranges    Date and time stuff - mostly for unit tests  Not sure I d use them in production     var birthday   19 June 1976   var workingDay   7 Hours     30 Minutes      Ranges and stepping - massive thanks to Marc Gravell for his operator stuff to make this possible   var evenNaturals   2 To int MaxValue  Step 2   var daysSinceBirth   birthday To DateTime Today  Step 1 Days       Comparisons   var myComparer   ProjectionComparer Create Person p   gt  p Name   var next   myComparer ThenBy p   gt  p Age   var reversed   myComparer Reverse      Argument checking   x ThrowIfNull  x      LINQ to XML applied to anonymous types  or other types with appropriate properties        lt Name gt Jon lt  Name gt  lt Age gt 32 lt  Age gt  new   Name  Jon   Age 32  ToXElements       Name  Jon  Age  32   as XAttributes  obviously  new   Name  Jon   Age 32  ToXAttributes     Push LINQ - would take too long to explain here  but search for it

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I have implemented a package of extension methods  available at http   foop codeplex com   and some of my daily used ones are      the most beloved extension method for me is Pipe   lt    variable Pipe x   gt  this SomeFunction x   Pipe y   gt                 return this SomeOtherFunction y        gt   var d   28 December 2009      some extension methods for creating DateTime DateTime justDatePart   d JustDate    TimeSpan justTimePart   d JustTime    var nextTime   d Add 5 Hours      using StreamReader reader   new StreamReader  lines-of-data-file-for-example                     for reading streams line by line and usable in LINQ     var query   from line in reader Lines                     where line Contains  today                  select new   Parts   PartsOf line   Time    now       500 Sleep     XmlSerialize and XmlDeserialize  IsNull and IsNotNull  IfTrue  IfFalse and Iff  true IfTrue      gt  Console WriteLine  it is true then      IfNull and IfNotNull

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The WhereIf   Method  var query   dc Reviewer       Where r   gt  r FacilityID    facilityID        WhereIf CheckBoxActive Checked  r   gt  r IsActive     public static IEnumerable lt TSource gt  WhereIf lt TSource gt       this IEnumerable lt TSource gt  source      bool condition  Func lt TSource  bool gt  predicate          if  condition           return source Where predicate        else          return source      public static IQueryable lt TSource gt  WhereIf lt TSource gt       this IQueryable lt TSource gt  source      bool condition  Expression lt Func lt TSource  bool gt  gt  predicate          if  condition           return source Where predicate        else          return source       I also added overloads for the index predicate in the Where   extension method  For more fun  add a flavor that includes an additional  else  predicate

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Below is an extension method that adapts Rick Strahl s code  and the comments too  to stop you having to guess or read the byte order mark of a byte array or text file each time you convert it to a string   The snippet allows you to simply do   byte   buffer   File ReadAllBytes   C  file txt    string content   buffer GetString      If you find any bugs please add to the comments  Feel free to include it in the Codeplex project   public static class Extensions            lt summary gt          Converts a byte array to a string  using its byte order mark to convert it to the right encoding          Original article  http   www west-wind com WebLog posts 197245 aspx          lt  summary gt           lt param name  buffer  gt An array of bytes to convert lt  param gt           lt returns gt The byte as a string  lt  returns gt      public static string GetString this byte   buffer                if  buffer    null    buffer Length    0              return                 Ansi as default         Encoding encoding   Encoding Default                                 EF BB BF    UTF-8              FF FE UTF-16    little endian              FE FF UTF-16    big endian              FF FE 00 00 UTF-32  little endian              00 00 FE FF UTF-32  big-endian                       if  buffer 0     0xef  amp  amp  buffer 1     0xbb  amp  amp  buffer 2     0xbf              encoding   Encoding UTF8          else if  buffer 0     0xfe  amp  amp  buffer 1     0xff              encoding   Encoding Unicode          else if  buffer 0     0xfe  amp  amp  buffer 1     0xff              encoding   Encoding BigEndianUnicode     utf-16be         else if  buffer 0     0  amp  amp  buffer 1     0  amp  amp  buffer 2     0xfe  amp  amp  buffer 3     0xff              encoding   Encoding UTF32          else if  buffer 0     0x2b  amp  amp  buffer 1     0x2f  amp  amp  buffer 2     0x76              encoding   Encoding UTF7           using  MemoryStream stream   new MemoryStream                          stream Write buffer  0  buffer Length               stream Seek 0  SeekOrigin Begin               using  StreamReader reader   new StreamReader stream  encoding                                 return reader ReadToEnd

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Here is another ThrowIfNull implementation    ThreadStatic  private static string lastMethodName   null    ThreadStatic  private static int lastParamIndex   0    MethodImpl MethodImplOptions NoInlining   public static void ThrowIfNull lt T gt  this T parameter        var currentStackFrame   new StackFrame 1       var props   currentStackFrame GetMethod   GetParameters         if   String IsNullOrEmpty lastMethodName             if  currentStackFrame GetMethod   Name    lastMethodName                lastParamIndex   0            else if  lastParamIndex  gt   props Length - 1                lastParamIndex   0            else               lastParamIndex                    else           lastParamIndex   0             if   typeof T  IsValueType            for  int i   lastParamIndex  i  amp lt  props Length  i                  if  props i  ParameterType IsValueType                    lastParamIndex                  else                   break                                     if  parameter    null            string paramName   props lastParamIndex  Name          throw new ArgumentNullException paramName              lastMethodName   currentStackFrame GetMethod   Name      It s not as efficient as the other impementations  but has cleaner usage   public void Foo         Bar 1  2   Hello    World      no exception     Bar 1  2   Hello   null     exception     Bar 1  2  null   World      exception    public void Bar int x  int y  string someString1  string someString2          will also work with comments removed       x ThrowIfNull          y ThrowIfNull        someString1 ThrowIfNull        someString2 ThrowIfNull           Do something incredibly useful here      Changing the parameters to int  will also work   -bill

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For ASP NET  I use these extensions to HttpSessionState to load objects in session  It allows you to load session objects in a clean manner  and will create and initialize them if they do not exist  I use the two extension methods like so   private bool CreateMode  private MyClass SomeClass   protected override void OnInit  EventArgs e        CreateMode   Session GetSessionValue lt bool gt    someKey1        gt  true       SomeClass   Session GetSessionClass lt MyClass gt    someKey2        gt  new MyClass                   MyProperty   123              Here are the extension classes   public static class SessionExtensions           public delegate object UponCreate         public static T GetSessionClass lt T gt   this HttpSessionState session          string key  UponCreate uponCreate  where T   class               if  null    session key                         var item   uponCreate    as T              session key    item              return item                    return session key  as T            public static T GetSessionValue lt T gt   this HttpSessionState session          string key  UponCreate uponCreate  where T   struct               if  null    session key                         var item   uponCreate                session key    item              return  T item                    return  T session key

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public static class StringExtensions             lt summary gt          Parses a string into an Enum          lt  summary gt           lt typeparam name  T  gt The type of the Enum lt  typeparam gt           lt param name  value  gt String value to parse lt  param gt           lt returns gt The Enum corresponding to the stringExtensions lt  returns gt      public static T EnumParse lt T gt  this string value            return StringExtensions EnumParse lt T gt  value  false              public static T EnumParse lt T gt  this string value  bool ignorecase             if  value    null                throw new ArgumentNullException  value                       value   value Trim             if  value Length    0                throw new ArgumentException  Must specify valid information for parsing in the string     value                       Type t   typeof T            if   t IsEnum                throw new ArgumentException  Type provided must be an Enum     T                       return  T Enum Parse t  value  ignorecase             Useful to parse a string into an Enum   public enum TestEnum       Bar      Test    public class Test       public void Test                 TestEnum foo    Test  EnumParse lt TestEnum gt                Credit goes to Scott Dorman  --- Edit for Codeplex project ---  I have asked Scott Dorman if he would mind us publishing his code in the Codeplex project  This is the reply I got from him      Thanks for the heads-up on both the SO post and the CodePlex project  I have upvoted your answer on the question  Yes  the code is effectively in the public domain currently under the CodeProject Open License  http   www codeproject com info cpol10 aspx        I have no problems with this being included in the CodePlex project  and if you want to add me to the project  username is sdorman  I will add that method plus some additional enum helper methods

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Some of my best method extensions  I have a lot      public static T ToEnum lt T gt  this string str  where T   struct       return  T Enum Parse typeof T   str        DayOfWeek sunday     Sunday  ToEnum lt DayOfWeek gt      public static string ToString lt T gt  this IEnumerable lt T gt  collection  string separator        return ToString collection  t   gt  t ToString    separator      public static string ToString lt T gt  this IEnumerable lt T gt  collection  Func lt T  string gt  stringElement  string separator        StringBuilder sb   new StringBuilder        foreach  var item in collection                sb Append stringElement item            sb Append separator             return sb ToString 0  Math Max 0  sb Length - separator Length        quita el ultimo separador      new    1 2 3  ToString i  gt i 2         -- gt   2  4  6    Also  the next ones are meant to be able to continue in the same line in almost any situation  not declaring new variables and then removing state   public static R Map lt T  R gt  this T t  Func lt T  R gt  func        return func t      ExpensiveFindWally   Map wally  gt wally FirstName         wally LastName   public static R TryCC lt T  R gt  this T t  Func lt T  R gt  func      where T   class     where R   class       if  t    null  return null      return func t      public static R  TryCS lt T  R gt  this T t  Func lt T  R gt  func      where T   class     where R   struct       if  t    null  return null      return func t      public static R  TryCS lt T  R gt  this T t  Func lt T  R  gt  func      where T   class     where R   struct       if  t    null  return null      return func t      public static R TrySC lt T  R gt  this T  t  Func lt T  R gt  func      where T   struct     where R   class       if  t    null  return null      return func t Value      public static R  TrySS lt T  R gt  this T  t  Func lt T  R gt  func      where T   struct     where R   struct       if  t    null  return null      return func t Value      public static R  TrySS lt T  R gt  this T  t  Func lt T  R  gt  func      where T   struct     where R   struct       if  t    null  return null      return func t Value        int  bossNameLength    Departament Boss TryCC b  gt b Name  TryCS s  gt s Length     public static T ThrowIfNullS lt T gt  this T  t  string mensaje      where T   struct       if  t    null          throw new NullReferenceException mensaje       return t Value     public static T ThrowIfNullC lt T gt  this T t  string mensaje      where T   class       if  t    null          throw new NullReferenceException mensaje       return t     public static T Do lt T gt  this T t  Action lt T gt  action        action t       return t       Button b   new Button Content    Click   Do b  gt Canvas SetColumn b 2     public static T TryDo lt T gt  this T t  Action lt T gt  action  where T   class       if  t    null          action t       return t     public static T  TryDoS lt T gt  this T  t  Action lt T gt  action  where T   struct       if  t    null          action t Value       return t      Hope it doesn t look like coming from Mars

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I find this one pretty useful   public static class PaulaBean       private static String paula    Brillant       public static String GetPaula lt T gt  this T obj            return paula            You may use it on CodePlex

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Easily serialize objects into XML   public static string ToXml lt T gt  this T obj  where T   class       XmlSerializer s   new XmlSerializer obj GetType         using  StringWriter writer   new StringWriter                  s Serialize writer  obj           return writer ToString               lt root gt  lt child gt foo lt  child lt  root gt   ToXml lt MyCustomType gt

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Sometimes its handy to write out a string on a selected element in a list with a custom seperator   For instance if you have a List lt Person gt  and want to loop out lastname seperated with a comma you could do this   string result   string Empty  foreach  var person in personList       result    person LastName           result   result Substring 0  result Length - 2   return result    Or you could use this handy extension method  public static string Join lt T gt  this IEnumerable lt T gt  collection  Func lt T  string gt  func  string separator      return String Join separator  collection Select func  ToArray         And use it like this  personList Join x   gt  x LastName           Which produces the same result  in this case a list of lastnames seperated by a comma

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Equivalent to Python s Join method        lt summary gt      same as python  join       lt  summary gt       lt typeparam name  T  gt list type lt  typeparam gt       lt param name  separator  gt string separator  lt  param gt       lt param name  list  gt list of objects to be ToString d lt  param gt       lt returns gt a concatenated list interleaved with separators lt  returns gt  static public string Join lt T gt  this string separator  IEnumerable lt T gt  list        var sb   new StringBuilder        bool first   true       foreach  T v in list                if   first              sb Append separator           first   false           if  v    null              sb Append v ToString                return sb ToString

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I actually just blogged this today  It s a strongly typed reactive wrapper around a INotifyPropertyChanged property   GetPropertyValues  returns an IObservable lt T gt  of the values as they change  starting with the current value  If ignore the current value  you can just call Skip 1  on the result   Usage is like so   IObservable lt int gt  values   viewModel GetPropertyValues x   gt  x IntProperty     Implementation   public static class NotifyPropertyChangeReactiveExtensions          Returns the values of property  an Expression  as they change          starting with the current value     public static IObservable lt TValue gt  GetPropertyValues lt TSource  TValue gt           this TSource source  Expression lt Func lt TSource  TValue gt  gt  property          where TSource   INotifyPropertyChanged               MemberExpression memberExpression   property Body as MemberExpression           if  memberExpression    null                        throw new ArgumentException                   property must directly access a property of the source                       string propertyName   memberExpression Member Name           Func lt TSource  TValue gt  accessor   property Compile             return source GetPropertyChangedEvents                Where x   gt  x EventArgs PropertyName    propertyName               Select x   gt  accessor source                StartWith accessor source                  This is a wrapper around FromEvent PropertyChanged      public static IObservable lt IEvent lt PropertyChangedEventArgs gt  gt          GetPropertyChangedEvents this INotifyPropertyChanged source                return Observable FromEvent lt PropertyChangedEventHandler               PropertyChangedEventArgs gt               h   gt  new PropertyChangedEventHandler h               h   gt  source PropertyChanged    h              h   gt  source PropertyChanged -  h

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Overwrite a portion of a string at a specified index   I have to work with a system that expects some input values to be fixed width  fixed position strings   public static string Overwrite this string s  int startIndex  string newStringValue      return s Remove startIndex  newStringValue Length  Insert startIndex  newStringValue       So I can do   string s   new String     60   s   s Overwrite 7  NewValue

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I use this one all the time   public static void DelimitedAppend this StringBuilder sb  string value  string delimiter        if  sb Length  gt  0          sb Append delimiter       sb Append value       This just ensures that the delimiter is not inserted when the string is empty  For example  to create a comma-seperated list of words   var farmAnimals   new     new   Species    Dog   IsTasty   false    new   Species    Cat   IsTasty   false    new   Species    Chicken   IsTasty   true       var soupIngredients   new StringBuilder    foreach  var edible in farmAnimals Where farmAnimal   gt  farmAnimal IsTasty       soupIngredients DelimitedAppend edible Species

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I find the following extension method quite useful   public static T GetService lt T gt  this IServiceProvider provider        return  T provider GetService typeof T        It makes it much easier to use the IServiceProvider interface  Compare   IProvideValueTarget target    IProvideValueTarget serviceProvider typeof IProvideValueTarget      and  var target   serviceProvider GetService lt IProvideValueTarget gt

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An easier way to load default settings from a collection  in real life I use it to populate the settings from any source  including the command line  ClickOnce URL parameters etc     public static void LoadFrom this ApplicationSettingsBase settings  NameValueCollection configuration        if  configuration    null          foreach  string key in configuration AllKeys              if   String IsNullOrEmpty key                   try                                       settings key    configuration Get key                                     catch  SettingsPropertyNotFoundException                                         handle bad arguments as you wish                       Example   Settings Default LoadFrom new NameValueCollection        Setting1    Value1        Setting2    Value2

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Are these any use   public static bool CoinToss this Random rng        return rng Next 2     0     public static T OneOf lt T gt  this Random rng  params T   things        return things rng Next things Length       Random rand  bool luckyDay   rand CoinToss    string babyName   rand OneOf  John    George    Radio XBR74 ROCKS

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I use the following extensions to extend all collections  maybe someone find these useful         lt summary gt      Collection Helper      lt  summary gt       lt remarks gt      Use IEnumerable by default  but when altering or getting item at index use IList       lt  remarks gt  public static class CollectionHelper         region Alter            lt summary gt          Swap item to another place          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  IndexA  gt Index a lt  param gt           lt param name  IndexB  gt Index b lt  param gt           lt returns gt New collection lt  returns gt      public static IList lt T gt  Swap lt T gt  this IList lt T gt   this  Int32 IndexA  Int32 IndexB                T Temp    this IndexA            this IndexA     this IndexB            this IndexB    Temp          return  this                  lt summary gt          Swap item to the left          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index lt  param gt           lt returns gt New collection lt  returns gt      public static IList lt T gt  SwapLeft lt T gt  this IList lt T gt   this  Int32 Index                return  this Swap Index  Index - 1                   lt summary gt          Swap item to the right          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index lt  param gt           lt returns gt New collection lt  returns gt      public static IList lt T gt  SwapRight lt T gt  this IList lt T gt   this  Int32 Index                return  this Swap Index  Index   1               endregion Alter        region Action            lt summary gt          Execute action at specified index          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index lt  param gt           lt param name  ActionAt  gt Action to execute lt  param gt           lt returns gt New collection lt  returns gt      public static IList lt T gt  ActionAt lt T gt  this IList lt T gt   this  Int32 Index  Action lt T gt  ActionAt                ActionAt  this Index            return  this              endregion Action        region Randomize            lt summary gt          Take random items          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Count  gt Number of items to take lt  param gt           lt returns gt New collection lt  returns gt      public static IEnumerable lt T gt  TakeRandom lt T gt  this IEnumerable lt T gt   this  Int32 Count                return  this Shuffle   Take Count                   lt summary gt          Take random item          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt returns gt Item lt  returns gt      public static T TakeRandom lt T gt  this IEnumerable lt T gt   this                return  this TakeRandom 1  Single                    lt summary gt          Shuffle list          lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt returns gt New collection lt  returns gt      public static IEnumerable lt T gt  Shuffle lt T gt  this IEnumerable lt T gt   this                return  this OrderBy Item   gt  Guid NewGuid                 endregion Randomize        region Navigate            lt summary gt          Get next item in collection and give first item  when last item is selected           lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index in collection lt  param gt           lt returns gt Next item lt  returns gt      public static T Next lt T gt  this IList lt T gt   this  ref Int32 Index                Index     Index  gt   0  amp  amp  Index  lt   this Count   Index   0          return  this Index                   lt summary gt          Get previous item in collection and give last item  when first item is selected           lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt param name  Index  gt Index in collection lt  param gt           lt returns gt Previous item lt  returns gt      public static T Previous lt T gt  this IList lt T gt   this  ref Int32 Index                Index   --Index  gt   0  amp  amp  Index  lt   this Count   Index    this Count - 1          return  this Index               endregion Navigate        region Clone            lt summary gt                    lt  summary gt           lt typeparam name  T  gt Collection type lt  typeparam gt           lt param name  this  gt Collection lt  param gt           lt returns gt Cloned collection lt  returns gt      public static IEnumerable lt T gt  Clone lt T gt  this IEnumerable lt T gt   this  where T   ICloneable               return  this Select Item   gt   T Item Clone                 endregion Clone        region String            lt summary gt          Joins multiple string with Separator          lt  summary gt           lt param name  this  gt Collection lt  param gt           lt param name  Separator  gt Separator lt  param gt           lt returns gt Joined string lt  returns gt      public static String Join this IEnumerable lt String gt   this  String Separator                     return String Join Separator   this               endregion String

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public static class StringHelper       public static String F this String str  params object   args                return String Format str  args             Using like    Say  0   F  Hello

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I am sure this has been done before  but I find myself using this method  and simpler derivatives  often   public static bool CompareEx this string strA  string strB  CultureInfo culture  bool ignoreCase        return string Compare strA  strB  ignoreCase  culture     0      You can write it in a number of ways  but I like it because it very quickly unifies my approach to comparing strings while saving me lines of code  or characters of code

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Converts a List to a datatable  public static class DataTableConverter            lt summary gt          Convert a List T  to a DataTable           lt  summary gt      public static DataTable ToDataTable lt T gt  this IList lt T gt  items                var tb   new DataTable typeof T  Name            PropertyInfo   props   typeof T  GetProperties BindingFlags Public   BindingFlags Instance            foreach  PropertyInfo prop in props                        Type t   GetCoreType prop PropertyType               tb Columns Add prop Name  t                      foreach  T item in items                        var values   new object props Length                for  int i   0  i  lt  props Length  i                                  values i    props i  GetValue item  null                              tb Rows Add values                      return tb                  lt summary gt          Determine of specified type is nullable          lt  summary gt      public static bool IsNullable Type t                return  t IsValueType     t IsGenericType  amp  amp  t GetGenericTypeDefinition      typeof Nullable lt  gt                     lt summary gt          Return underlying type if type is Nullable otherwise return the type          lt  summary gt      public static Type GetCoreType Type t                if  t    null  amp  amp  IsNullable t                         if   t IsValueType                                return t                            else                               return Nullable GetUnderlyingType t                                   else                       return t                      Usage       IList lt MyClass gt  myClassList   new List lt MyClass gt         DataTable myClassDataTable   myClassList ToDataTable

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Another useful one for me        lt summary gt      Converts any type in to an Int32      lt  summary gt       lt typeparam name  T  gt Any Object lt  typeparam gt       lt param name  value  gt Value to convert lt  param gt       lt returns gt The integer  0 if unsuccessful lt  returns gt  public static int ToInt32 lt T gt  this T value      int result    if  int TryParse value ToString    out result           return result        return 0          lt summary gt      Converts any type in to an Int32 but if null then returns the default      lt  summary gt       lt param name  value  gt Value to convert lt  param gt       lt typeparam name  T  gt Any Object lt  typeparam gt       lt param name  defaultValue  gt Default to use lt  param gt       lt returns gt The defaultValue if unsuccessful lt  returns gt  public static int ToInt32 lt T gt  this T value  int defaultValue      int result    if  int TryParse value ToString    out result           return result        return defaultValue      Example   int number    123  ToInt32      or   int badNumber    a  ToInt32 100      Returns 100 since a is nan

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In the recent searches section on my blog stats page  I had removed all duplicates  but needed a way to remove nearly-duplicate lines  I d get tons of similar but not quite the same Google queries    I ended up using an anonymous type instead of a dictionary  but wanted a way to create a List of that anonymous type  You can t do that  but you can create a List lt dynamic gt  in  NET 4 0     Mostly I like it because I effectively get a List lt AnonymousType 1 gt           lt summary gt Remove extraneous entries for common word permutations lt  summary gt       lt param name  input  gt Incoming series of words to be filtered lt  param gt       lt param name  MaxIgnoreLength  gt Words this long or shorter will not count as duplicates lt  param gt       lt param name  words2  gt Instance list from BuildInstanceList   lt  param gt       lt returns gt Filtered list of lines from input  based on filter info in words2 lt  returns gt  private static List lt string gt  FilterNearDuplicates List lt string gt  input  int MaxIgnoreLength  List lt dynamic gt  words2        List lt string gt  output   new List lt string gt         foreach  string line in input                int Dupes   0          foreach  string word in line Split new char                                            r     n                       Where p   gt  p Length  gt  MaxIgnoreLength               Distinct                          int Instances   0              foreach  dynamic dyn in words2              if  word    dyn Word                                Instances   dyn Instances                  if  Instances  gt  1                      Dupes                    break                                  if  Dupes    0              output Add line             return output         lt summary gt Builds a list of words and how many times they occur in the overall list lt  summary gt       lt param name  input  gt Incoming series of words to be counted lt  param gt       lt returns gt  lt  returns gt  private static List lt dynamic gt  BuildInstanceList List lt string gt  input        List lt dynamic gt  words2   new List lt object gt         foreach  string line in input      foreach  string word in line Split new char                                            r     n                         if  string IsNullOrEmpty word               continue          else if  ExistsInList word  words2               for  int i   words2 Count - 1  i  gt   0  i--                                if  words2 i  Word    word                      words2 i    new   Word   words2 i  Word  Instances   words2 i  Instances   1                          else             words2 Add new   Word   word  Instances   1                return words2         lt summary gt Determines whether a dynamic Word object exists in a List of this dynamic type  lt  summary gt       lt param name  word  gt Word to look for lt  param gt       lt param name  words  gt Word dynamics to search through lt  param gt       lt returns gt Indicator of whether the word exists in the list of words lt  returns gt  private static bool ExistsInList string word  List lt dynamic gt  words        foreach  dynamic dyn in words          if  dyn Word    word              return true      return false

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Two little ones  some people find them silly  that I put in all my projects are   public static bool IsNull this object o     return o    null      and  public static bool IsNullOrEmpty this string s     return string IsNullOrEmpty s       It makes my code so much more fluent    if  myClassInstance IsNull          do something  if  myString IsNullOrEmpty          do something   I think these would make really nice extension properties  if we ever get those

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The ThrowIfArgumentIsNull is a nice way to do that null check we all should do   public static class Extensions       public static void ThrowIfArgumentIsNull lt T gt  this T obj  string parameterName  where T   class               if  obj    null  throw new ArgumentNullException parameterName     not allowed to be null              Below is the way to use it and it works on all classes in your namespace or wherever you use the namespace its within   internal class Test       public Test string input1                input1 ThrowIfArgumentIsNull  input1              It s ok to use this code on the CodePlex project

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These extension methods are pretty useful for me when parsing form input before putting into the database  public static int  ToInt this string input         int val      if  int TryParse input  out val           return val      return null     public static DateTime  ToDate this string input        DateTime val      if  DateTime TryParse input  out val           return val      return null     public static decimal  ToDecimal this string input        decimal val      if  decimal TryParse input  out val           return val      return null

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For Winform Controls        lt summary gt      Returns whether the function is being executed during design time in Visual Studio       lt  summary gt  public static bool IsDesignTime this Control control        if  LicenseManager UsageMode    LicenseUsageMode Designtime                return true             if  control Site    null  amp  amp  control Site DesignMode                return true             var parent   control Parent      while  parent    null                if  parent Site    null  amp  amp  parent Site DesignMode                        return true                    parent   parent Parent            return false          lt summary gt      Sets the DropDownWidth to ensure that no item s text is cut off       lt  summary gt  public static void SetDropDownWidth this ComboBox comboBox        var g   comboBox CreateGraphics        var font   comboBox Font      float maxWidth   0       foreach  var item in comboBox Items                maxWidth   Math Max maxWidth  g MeasureString item ToString    font  Width              if  comboBox Items Count  gt  comboBox MaxDropDownItems                maxWidth    SystemInformation VerticalScrollBarWidth             comboBox DropDownWidth   Math Max comboBox Width  Convert ToInt32 maxWidth        IsDesignTime Usage   public class SomeForm   Form       public SomeForm                 InitializeComponent             if  this IsDesignTime                          return                        Do something that makes the visual studio crash or hang if we re in design time             but any other time executes just fine           SetDropdownWidth Usage   ComboBox cbo   new ComboBox   Width   50    cbo Items Add  Short    cbo Items Add  A little longer    cbo Items Add  Holy cow  this is a really  really long item  How in the world will it fit     cbo SetDropDownWidth      I forgot to mention  feel free to use these on Codeplex

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public static class ComparableExtensions     public static bool Between lt T gt  this T actual  T lower  T upper  where T   IComparable lt T gt          return actual CompareTo lower   gt   0  amp  amp  actual CompareTo upper   lt  0          Example   if  myNumber Between 3 7

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gitorious org cadenza is a full library of some of the most useful extension methods I ve seen

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For ASP NET  I use these extensions to HttpSessionState to load objects in session  It allows you to load session objects in a clean manner  and will create and initialize them if they do not exist  I use the two extension methods like so   private bool CreateMode  private MyClass SomeClass   protected override void OnInit  EventArgs e        CreateMode   Session GetSessionValue lt bool gt    someKey1        gt  true       SomeClass   Session GetSessionClass lt MyClass gt    someKey2        gt  new MyClass                   MyProperty   123              Here are the extension classes   public static class SessionExtensions           public delegate object UponCreate         public static T GetSessionClass lt T gt   this HttpSessionState session          string key  UponCreate uponCreate  where T   class               if  null    session key                         var item   uponCreate    as T              session key    item              return item                    return session key  as T            public static T GetSessionValue lt T gt   this HttpSessionState session          string key  UponCreate uponCreate  where T   struct               if  null    session key                         var item   uponCreate                session key    item              return  T item                    return  T session key

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If you need for check your string for Is All char is 0     static public bool   IsAllZero             this string input                        if string IsNullOrEmpty input                   return true              foreach  char ch in input                                if ch     0                       return false                            return true

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I thought I d seen this somewhere before  but couldn t find it suggested anywhere here   MS has a TryGetValue function on the IDictionary interface  but it returns a bool and gives the value in an out parameter  so here s a simpler  cleaner implementation   public static TVal GetValueOrDefault lt TKey  TVal gt  this IDictionary lt TKey  TVal gt  d  TKey key      if  d ContainsKey key       return d key     return default TVal

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This is an extension method for the ASP Net MVC action link helper method that allows it to use the controller s authorize attributes to decide if the link should be enabled  disabled or hidden from the current user s view   I saves you from having to enclose your restricted actions in  if  clauses that check for user membership in all the views  Thanks to Maarten Balliauw for the idea and the code bits that showed me the way     using System  using System Collections Generic  using System Linq  using System Web  using System Security Principal  using System Web Routing  using System Web Mvc  using System Collections  using System Reflection  namespace System Web Mvc Html       public static class HtmlHelperExtensions                     lt summary gt              Shows or hides an action link based on the user s membership status             and the controller s authorize attributes              lt  summary gt               lt param name  linkText  gt The link text  lt  param gt               lt param name  action  gt The controller action name  lt  param gt               lt param name  controller  gt The controller name  lt  param gt               lt returns gt  lt  returns gt          public static string SecurityTrimmedActionLink              this HtmlHelper htmlHelper              string linkText              string action              string controller                        return SecurityTrimmedActionLink htmlHelper  linkText  action  controller  false  null                           lt summary gt              Enables  disables or hides an action link based on the user s membership status             and the controller s authorize attributes              lt  summary gt               lt param name  linkText  gt The link text  lt  param gt               lt param name  action  gt The action name  lt  param gt               lt param name  controller  gt The controller name  lt  param gt               lt param name  showDisabled  gt if set to  lt c gt true lt  c gt   show link as disabled -              using a span tag instead of an anchor tag    lt  param gt               lt param name  disabledAttributeText  gt Use this to add attributes to the disabled             span tag  lt  param gt               lt returns gt  lt  returns gt          public static string SecurityTrimmedActionLink              this HtmlHelper htmlHelper               string linkText               string action               string controller               bool showDisabled               string disabledAttributeText                        if  IsAccessibleToUser action  controller  HttpContext Current                                  return htmlHelper ActionLink linkText  action  controller                             else                               return showDisabled                        String Format                            lt span 1  gt  0  lt  span gt                             linkText                           disabledAttributeText  null        disabledAttributeText                                                                  private static IController GetControllerInstance string controllerName                        Assembly assembly   Assembly GetExecutingAssembly                Type controllerType   GetControllerType controllerName               return  IController Activator CreateInstance controllerType                      private static ArrayList GetControllerAttributes string controllerName  HttpContext context                        if  context Cache controllerName     ControllerAttributes      null                                var controller   GetControllerInstance controllerName                    context Cache Add                      controllerName     ControllerAttributes                       new ArrayList controller GetType   GetCustomAttributes typeof AuthorizeAttribute   true                        null                      Caching Cache NoAbsoluteExpiration                      Caching Cache NoSlidingExpiration                      Caching CacheItemPriority Default                      null                              return  ArrayList context Cache controllerName     ControllerAttributes                        private static ArrayList GetMethodAttributes string controllerName  string actionName  HttpContext context                        if  context Cache controllerName         actionName     ActionAttributes      null                                ArrayList actionAttrs   new ArrayList                    var controller   GetControllerInstance controllerName                   MethodInfo   methods   controller GetType   GetMethods                     foreach  MethodInfo method in methods                                        object   attributes   method GetCustomAttributes typeof ActionNameAttribute   true                        if   attributes Length    0  amp  amp  method Name    actionName                                                      attributes Length  gt  0  amp  amp    ActionNameAttribute attributes 0   Name    actionName                                                 actionAttrs AddRange method GetCustomAttributes typeof AuthorizeAttribute   true                                                             context Cache Add                      controllerName         actionName     ActionAttributes                       actionAttrs                      null                      Caching Cache NoAbsoluteExpiration                      Caching Cache NoSlidingExpiration                      Caching CacheItemPriority Default                      null                               return  ArrayList context Cache controllerName         actionName    ActionAttributes                        public static bool IsAccessibleToUser string actionToAuthorize  string controllerToAuthorize  HttpContext context                        IPrincipal principal   context User                 cache the attribute list for both controller class and it s methods              ArrayList controllerAttributes   GetControllerAttributes controllerToAuthorize  context                ArrayList actionAttributes   GetMethodAttributes controllerToAuthorize  actionToAuthorize  context                                        if  controllerAttributes Count    0  amp  amp  actionAttributes Count    0                  return true               string roles                   string users                   if  controllerAttributes Count  gt  0                                AuthorizeAttribute attribute   controllerAttributes 0  as AuthorizeAttribute                  roles    attribute Roles                  users    attribute Users                            if  actionAttributes Count  gt  0                                AuthorizeAttribute attribute   actionAttributes 0  as AuthorizeAttribute                  roles    attribute Roles                  users    attribute Users                             if  string IsNullOrEmpty roles   amp  amp  string IsNullOrEmpty users   amp  amp  principal Identity IsAuthenticated                  return true               string   roleArray   roles Split                   string   usersArray   users Split                   foreach  string role in roleArray                                if  role           principal IsInRole role                       return true                            foreach  string user in usersArray                                if  user         amp  amp   principal Identity Name    user                       return true                            return false                     private static Type GetControllerType string controllerName                        Assembly assembly   Assembly GetExecutingAssembly                foreach  Type type in assembly GetTypes                                  if                       type BaseType  null                       amp  amp  type BaseType Name     Controller                        amp  amp   type Name ToUpper       controllerName ToUpper      Controller  ToUpper                                            return type                                              return null

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Values ordered true false    True false values separated by a capital letter    Only two values allowed    ---------------------------    Limited  but could be useful public enum BooleanFormat       OneZero      YN      YesNo      TF      TrueFalse      PassFail      YepNope    public static class BooleanExtension            lt summary gt          Converts the boolean value of this instance to the specified string value            lt  summary gt      private static string ToString this bool value  string passValue  string failValue                return value   passValue   failValue                  lt summary gt          Converts the boolean value of this instance to a string            lt  summary gt           lt param name  booleanFormat  gt A BooleanFormat value           Example  BooleanFormat PassFail would return  Pass  if true and  Fail  if false  lt  param gt           lt returns gt Boolean formatted string lt  returns gt      public static string ToString this bool value  BooleanFormat booleanFormat                string booleanFormatString   Enum GetName booleanFormat GetType    booleanFormat           return ParseBooleanString value  booleanFormatString                       Parses boolean format strings  not optimized     private static string ParseBooleanString bool value  string booleanFormatString                StringBuilder trueString   new StringBuilder            StringBuilder falseString   new StringBuilder             int charCount   booleanFormatString Length           bool isTrueString   true           for  int i   0  i    charCount  i                          if  char IsUpper booleanFormatString i    amp  amp  i    0                  isTrueString   false               if  isTrueString                  trueString Append booleanFormatString i                else                 falseString Append booleanFormatString i                       return  value    true   trueString ToString     falseString ToString

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Are these any use   public static bool CoinToss this Random rng        return rng Next 2     0     public static T OneOf lt T gt  this Random rng  params T   things        return things rng Next things Length       Random rand  bool luckyDay   rand CoinToss    string babyName   rand OneOf  John    George    Radio XBR74 ROCKS

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I didn t want to add anything that was already said  so here are some that I use that haven t been mentioned   Sorry if this is too lengthy    public static class MyExtensions       public static bool IsInteger this string input                int temp           return int TryParse input  out temp              public static bool IsDecimal this string input                decimal temp           return decimal TryParse input  out temp              public static int ToInteger this string input  int defaultValue                int temp           return  int TryParse input  out temp     temp   defaultValue             public static decimal ToDecimal this string input  decimal defaultValue                decimal temp           return  decimal TryParse input  out temp     temp   defaultValue             public static DateTime ToFirstOfTheMonth this DateTime input                return input Date AddDays -1   input Day   1                 Intentionally returns 0 if the target date is before the input date      public static int MonthsUntil this DateTime input  DateTime targetDate                input   input ToFirstOfTheMonth             targetDate   targetDate ToFirstOfTheMonth             int result   0           while  input  lt  targetDate                    input   input AddMonths 1               result                       return result                Used for backwards compatibility in a system built before my time      public static DataTable ToDataTable this IEnumerable input                   too much code to show here right now

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This one is not fully baked as we just came up with it this morning  It will generate a full class definition for a Type  Useful for situations where you have a a large class and want to create a subset or full definition but don t have access to it  For example  to store the object in a database etc   public static class TypeExtensions       public static string GenerateClassDefinition this Type type                var properties   type GetFields            var sb   new StringBuilder            var classtext     private class  name                     props             foreach  var p in GetTypeInfo type                         sb AppendFormat    public  0   1     p Item2  p Item1  AppendLine     get  set                          return classtext Replace   name   type Name  Replace   props   sb ToString                 region Private Methods     private static List lt Tuple lt string  string gt  gt  GetTypeInfo Type type                var ret   new List lt Tuple lt string  string gt  gt             var fields   type GetFields            var props   type GetProperties             foreach var p in props  ret Add new Tuple lt string  string gt  p Name  TranslateType p PropertyType                 foreach var f in fields  ret Add new Tuple lt string  string gt  f Name  TranslateType f FieldType              return ret              private static string TranslateType Type input                string ret           if  Nullable GetUnderlyingType input     null                        ret   string Format   0     TranslateType Nullable GetUnderlyingType input                       else                       switch  input Name                                case  Int32   ret    int   break                  case  Int64   ret    long   break                  case  IntPtr   ret    long   break                  case  Boolean   ret    bool   break                  case  String                   case  Char                   case  Decimal                       ret   input Name ToLower    break                  default  ret   input Name  break                                   return ret             endregion     Example usage   Process GetProcesses   First   GetType   GenerateClassDefinition      Becomes even more handy if using linqpad   Process GetProcesses   First   GetType   GenerateClassDefinition   Dump

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Timespan-related extensions like   public static TimeSpan Seconds this int seconds      return TimeSpan FromSeconds seconds      public static TimeSpan Minutes this int minutes      return TimeSpan FromMinutes minutes       That allow to use   1 Seconds   20 Minutes     Lock extensions like   public static IDisposable GetReadLock this ReaderWriterLockSlim slimLock      slimLock EnterReadLock      return new DisposableAction slimLock ExitReadLock      public static IDisposable GetWriteLock this ReaderWriterLockSlim slimLock      slimLock EnterWriteLock      return new DisposableAction slimLock ExitWriteLock      public static IDisposable GetUpgradeableReadLock this ReaderWriterLockSlim slimLock      slimLock EnterUpgradeableReadLock      return new DisposableAction slimLock ExitUpgradeableReadLock       That allow to use locks like   using  lock GetUpgradeableReadLock           try read   using  lock GetWriteLock              do write         And many other from the Lokad Shared Libraries

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Another useful one for me        lt summary gt      Converts any type in to an Int32      lt  summary gt       lt typeparam name  T  gt Any Object lt  typeparam gt       lt param name  value  gt Value to convert lt  param gt       lt returns gt The integer  0 if unsuccessful lt  returns gt  public static int ToInt32 lt T gt  this T value      int result    if  int TryParse value ToString    out result           return result        return 0          lt summary gt      Converts any type in to an Int32 but if null then returns the default      lt  summary gt       lt param name  value  gt Value to convert lt  param gt       lt typeparam name  T  gt Any Object lt  typeparam gt       lt param name  defaultValue  gt Default to use lt  param gt       lt returns gt The defaultValue if unsuccessful lt  returns gt  public static int ToInt32 lt T gt  this T value  int defaultValue      int result    if  int TryParse value ToString    out result           return result        return defaultValue      Example   int number    123  ToInt32      or   int badNumber    a  ToInt32 100      Returns 100 since a is nan

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Comes in useful for unit testing   public static IList lt T gt  Clone lt T gt  this IList lt T gt  list  where T   ICloneable       var ret   new List lt T gt  list Count       foreach  var item in list          ret Add  T item Clone             done     return ret      A series of these like TWith2Sugars  alternate shortened syntax   public static long  ToNullableInt64 this string val        long ret      return Int64 TryParse val  out ret    ret   new long         And finally this - is there something already in the BCL that does the following   public static void Split lt T gt  this T   array       Func lt T bool gt  determinator       IList lt T gt  onTrue       IList lt T gt  onFalse        if  onTrue    null          onTrue   new List lt T gt         else         onTrue Clear         if  onFalse    null          onFalse   new List lt T gt         else         onFalse Clear         if  determinator    null          return       foreach  var item in array                if  determinator item               onTrue Add item           else             onFalse Add item

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Convert any string to type Int32     Calls the underlying int TryParse method to convert a string    representation of a number to its 32-bit signed integer equivalent     Returns Zero if conversion fails   public static int ToInt32 this string s        int retInt      int TryParse s  out retInt       return retInt      SAMPLE USE  string s    999   int i   s ToInt32

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Perhaps the most useful extension methods I ve written and used are here   http   www codeproject com KB cs fun-with-cs-extensions aspx msg 2838918 xx2838918xx

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I have an extension method for logging exceptions   public static void Log this Exception obj        your logging logic here     And it is used like this   try         Your stuff here   catch Exception ex        ex Log         sorry for posting twice  the 2nd one is better designed  -

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My conversion extensions which allow you to do   int i   myString To lt int gt       Here it is  as posted on TheSoftwareJedi com  public static T To lt T gt  this IConvertible obj      return  T Convert ChangeType obj  typeof T       public static T ToOrDefault lt T gt                this IConvertible obj        try               return To lt T gt  obj             catch               return default T            public static bool ToOrDefault lt T gt                       this IConvertible obj                       out T newObj        try               newObj   To lt T gt  obj            return true            catch               newObj   default T            return false           public static T ToOrOther lt T gt                          this IConvertible obj                         T other      try           return To lt T gt obj         catch           return other         public static bool ToOrOther lt T gt                            this IConvertible obj                           out T newObj                           T other        try               newObj   To lt T gt  obj           return true            catch               newObj   other          return false           public static T ToOrNull lt T gt                         this IConvertible obj                        where T   class       try               return To lt T gt  obj             catch               return null           public static bool ToOrNull lt T gt                     this IConvertible obj                    out T newObj                    where T   class       try               newObj   To lt T gt  obj           return true            catch               newObj   null          return false            You can ask for default  calls blank constructor or  0  for numerics  on failure  specify a  default  value  I call it  other    or ask for null  where T   class   I ve also provided both silent exception models  and a typical TryParse model that returns a bool indicating the action taken  and an out param holds the new value  So our code can do things like this  int i   myString To lt int gt     string a   myInt ToOrDefault lt string gt       note type inference DateTime d   myString ToOrOther DateTime MAX VALUE   double d    note type inference bool didItGiveDefault   myString ToOrDefault out d   string s   myDateTime ToOrNull lt string gt       I couldn t get Nullable types to roll into the whole thing very cleanly  I tried for about 20 minutes before I threw in the towel

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I miss the Visual Basic s With statement when moving to C   so here it goes   public static void With lt T gt  this T obj  Action lt T gt  act    act obj       And here s how to use it in C    someVeryVeryLonggggVariableName With x   gt        x Int   123      x Str    Hello       x Str2     World          Saves a lot of typing   Compare this to   someVeryVeryLonggggVariableName Int   123  someVeryVeryLonggggVariableName Str    Hello   someVeryVeryLonggggVariableName Str2     World      put in codeplex project

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We have a deployment tool to deploy between environments  Since the files could be marked as modified but not actually different I came up with this        lt summary gt      Compares the files to see if they are different       First checks file size     Then modified if the file is larger than the specified size     Then compares the bytes      lt  summary gt       lt param name  file1  gt The source file lt  param gt       lt param name  file2  gt The destination file lt  param gt       lt param name  mb  gt Skip the smart check if the file is larger than this many megabytes  Default is 10  lt  param gt       lt returns gt  lt  returns gt  public static bool IsDifferentThan this FileInfo file1  FileInfo file2  int mb   10      var ret   false        different size is a different file   if file1 Length    file2 Length  return true        if the file times are different and the file is bigger than 10mb flag it for updating   if file1 LastWriteTimeUtc  gt  file2 LastWriteTimeUtc  amp  amp  file1 Length  gt    mb 1024  1024   return true     var f1   File ReadAllBytes file1 FullName     var f2   File ReadAllBytes file2 FullName         loop through backwards because if they are different      it is more likely that the last few bytes will be different      than the first few   for var i   file1 Length - 1  i  gt  0  i--          if f1 i     f2 i               ret   true        break               return ret

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One of my favorites is an IsLike   extension on String   IsLike   matches VB s Like operator  and is handy when you don t want to write a full-on regex to solve your problem   Usage would be something like this    abc  IsLike  a        true  Abc  IsLike   A-Z  a-z  a-z        true  abc123  IsLike             true  hat  IsLike   at       true  joe  IsLike    aeiou         true   joe  IsLike   at       false  joe  IsLike   A-Z  a-z  a-z        false   Here s the code  public static class StringEntentions            lt summary gt          Indicates whether the current string matches the supplied wildcard pattern   Behaves the same         as VB s  Like  Operator           lt  summary gt           lt param name  s  gt The string instance where the extension method is called lt  param gt           lt param name  wildcardPattern  gt The wildcard pattern to match   Syntax matches VB s Like operator  lt  param gt           lt returns gt true if the string matches the supplied pattern  false otherwise  lt  returns gt           lt remarks gt See http   msdn microsoft com en-us library swf8kaxw v VS 100  aspx lt  remarks gt      public static bool IsLike this string s  string wildcardPattern            if  s    null    String IsNullOrEmpty wildcardPattern   return false             turn into regex pattern  and match the whole string with            var regexPattern         Regex Escape wildcardPattern                     add support for              and             regexPattern   regexPattern Replace                                                   Replace                                                 Replace                                                 Replace                                                 Replace                                                  Replace           d             var result   false          try               result   Regex IsMatch s  regexPattern                     catch  ArgumentException ex                throw new ArgumentException String Format  Invalid pattern   0    wildcardPattern   ex                     return result

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Comes in useful for unit testing   public static IList lt T gt  Clone lt T gt  this IList lt T gt  list  where T   ICloneable       var ret   new List lt T gt  list Count       foreach  var item in list          ret Add  T item Clone             done     return ret      A series of these like TWith2Sugars  alternate shortened syntax   public static long  ToNullableInt64 this string val        long ret      return Int64 TryParse val  out ret    ret   new long         And finally this - is there something already in the BCL that does the following   public static void Split lt T gt  this T   array       Func lt T bool gt  determinator       IList lt T gt  onTrue       IList lt T gt  onFalse        if  onTrue    null          onTrue   new List lt T gt         else         onTrue Clear         if  onFalse    null          onFalse   new List lt T gt         else         onFalse Clear         if  determinator    null          return       foreach  var item in array                if  determinator item               onTrue Add item           else             onFalse Add item

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Takes a camelCaseWord or PascalCaseWord and  wordifies  it  ie camelCaseWord    camel Case Word  public static string Wordify  this string camelCaseWord            if the word is all upper  just return it     if   Regex IsMatch  camelCaseWord    a-z               return camelCaseWord       return string Join       Regex Split  camelCaseWord       lt        A-Z             I often use it in conjuction with Capitalize  public static string Capitalize  this string word         return word 0  ToString    ToUpper      word Substring  1        Example usage  SomeEntityObject entity   DataAccessObject GetSomeEntityObject  id    List lt PropertyInfo gt  properties   entity GetType   GetPublicNonCollectionProperties         wordify the property names to act as column headers for an html table or something List lt string gt  columns   properties Select  p   gt  p Name Capitalize    Wordify      ToList       Free to use in codeplex project

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Inspired by String IsNullOrEmpty  To validate the given List is null or empty  public static bool IsNullOrEmpty lt TSource gt  this List lt TSource gt  src                    return  src    null    src Count    0       And this one is to validate given 2 files and properties  public static bool Compare this FileInfo f1  FileInfo f2  string propertyName        try               PropertyInfo p1   f1 GetType   GetProperty propertyName           PropertyInfo p2   f2 GetType   GetProperty propertyName            if  p1 GetValue f1  null     p2 GetValue f1  null               return true            catch  Exception ex                return false            return false      And use it like this  FileInfo fo   new FileInfo  c   netlog txt    FileInfo f1   new FileInfo  c   regkey txt     fo compare f1   CreationTime

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FindControl with built-in casting   public static T FindControl lt T gt  this Control control  string id  where T   Control       return  T control FindControl id       It s nothing amazing  but I feel it makes for cleaner code        With extension method container FindControl lt TextBox gt   myTextBox   SelectedValue    Hello world        Without extension method   TextBox container FindControl  myTextBox    SelectedValue    Hello world      This can be put this in the codeplex project  if so desired

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Here is one I use frequently for presentation formatting   public static string ToTitleCase this string mText        if  mText    null  return mText       System Globalization CultureInfo cultureInfo   System Threading Thread CurrentThread CurrentCulture      System Globalization TextInfo textInfo   cultureInfo TextInfo          TextInfo ToTitleCase only operates on the string if is all lower case  otherwise it returns the string unchanged      return textInfo ToTitleCase mText ToLower

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string Format shortcut   public static class StringExtensions          Enable quick and more natural string Format calls     public static string F this string s  params object   args                return string Format s  args             Example   var s    The co-ordinate is   0    1    F point X  point Y     For quick copy-and-paste go here   Don t you find it more natural to type  some string  F  param   instead of string Format  some string    param      For a more readable name  try one of these suggestion   s    Hello  0  world  1    Fmt  Stack    Overflow    s    Hello  0  world  1    FormatBy  Stack    Overflow    s    Hello  0  world  1    FormatWith  Stack    Overflow    s    Hello  0  world  1    Display  Stack    Overflow    s    Hello  0  world  1    With  Stack    Overflow

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Perhaps the most useful extension methods I ve written and used are here   http   www codeproject com KB cs fun-with-cs-extensions aspx msg 2838918 xx2838918xx

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I find myself doing this  over and over  again     public static bool EqualsIgnoreCase this string a  string b        return string Equals a  b  StringComparison OrdinalIgnoreCase          followed by StartsWithIgnoreCase  EndsWithIgnoreCase and ContainsIgnoreCase

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Some handy string helpers   Usage   I hate unwanted spaces trailing or leading strings and since string can take on a null value  it can be tricky  so i use this   public bool IsGroup   get   return  this GroupName IsNullOrTrimEmpty          Here is another extention method that i use for a new validation framework i m trialing  You can see the regex extensions within that help clean otherwise messy regex   public static bool IsRequiredWithLengthLessThanOrEqualNoSpecial this String str  int length        return  str IsNullOrTrimEmpty    amp  amp          str RegexMatch                  -  r n            amp         w   1  0     RegexReplace     0     length ToString                 RegexOptions Multiline     str      Source   public static class StringHelpers            lt summary gt          Same as String IsNullOrEmpty except that         it captures the Empty state for whitespace         strings by Trimming first           lt  summary gt      public static bool IsNullOrTrimEmpty this String helper                if  helper    null              return true          else             return String Empty    helper Trim               public static int TrimLength this String helper                return helper Trim   Length                  lt summary gt          Returns the matched string from the regex pattern  The         groupName is for named group match values in the form    lt name gt group            lt  summary gt      public static string RegexMatch this String helper  string pattern  RegexOptions options  string groupName                if  groupName IsNullOrTrimEmpty                return Regex Match helper  pattern  options  Value          else             return Regex Match helper  pattern  options  Groups groupName  Value             public static string RegexMatch this String helper  string pattern                return RegexMatch helper  pattern  RegexOptions None  null              public static string RegexMatch this String helper  string pattern  RegexOptions options                return RegexMatch helper  pattern  options  null              public static string RegexMatch this String helper  string pattern  string groupName                return RegexMatch helper  pattern  RegexOptions None  groupName                   lt summary gt          Returns true if there is a match from the regex pattern          lt  summary gt      public static bool IsRegexMatch this String helper  string pattern  RegexOptions options                return helper RegexMatch pattern  options  Length  gt  0             public static bool IsRegexMatch this String helper  string pattern                return helper IsRegexMatch pattern  RegexOptions None                   lt summary gt          Returns a string where matching patterns are replaced by the replacement string           lt  summary gt           lt param name  pattern  gt The regex pattern for matching the items to be replaced lt  param gt           lt param name  replacement  gt The string to replace matching items lt  param gt           lt returns gt  lt  returns gt      public static string RegexReplace this String helper  string pattern  string replacement  RegexOptions options                return Regex Replace helper  pattern  replacement  options              public static string RegexReplace this String helper  string pattern  string replacement                return Regex Replace helper  pattern  replacement  RegexOptions None             I like to do a lot of regex so i consider these easier than adding the using statement and the extra code to handle named groups

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Gets the root domain of a URI        lt summary gt Gets the root domain of any URI lt  summary gt       lt param name  uri  gt URI to get root domain of lt  param gt       lt returns gt Root domain with TLD lt  returns gt  public static string GetRootDomain this System Uri uri        if  uri    null          return null       string Domain   uri Host      while  System Text RegularExpressions Regex Matches Domain           Count  gt  1          Domain   Domain Substring Domain IndexOf        1       Domain   Domain Substring 0  Domain IndexOf            return Domain

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gitorious org cadenza is a full library of some of the most useful extension methods I ve seen

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Overwrite a portion of a string at a specified index   I have to work with a system that expects some input values to be fixed width  fixed position strings   public static string Overwrite this string s  int startIndex  string newStringValue      return s Remove startIndex  newStringValue Length  Insert startIndex  newStringValue       So I can do   string s   new String     60   s   s Overwrite 7  NewValue

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Perhaps the most useful extension methods I ve written and used are here   http   www codeproject com KB cs fun-with-cs-extensions aspx msg 2838918 xx2838918xx

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Timespan-related extensions like   public static TimeSpan Seconds this int seconds      return TimeSpan FromSeconds seconds      public static TimeSpan Minutes this int minutes      return TimeSpan FromMinutes minutes       That allow to use   1 Seconds   20 Minutes     Lock extensions like   public static IDisposable GetReadLock this ReaderWriterLockSlim slimLock      slimLock EnterReadLock      return new DisposableAction slimLock ExitReadLock      public static IDisposable GetWriteLock this ReaderWriterLockSlim slimLock      slimLock EnterWriteLock      return new DisposableAction slimLock ExitWriteLock      public static IDisposable GetUpgradeableReadLock this ReaderWriterLockSlim slimLock      slimLock EnterUpgradeableReadLock      return new DisposableAction slimLock ExitUpgradeableReadLock       That allow to use locks like   using  lock GetUpgradeableReadLock           try read   using  lock GetWriteLock              do write         And many other from the Lokad Shared Libraries

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Function to compare Files Directories through the OS File System Info  This is useful to compare shares with local files     Usage   DirectoryInfo dir   new DirectoryInfo   C  test myShareDir    Console WriteLine dir IsSameFileAs     myMachineName myShareDir      FileInfo file   new FileInfo   C  test myShareDir file txt    Console WriteLine file IsSameFileAs     myMachineName myShareDir file txt       Code   public static class FileExtensions       struct BY HANDLE FILE INFORMATION               public uint FileAttributes          public System Runtime InteropServices ComTypes FILETIME CreationTime          public System Runtime InteropServices ComTypes FILETIME LastAccessTime          public System Runtime InteropServices ComTypes FILETIME LastWriteTime          public uint VolumeSerialNumber          public uint FileSizeHigh          public uint FileSizeLow          public uint NumberOfLinks          public uint FileIndexHigh          public uint FileIndexLow                       CreateFile constants            const uint FILE SHARE READ   0x00000001      const uint OPEN EXISTING   3      const uint GENERIC READ    0x80000000       const uint FILE FLAG BACKUP SEMANTICS   0x02000000         DllImport  kernel32 dll   SetLastError   true       static extern IntPtr CreateFile          string lpFileName          uint dwDesiredAccess          uint dwShareMode          IntPtr lpSecurityAttributes          uint dwCreationDisposition          uint dwFlagsAndAttributes          IntPtr hTemplateFile         DllImport  kernel32 dll   SetLastError   true       static extern bool GetFileInformationByHandle IntPtr hFile  out BY HANDLE FILE INFORMATION lpFileInformation        public static bool IsSameFileAs this FileSystemInfo file  string path                BY HANDLE FILE INFORMATION fileInfo1  fileInfo2          IntPtr ptr1   CreateFile file FullName  GENERIC READ  FILE SHARE READ  IntPtr Zero  OPEN EXISTING  FILE FLAG BACKUP SEMANTICS  IntPtr Zero           if   int ptr1    -1                        System ComponentModel Win32Exception e   new System ComponentModel Win32Exception Marshal GetLastWin32Error                 throw e                    IntPtr ptr2   CreateFile path  GENERIC READ  FILE SHARE READ  IntPtr Zero  OPEN EXISTING  FILE FLAG BACKUP SEMANTICS  IntPtr Zero           if   int ptr2    -1                        System ComponentModel Win32Exception e   new System ComponentModel Win32Exception Marshal GetLastWin32Error                 throw e                    GetFileInformationByHandle ptr1  out fileInfo1           GetFileInformationByHandle ptr2  out fileInfo2            return   fileInfo1 FileIndexHigh    fileInfo2 FileIndexHigh   amp  amp               fileInfo1 FileIndexLow    fileInfo2 FileIndexLow

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It irritated me that LINQ gives me an OrderBy that takes a class implementing IComparer as an argument  but does not support passing in a simple anonymous comparer function  I rectified that   This class creates an IComparer from your comparer function          lt summary gt          Creates an  lt see cref  IComparer T    gt  instance for the given         delegate function       lt  summary gt  internal class ComparerFactory lt T gt    IComparer lt T gt        public static IComparer lt T gt  Create Func lt T  T  int gt  comparison                return new ComparerFactory lt T gt  comparison              private readonly Func lt T  T  int gt   comparison       private ComparerFactory Func lt T  T  int gt  comparison                 comparison   comparison              region IComparer lt T gt  Members      public int Compare T x  T y                return  comparison x  y               endregion        and these extension methods expose my new OrderBy overloads on enumerables  I doubt this works for LINQ to SQL  but it s great for LINQ to Objects   public static class EnumerableExtensions            lt summary gt          Sorts the elements of a sequence in ascending order by using a specified comparison delegate           lt  summary gt      public static IOrderedEnumerable lt TSource gt  OrderBy lt TSource  TKey gt  this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector                                                                       Func lt TKey  TKey  int gt  comparison                var comparer   ComparerFactory lt TKey gt  Create comparison           return source OrderBy keySelector  comparer                   lt summary gt          Sorts the elements of a sequence in descending order by using a specified comparison delegate           lt  summary gt      public static IOrderedEnumerable lt TSource gt  OrderByDescending lt TSource  TKey gt  this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector                                                                                 Func lt TKey  TKey  int gt  comparison                var comparer   ComparerFactory lt TKey gt  Create comparison           return source OrderByDescending keySelector  comparer             You re welcome to put this on codeplex if you like

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The Substring method on the string class has always felt inadequate to me  Usually when you do a substring  you know the character s  from where you want to start  and the charachter s  where you want to end  Thus  I ve always felt that have to specify length as the second parameter is stupid  Therefore  I ve written my own extension methods  One that takes a startIndex and an endIndex  And one  that takes a startText  string  and endText  string  so you can just specify the text from where to start the substring  and the text for where to end it   NOTE  I couldn t name the method Substring as in  NET because my first overload takes the same parameter types as one of the  NET overloads  Therefore I named them Subsetstring  Feel free to add to the CodePlex     public static class StringExtensions            lt summary gt          Returns a Subset string starting at the specified start index and ending and the specified end         index           lt  summary gt           lt param name  s  gt The string to retrieve the subset from  lt  param gt           lt param name  startIndex  gt The specified start index for the subset  lt  param gt           lt param name  endIndex  gt The specified end index for the subset  lt  param gt           lt returns gt A Subset string starting at the specified start index and ending and the specified end         index  lt  returns gt      public static string Subsetstring this string s  int startIndex  int endIndex                if  startIndex  gt  endIndex                        throw new InvalidOperationException  End Index must be after Start Index                        if  startIndex  lt  0                        throw new InvalidOperationException  Start Index must be a positive number                        if endIndex  lt 0                        throw new InvalidOperationException  End Index must be a positive number                        return s Substring startIndex   endIndex - startIndex                    lt summary gt          Finds the specified Start Text and the End Text in this string instance  and returns a string         containing all the text starting from startText  to the begining of endText   endText is not         included            lt  summary gt           lt param name  s  gt The string to retrieve the subset from  lt  param gt           lt param name  startText  gt The Start Text to begin the Subset from  lt  param gt           lt param name  endText  gt The End Text to where the Subset goes to  lt  param gt           lt param name  ignoreCase  gt Whether or not to ignore case when comparing startText endText to the string  lt  param gt           lt returns gt A string containing all the text starting from startText  to the begining of endText  lt  returns gt      public static string Subsetstring this string s  string startText  string endText  bool ignoreCase                if  string IsNullOrEmpty startText     string IsNullOrEmpty endText                         throw new ArgumentException  Start Text and End Text cannot be empty                       string temp   s          if  ignoreCase                        temp   s ToUpperInvariant                startText   startText ToUpperInvariant                endText   endText ToUpperInvariant                      int start   temp IndexOf startText           int end   temp IndexOf endText  start           return Subsetstring s  start  end             Usage   string s    This is a tester for my cool extension method            s   s Subsetstring  tester    cool  true     Output   tester for my

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I created a nice Each extension that has the same behavior as jQuery s each function   It allows something like below  where you can get the index of the current value and break out of the loop by returning false   new      first    second    third    Each  value  index    gt        if  value Contains  d            return false      Console Write value       return true        Here s the code       lt summary gt      Generic iterator function that is useful to replace a foreach loop with at your discretion   A provided action is performed on each element       lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  source  gt  lt  param gt       lt param name  action  gt Function that takes in the current value in the sequence        lt returns gt  lt  returns gt  public static IEnumerable lt T gt  Each lt T gt  this IEnumerable lt T gt  source  Action lt T gt  action        return source Each  value  index    gt                action value           return true                   lt summary gt      Generic iterator function that is useful to replace a foreach loop with at your discretion   A provided action is performed on each element       lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  source  gt  lt  param gt       lt param name  action  gt Function that takes in the current value and its index in the sequence         lt returns gt  lt  returns gt  public static IEnumerable lt T gt  Each lt T gt  this IEnumerable lt T gt  source  Action lt T  int gt  action        return source Each  value  index    gt                action value  index           return true                  lt summary gt      Generic iterator function that is useful to replace a foreach loop with at your discretion   A provided action is performed on each element       lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  source  gt  lt  param gt       lt param name  action  gt Function that takes in the current value in the sequence   Returns a value indicating whether the iteration should continue   So return false if you don t want to iterate anymore  lt  param gt       lt returns gt  lt  returns gt  public static IEnumerable lt T gt  Each lt T gt  this IEnumerable lt T gt  source  Func lt T  bool gt  action        return source Each  value  index    gt                return action value                   lt summary gt      Generic iterator function that is useful to replace a foreach loop with at your discretion   A provided action is performed on each element       lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  source  gt  lt  param gt       lt param name  action  gt Function that takes in the current value and its index in the sequence   Returns a value indicating whether the iteration should continue   So return false if you don t want to iterate anymore  lt  param gt       lt returns gt  lt  returns gt  public static IEnumerable lt T gt  Each lt T gt  this IEnumerable lt T gt  source  Func lt T  int  bool gt  action        if  source    null          return source       int index   0      foreach  var sourceItem in source                if   action sourceItem  index               break          index              return source

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Find more examples here  www extensionmethod net

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I miss the Visual Basic s With statement when moving to C   so here it goes   public static void With lt T gt  this T obj  Action lt T gt  act    act obj       And here s how to use it in C    someVeryVeryLonggggVariableName With x   gt        x Int   123      x Str    Hello       x Str2     World          Saves a lot of typing   Compare this to   someVeryVeryLonggggVariableName Int   123  someVeryVeryLonggggVariableName Str    Hello   someVeryVeryLonggggVariableName Str2     World      put in codeplex project

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I m using this one quite a lot     Original code   if  guid    Guid Empty  return guid  else return Guid NewGuid      New code   return guid NewGuidIfEmpty      Extension method   public static Guid NewGuidIfEmpty this Guid uuid        return  uuid    Guid Empty   uuid   Guid NewGuid

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Two color extensions I use  mostly for control development   public static class ColorExtensions        Gets a color that will be readable on top of a given background color   public static Color GetForegroundColor this Color input             Math taken from one of the replies to        http   stackoverflow com questions 2241447 make-foregroundcolor-black-or-white-depending-on-background     if  Math Sqrt input R   input R    241   input G   input G    691   input B   input B    068   gt  128        return Color Black      else       return Color White            Converts a given Color to gray   public static Color ToGray this Color input          int g    int  input R    299     int  input G    587     int  input B    114       return Color FromArgb input A  g  g  g           Usage   Color foreColor   someBackColor GetForegroundColor    Color grayColor   someBackColor ToGray

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These extension methods are pretty useful for me when parsing form input before putting into the database  public static int  ToInt this string input         int val      if  int TryParse input  out val           return val      return null     public static DateTime  ToDate this string input        DateTime val      if  DateTime TryParse input  out val           return val      return null     public static decimal  ToDecimal this string input        decimal val      if  decimal TryParse input  out val           return val      return null

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In ASP NET I always get fed up using FindControl and then having to cast and check if the value is null before referencing  So  I added a TryParse   method to Control that mirrors the similar ones in the framework for Int32 etc   public static bool TryParse lt T gt  this Control control  string id  out T result       where T   Control       result   control FindControl id  as T      return result    null      So now you can do this in ASP NET web-form pages   Label lbl  if  Page TryParse  Label1   out lbl         lbl Text    Safely set text

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If you have persian language and must show the numbers to users in persian language   static public string ToFaString          this string value                           1728   1584             string result                   if  value    null                                char   resChar   value ToCharArray                    for  int i   0  i  lt  resChar Length  i                                          if  resChar i   gt    0   amp  amp  resChar i   lt    9                           result     char  resChar i    1728                       else                         result    resChar i                                               return result

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ForEach for IEnumerables  public static class FrameworkExtensions          a map function     public static void ForEach lt T gt  this IEnumerable lt T gt   enum  Action lt T gt  mapFunction                foreach  var item in  enum  mapFunction item             Naive example   var buttons   GetListOfButtons   as IEnumerable lt Button gt       click all buttons buttons ForEach b   gt  b Click       Cool example      no need to type the same assignment 3 times  just    new   up an array and use foreach   lambda    everything is properly inferred by csc  -  new   itemA  itemB  itemC        ForEach item   gt            item Number   1          item Str    Hello World              Note   This is not like Select because Select expects your function to return something as for transforming into another list   ForEach simply allows you to execute something for each of the items without any transformations data manipulation   I made this so I can program in a more functional style and I was surprised that List has a ForEach while IEnumerable does not   Put this in the codeplex project

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For Winform Controls        lt summary gt      Returns whether the function is being executed during design time in Visual Studio       lt  summary gt  public static bool IsDesignTime this Control control        if  LicenseManager UsageMode    LicenseUsageMode Designtime                return true             if  control Site    null  amp  amp  control Site DesignMode                return true             var parent   control Parent      while  parent    null                if  parent Site    null  amp  amp  parent Site DesignMode                        return true                    parent   parent Parent            return false          lt summary gt      Sets the DropDownWidth to ensure that no item s text is cut off       lt  summary gt  public static void SetDropDownWidth this ComboBox comboBox        var g   comboBox CreateGraphics        var font   comboBox Font      float maxWidth   0       foreach  var item in comboBox Items                maxWidth   Math Max maxWidth  g MeasureString item ToString    font  Width              if  comboBox Items Count  gt  comboBox MaxDropDownItems                maxWidth    SystemInformation VerticalScrollBarWidth             comboBox DropDownWidth   Math Max comboBox Width  Convert ToInt32 maxWidth        IsDesignTime Usage   public class SomeForm   Form       public SomeForm                 InitializeComponent             if  this IsDesignTime                          return                        Do something that makes the visual studio crash or hang if we re in design time             but any other time executes just fine           SetDropdownWidth Usage   ComboBox cbo   new ComboBox   Width   50    cbo Items Add  Short    cbo Items Add  A little longer    cbo Items Add  Holy cow  this is a really  really long item  How in the world will it fit     cbo SetDropDownWidth      I forgot to mention  feel free to use these on Codeplex

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By all means put this in the codeplex project   Serializing   Deserializing objects to XML        lt summary gt Serializes an object of type T in to an xml string lt  summary gt       lt typeparam name  T  gt Any class type lt  typeparam gt       lt param name  obj  gt Object to serialize lt  param gt       lt returns gt A string that represents Xml  empty otherwise lt  returns gt  public static string XmlSerialize lt T gt  this T obj  where T   class  new         if  obj    null  throw new ArgumentNullException  obj         var serializer   new XmlSerializer typeof T        using  var writer   new StringWriter                  serializer Serialize writer  obj           return writer ToString                  lt summary gt Deserializes an xml string in to an object of Type T lt  summary gt       lt typeparam name  T  gt Any class type lt  typeparam gt       lt param name  xml  gt Xml as string to deserialize from lt  param gt       lt returns gt A new object of type T is successful  null if failed lt  returns gt  public static T XmlDeserialize lt T gt  this string xml  where T   class  new         if  xml    null  throw new ArgumentNullException  xml         var serializer   new XmlSerializer typeof T        using  var reader   new StringReader xml                 try   return  T serializer Deserialize reader             catch   return null       Could not be deserialized to this type

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Gets the root domain of a URI        lt summary gt Gets the root domain of any URI lt  summary gt       lt param name  uri  gt URI to get root domain of lt  param gt       lt returns gt Root domain with TLD lt  returns gt  public static string GetRootDomain this System Uri uri        if  uri    null          return null       string Domain   uri Host      while  System Text RegularExpressions Regex Matches Domain           Count  gt  1          Domain   Domain Substring Domain IndexOf        1       Domain   Domain Substring 0  Domain IndexOf            return Domain

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Some extensions for working with lists        lt summary gt      Wrap an object in a list      lt  summary gt  public static IList lt T gt  WrapInList lt T gt  this T item        List lt T gt  result   new List lt T gt         result Add item       return result      use eg   myList   someObject InList      To make an IEnumerable that contains items from one or more sources  in order to make IEnumerable work more like lists  This may not be a good idea for high-performance code but useful for making tests   public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  enumerable  T newItem        foreach  T item in enumerable                yield return item             yield return newItem     public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  enumerable  params T   newItems        foreach  T item in enumerable                yield return item             foreach  T newItem in newItems                yield return newItem            use e g   someEnumeration   someEnumeration Append newItem     Other variations of this are possible - e g   someEnumeration   someEnumeration Append otherEnumeration     If you are cloning items  you may also want to clone lists of them   public static IList lt T gt  Clone lt T gt  this IEnumerable lt T gt  source  where T  ICloneable       List lt T gt  result   new List lt T gt          foreach  T item in source                result Add  T item Clone                return result      When I am working with ObservableCollection lt T gt   I generally extend it with an AddRange method  Other answers here give implementations of this   You may put this code in the Codeplex project if you want

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gitorious org cadenza is a full library of some of the most useful extension methods I ve seen

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The extension method   public static void AddRange lt T  S gt  this ICollection lt T gt  list  params S   values      where S   T       foreach  S value in values          list Add value       The method applies for all types and lets you add a range of items to a list as parameters   Example   var list   new List lt Int32 gt     list AddRange 5  4  8  4  2

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I just went through all 4 pages of this so far  and I was rather surprised that I didn t see this way to shorten a check for InvokeRequired   using System  using System Windows Forms        lt summary gt      Extension methods acting on Control objects       lt  summary gt  internal static class ControlExtensionMethods            lt summary gt          Invokes the given action on the given control s UI thread  if invocation is needed           lt  summary gt           lt param name  control  gt Control on whose UI thread to possibly invoke  lt  param gt           lt param name  action  gt Action to be invoked on the given control  lt  param gt      public static void MaybeInvoke this Control control  Action action                if  control    null  amp  amp  control InvokeRequired                        control Invoke action                     else                       action                              lt summary gt          Maybe Invoke a Func that returns a value           lt  summary gt           lt typeparam name  T  gt Return type of func  lt  typeparam gt           lt param name  control  gt Control on which to maybe invoke  lt  param gt           lt param name  func  gt Function returning a value  to invoke  lt  param gt           lt returns gt The result of the call to func  lt  returns gt      public static T MaybeInvoke lt T gt  this Control control  Func lt T gt  func                if  control    null  amp  amp  control InvokeRequired                        return  T  control Invoke func                      else                       return func                        Usage   myForm MaybeInvoke      gt  this Text    Hello world        Sometimes the control might be null  but that s okay  var dialogResult   this Parent MaybeInvoke      gt  MessageBox Show this   Yes or no     Choice   MessageBoxButtons YesNo

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And one more   public enum ParseFailBehavior      ReturnNull     ReturnDefault     ThrowException    public static T  ParseNullableEnum lt T gt  this string theValue  ParseFailBehavior desiredBehavior   ParseFailBehavior ReturnNull  where T struct      T output     T  result   Enum TryParse lt T gt  theValue  out output            T  output         desiredBehavior    ParseFailBehavior ReturnDefault             T  default T             null      if result    null  amp  amp  desiredBehavior    ParseFailBehavior ThrowException        throw new ArgumentException  Parse Failed for value  0  of enum type  1             FormatWith theValue  typeof T  Name               This version requires  NET 4 0  in 3 5 you have no TryParse and no optional parameters  you re stuck with Enum Parse   which you have to try-catch  It s still totally doable in 3 5  and much more useful as Enum Parse   is oogly and your only other option    public static T  ParseNummableEnum lt T gt  this string theValue        return theValue ParseNullableEnum lt T gt  ParseFailBehavior ReturnNull      public static T  ParseNullableEnum lt T gt  this string theValue       ParseFailBehavior desiredBehavior  where T struct       try               return  T   Enum Parse typeof  T   theValue             catch  Exception                if desiredBehavior    ParseFailBehavior ThrowException  throw             return desiredBehavior    ParseFailBehavior ReturnDefault    T  default T    null      Usage     returns null if OptionOne isn t an enum constant var myEnum    OptionOne  ParseNullableEnum lt OptionEnum gt         guarantees a return value IF the enum has a  zero  constant value  generally a good practice  var myEnum    OptionTwo  ParseNullableEnum lt OptionEnum gt  ParseFailBehavior ReturnDefault  Value

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static string Format  this string str                        params Expression lt Func lt string object gt  gt    args        var parameters   args ToDictionary                           e  gt string Format     0     e Parameters 0  Name                            e  gt e Compile   e Parameters 0  Name         var sb   new StringBuilder str       foreach var kv in parameters                sb Replace  kv Key                     kv Value    null   kv Value ToString                     return sb ToString        With the above extension you can write this   var str     foo   bar   baz   Format foo  gt foo  bar  gt 2  baz  gt new object       and you ll get  foo 2 System Object

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string Format shortcut   public static class StringExtensions          Enable quick and more natural string Format calls     public static string F this string s  params object   args                return string Format s  args             Example   var s    The co-ordinate is   0    1    F point X  point Y     For quick copy-and-paste go here   Don t you find it more natural to type  some string  F  param   instead of string Format  some string    param      For a more readable name  try one of these suggestion   s    Hello  0  world  1    Fmt  Stack    Overflow    s    Hello  0  world  1    FormatBy  Stack    Overflow    s    Hello  0  world  1    FormatWith  Stack    Overflow    s    Hello  0  world  1    Display  Stack    Overflow    s    Hello  0  world  1    With  Stack    Overflow

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My conversion extensions which allow you to do   int i   myString To lt int gt       Here it is  as posted on TheSoftwareJedi com  public static T To lt T gt  this IConvertible obj      return  T Convert ChangeType obj  typeof T       public static T ToOrDefault lt T gt                this IConvertible obj        try               return To lt T gt  obj             catch               return default T            public static bool ToOrDefault lt T gt                       this IConvertible obj                       out T newObj        try               newObj   To lt T gt  obj            return true            catch               newObj   default T            return false           public static T ToOrOther lt T gt                          this IConvertible obj                         T other      try           return To lt T gt obj         catch           return other         public static bool ToOrOther lt T gt                            this IConvertible obj                           out T newObj                           T other        try               newObj   To lt T gt  obj           return true            catch               newObj   other          return false           public static T ToOrNull lt T gt                         this IConvertible obj                        where T   class       try               return To lt T gt  obj             catch               return null           public static bool ToOrNull lt T gt                     this IConvertible obj                    out T newObj                    where T   class       try               newObj   To lt T gt  obj           return true            catch               newObj   null          return false            You can ask for default  calls blank constructor or  0  for numerics  on failure  specify a  default  value  I call it  other    or ask for null  where T   class   I ve also provided both silent exception models  and a typical TryParse model that returns a bool indicating the action taken  and an out param holds the new value  So our code can do things like this  int i   myString To lt int gt     string a   myInt ToOrDefault lt string gt       note type inference DateTime d   myString ToOrOther DateTime MAX VALUE   double d    note type inference bool didItGiveDefault   myString ToOrDefault out d   string s   myDateTime ToOrNull lt string gt       I couldn t get Nullable types to roll into the whole thing very cleanly  I tried for about 20 minutes before I threw in the towel

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on serializing and configs there is better using long as DateTime  so       public static readonly DateTime Epoch   new DateTime 1970  1  1  0  0  0        public static long ToUnixTimestamp this DateTime dateTime                return  long   dateTime - Epoch  TotalSeconds             public static long ToUnixUltraTimestamp this DateTime dateTime                return  long   dateTime - Epoch  TotalMilliseconds          and backwards      public static DateTime ToDateTime this long unixDateTime                return Epoch AddSeconds unixDateTime              public static DateTime ToDateTimeUltra this long unixUltraDateTime                return Epoch AddMilliseconds unixUltraDateTime

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Compare the equality of two objects without  necessarily  overriding Equals or implementing IEquatable lt     Why would you want to do this  When you really want to know if two objects are equal  but you re too lazy to override Equals object  or implement IEquatable lt T gt   Or  more realistically  if you have a terribly complex class and implementing Equals by hand would be extremely tedious  error prone  and not fun to maintain  It also helps if you don t care too much about performance   I am currently using IsEqualTo because of the second reason - I have a class with many properties whose types are other user-defined classes  each of which has many other properties whose types are other user-defined classes  ad infinitum  Throw in a bunch of collections in many of these classes  and implementing Equals object  truly becomes a nightmare   Usage   if  myTerriblyComplexObject IsEqualTo myOtherTerriblyComplexObject            Do something terribly interesting      In order to determine equality  I make numerous comparisons  I make every attempt to do the  right  one in the  right  order  The comparisons  in order are    Use the static Equals object  object  method  If it returns true  return true  It will return true if the references are the same  It will also return true if thisObject overrides Equals object   If thisObject is null  return false  No further comparisons can be made if it is null  If thisObject has overridden Equals object   return false  Since it overrides Equals  it must mean that Equals was executed at step  1 and returned false  If someone has bothered to override Equals  we should respect that and return what Equals returns  If thisObject inherits from IEquatable lt T gt   where otherObject can be assigned to T  get the Equals T  method using reflection  Invoke that method and return its return value  If both objects are IEnumerable  return whether contain the same items  in the same order  using IsEqualTo to compare the items  If the objects have different types  return false  Since we know now that thisObject does not have an Equals method  there isn t any way to realistically evaluate two object of different types to be true  If the objects are a value type  primitive or struct  or a string  return false  We have already failed the Equals object  test - enough said  For each property of thisObject  test its value with IsEqualTo  If any return false  return false  If all return true  return true    String comparisons could be better  but easy to implement  Also  I m not 100  sure I m handling structs right   Without further ado  here is the extension method        lt summary gt      Provides extension methods to determine if objects are equal       lt  summary gt  public static class EqualsEx            lt summary gt          The  lt see cref  Type   gt  of  lt see cref  string   gt            lt  summary gt      private static readonly Type StringType   typeof string             lt summary gt          The  lt see cref  Type   gt  of  lt see cref  object   gt            lt  summary gt      private static readonly Type ObjectType   typeof object             lt summary gt          The  lt see cref  Type   gt  of  lt see cref  IEquatable T    gt            lt  summary gt      private static readonly Type EquatableType   typeof IEquatable lt  gt              lt summary gt          Determines whether  lt paramref name  thisObject   gt  is equal to  lt paramref name  otherObject   gt            lt  summary gt           lt param name  thisObject  gt          This object           lt  param gt           lt param name  otherObject  gt          The other object           lt  param gt           lt returns gt          True  if they are equal  otherwise false           lt  returns gt      public static bool IsEqualTo this object thisObject  object otherObject                if  Equals thisObject  otherObject                            Always check Equals first  If the object has overridden Equals  use it  This will also capture the case where both are the same reference              return true                     if  thisObject    null                           Because Equals object  object  returns true if both are null  if either is null  return false              return false                     var thisObjectType   thisObject GetType            var equalsMethod   thisObjectType GetMethod  Equals   BindingFlags Public   BindingFlags Instance  null  new     ObjectType    null           if  equalsMethod DeclaringType    thisObjectType                           thisObject overrides Equals  and we have already failed the Equals test  so return false              return false                     var otherObjectType   otherObject    null   null   otherObject GetType                If thisObject inherits from IEquatable lt  gt   and otherObject can be passed into its Equals method  use it          var equatableTypes   thisObjectType GetInterfaces   Where                                              Get interfaces of thisObjectType that                i   gt  i IsGenericType                                                                                  are generic                 amp  amp  i GetGenericTypeDefinition      EquatableType                                                      and are IEquatable of some type                 amp  amp   otherObjectType     null    i GetGenericArguments   0  IsAssignableFrom otherObjectType           and otherObjectType can be assigned to the IEquatable s type           if  equatableTypes Any                             If we found any interfaces that meed our criteria  invoke the Equals method for each interface                 If any return true  return true  If all return false  return false              return equatableTypes                  Select equatableType   gt  equatableType GetMethod  Equals   BindingFlags Public   BindingFlags Instance                    Any equatableEqualsMethod   gt   bool equatableEqualsMethod Invoke thisObject  new     otherObject                         if  thisObjectType    StringType  amp  amp  thisObject is IEnumerable  amp  amp  otherObject is IEnumerable                           If both are IEnumerable  check their items              var thisEnumerable     IEnumerable thisObject  Cast lt object gt                 var otherEnumerable     IEnumerable otherObject  Cast lt object gt                  return thisEnumerable SequenceEqual otherEnumerable  IsEqualToComparer Instance                      if  thisObjectType    otherObjectType                           If they have different types  they cannot be equal              return false                     if  thisObjectType IsValueType    thisObjectType    StringType                           If it is a value type  we have already determined that they are not equal  so return false              return false                        Recurse into each public property  if any are not equal  return false  If all are true  return true          return   from propertyInfo in thisObjectType GetProperties                    let thisPropertyValue   propertyInfo GetValue thisObject  null                   let otherPropertyValue   propertyInfo GetValue otherObject  null                   where  thisPropertyValue IsEqualTo otherPropertyValue                   select thisPropertyValue  Any                    lt summary gt          A  lt see cref  IEqualityComparer T    gt  to be used when comparing sequences of collections           lt  summary gt      private class IsEqualToComparer   IEqualityComparer lt object gt                     lt summary gt              The singleton instance of  lt see cref  IsEqualToComparer   gt                lt  summary gt          public static readonly IsEqualToComparer Instance                lt summary gt              Initializes static members of the  lt see cref  EqualsEx IsEqualToComparer   gt  class               lt  summary gt          static IsEqualToComparer                         Instance   new IsEqualToComparer                            lt summary gt              Prevents a default instance of the  lt see cref  EqualsEx IsEqualToComparer   gt  class from being created               lt  summary gt          private IsEqualToComparer                                     lt summary gt              Determines whether the specified objects are equal               lt  summary gt               lt param name  x  gt              The first object to compare               lt  param gt               lt param name  y  gt              The second object to compare               lt  param gt               lt returns gt              true if the specified objects are equal  otherwise  false               lt  returns gt          bool IEqualityComparer lt object gt  Equals object x  object y                        return x IsEqualTo y                           lt summary gt              Not implemented - throws an  lt see cref  NotImplementedException   gt                lt  summary gt               lt param name  obj  gt              The  lt see cref  object   gt  for which a hash code is to be returned               lt  param gt               lt returns gt              A hash code for the specified object               lt  returns gt          int IEqualityComparer lt object gt  GetHashCode object obj                        throw new NotImplementedException

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I m disappointed that the  NET Framework prefers that files and directories be represented as strings rather than objects  and that the FileInfo and DirectoryInfo types aren t as powerful as I d wish   So  I started to write fluent extension methods as I needed them  e g    public static FileInfo SetExtension this FileInfo fileInfo  string extension        return new FileInfo Path ChangeExtension fileInfo FullName  extension       public static FileInfo SetDirectory this FileInfo fileInfo  string directory        return new FileInfo Path Combine directory  fileInfo Name        Yes  you can put this in the codeplex

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Values ordered true false    True false values separated by a capital letter    Only two values allowed    ---------------------------    Limited  but could be useful public enum BooleanFormat       OneZero      YN      YesNo      TF      TrueFalse      PassFail      YepNope    public static class BooleanExtension            lt summary gt          Converts the boolean value of this instance to the specified string value            lt  summary gt      private static string ToString this bool value  string passValue  string failValue                return value   passValue   failValue                  lt summary gt          Converts the boolean value of this instance to a string            lt  summary gt           lt param name  booleanFormat  gt A BooleanFormat value           Example  BooleanFormat PassFail would return  Pass  if true and  Fail  if false  lt  param gt           lt returns gt Boolean formatted string lt  returns gt      public static string ToString this bool value  BooleanFormat booleanFormat                string booleanFormatString   Enum GetName booleanFormat GetType    booleanFormat           return ParseBooleanString value  booleanFormatString                       Parses boolean format strings  not optimized     private static string ParseBooleanString bool value  string booleanFormatString                StringBuilder trueString   new StringBuilder            StringBuilder falseString   new StringBuilder             int charCount   booleanFormatString Length           bool isTrueString   true           for  int i   0  i    charCount  i                          if  char IsUpper booleanFormatString i    amp  amp  i    0                  isTrueString   false               if  isTrueString                  trueString Append booleanFormatString i                else                 falseString Append booleanFormatString i                       return  value    true   trueString ToString     falseString ToString

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I got tired of tedious null-checking while pulling values from MySqlDataReader  so   public static DateTime  GetNullableDateTime this MySqlDataReader dr  string fieldName        DateTime  nullDate   null      return dr IsDBNull dr GetOrdinal fieldName     nullDate   dr GetDateTime fieldName      public static string GetNullableString this MySqlDataReader dr  string fieldName        return dr IsDBNull dr GetOrdinal fieldName     String Empty   dr GetString fieldName      public static char  GetNullableChar this MySqlDataReader dr  string fieldName        char  nullChar   null      return dr IsDBNull dr GetOrdinal fieldName     nullChar   dr GetChar fieldName       Of course this could be used with any SqlDataReader     Both hangy and Joe had some good comments on how to do this  and I have since had an opportunity to implement something similar in a different context  so here is another version   public static int  GetNullableInt32 this IDataRecord dr  int ordinal        int  nullInt   null      return dr IsDBNull ordinal    nullInt   dr GetInt32 ordinal      public static int  GetNullableInt32 this IDataRecord dr  string fieldname        int ordinal   dr GetOrdinal fieldname       return dr GetNullableInt32 ordinal      public static bool  GetNullableBoolean this IDataRecord dr  int ordinal        bool  nullBool   null      return dr IsDBNull ordinal    nullBool   dr GetBoolean ordinal      public static bool  GetNullableBoolean this IDataRecord dr  string fieldname        int ordinal   dr GetOrdinal fieldname       return dr GetNullableBoolean ordinal

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Here is one I use frequently for presentation formatting   public static string ToTitleCase this string mText        if  mText    null  return mText       System Globalization CultureInfo cultureInfo   System Threading Thread CurrentThread CurrentCulture      System Globalization TextInfo textInfo   cultureInfo TextInfo          TextInfo ToTitleCase only operates on the string if is all lower case  otherwise it returns the string unchanged      return textInfo ToTitleCase mText ToLower

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The ThrowIfArgumentIsNull is a nice way to do that null check we all should do   public static class Extensions       public static void ThrowIfArgumentIsNull lt T gt  this T obj  string parameterName  where T   class               if  obj    null  throw new ArgumentNullException parameterName     not allowed to be null              Below is the way to use it and it works on all classes in your namespace or wherever you use the namespace its within   internal class Test       public Test string input1                input1 ThrowIfArgumentIsNull  input1              It s ok to use this code on the CodePlex project

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static string Format  this string str                        params Expression lt Func lt string object gt  gt    args        var parameters   args ToDictionary                           e  gt string Format     0     e Parameters 0  Name                            e  gt e Compile   e Parameters 0  Name         var sb   new StringBuilder str       foreach var kv in parameters                sb Replace  kv Key                     kv Value    null   kv Value ToString                     return sb ToString        With the above extension you can write this   var str     foo   bar   baz   Format foo  gt foo  bar  gt 2  baz  gt new object       and you ll get  foo 2 System Object

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Here s another control-extension i ve been using though i don t know if it s posted here before   public static class ControlExtensions       public static void DoubleBuffer this Control control                    http   stackoverflow com questions 76993 how-to-double-buffer-net-controls-on-a-form 77233 77233            Taxes  Remote Desktop Connection and painting  http   blogs msdn com oldnewthing archive 2006 01 03 508694 aspx          if  System Windows Forms SystemInformation TerminalServerSession  return          System Reflection PropertyInfo dbProp   typeof System Windows Forms Control  GetProperty  DoubleBuffered   System Reflection BindingFlags NonPublic   System Reflection BindingFlags Instance           dbProp SetValue control  true  null             usage   this someControl DoubleBuffer

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public static class ComparableExtensions     public static bool Between lt T gt  this T actual  T lower  T upper  where T   IComparable lt T gt          return actual CompareTo lower   gt   0  amp  amp  actual CompareTo upper   lt  0          Example   if  myNumber Between 3 7

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There is somethimes need to have instance of class no matter if valid but not null

public static T Safe<T>(this T obj) where T : new()
{
    if (obj == null)
    {
        obj = new T();
    }

    return obj;
}

usage will be like:

MyClass myClass = Provider.GetSomeResult();
string temp = myClass.Safe().SomeValue;

instead of:

MyClass myClass = Provider.GetSomeResult();
string temp = "some default value";
if (myClass != null)
{
        temp = myClass.SomeValue;
}

sorry if it is a duplicity, but I dont find it.

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I have various extension methods in my MiscUtil project  full source is available there - I m not going to repeat it here   My favourites  some of which involve other classes  such as ranges    Date and time stuff - mostly for unit tests  Not sure I d use them in production     var birthday   19 June 1976   var workingDay   7 Hours     30 Minutes      Ranges and stepping - massive thanks to Marc Gravell for his operator stuff to make this possible   var evenNaturals   2 To int MaxValue  Step 2   var daysSinceBirth   birthday To DateTime Today  Step 1 Days       Comparisons   var myComparer   ProjectionComparer Create Person p   gt  p Name   var next   myComparer ThenBy p   gt  p Age   var reversed   myComparer Reverse      Argument checking   x ThrowIfNull  x      LINQ to XML applied to anonymous types  or other types with appropriate properties        lt Name gt Jon lt  Name gt  lt Age gt 32 lt  Age gt  new   Name  Jon   Age 32  ToXElements       Name  Jon  Age  32   as XAttributes  obviously  new   Name  Jon   Age 32  ToXAttributes     Push LINQ - would take too long to explain here  but search for it

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Here s a to-and-from for Roman numerals   Not often used  but could be handy   Usage   if   IV  IsValidRomanNumeral            Do useful stuff with the number 4     Console WriteLine  MMMDCCCLXXXVIII  ParseRomanNumeral     Console WriteLine 3888 ToRomanNumeralString       The source       public static class RomanNumeralExtensions               private const int NumberOfRomanNumeralMaps   13           private static readonly Dictionary lt string  int gt  romanNumerals               new Dictionary lt string  int gt  NumberOfRomanNumeralMaps                                   M   1000                        CM   900                        D   500                        CD   400                        C   100                        XC   90                        L   50                        XL   40                        X   10                        IX   9                        V   5                        IV   4                        I   1                           private static readonly Regex validRomanNumeral   new Regex                  i     MDCLXVI    M 0 3    C DM    D C 0 3                       X LC    L XX 0 2   L    I VX    V  II 0 2    V                     RegexOptions Compiled            public static bool IsValidRomanNumeral this string value                        return validRomanNumeral IsMatch value                      public static int ParseRomanNumeral this string value                        if  value    null                                throw new ArgumentNullException  value                               value   value ToUpperInvariant   Trim                 var length   value Length               if   length    0      value IsValidRomanNumeral                                  throw new ArgumentException  Empty or invalid Roman numeral string     value                               var total   0              var i   length               while  i  gt  0                                var digit   romanNumerals value --i  ToString                      if  i  gt  0                                        var previousDigit   romanNumerals value i - 1  ToString                          if  previousDigit  lt  digit                                                digit -  previousDigit                          i--                                                           total    digit                             return total                     public static string ToRomanNumeralString this int value                        const int MinValue   1              const int MaxValue   3999               if   value  lt  MinValue      value  gt  MaxValue                                 throw new ArgumentOutOfRangeException  value   value   Argument out of Roman numeral range                                const int MaxRomanNumeralLength   15              var sb   new StringBuilder MaxRomanNumeralLength                foreach  var pair in romanNumerals                                while  value   pair Value  gt  0                                        sb Append pair Key                       value -  pair Value                                               return sb ToString

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I haven t seen any answer with this one yet     public static string   Split this string value  string regexPattern        return value Split regexPattern  RegexOptions None      public static string   Split this string value  string regexPattern       RegexOptions options        return Regex Split value  regexPattern  options       Usage   var obj    test1 test2 test3   string   arrays   obj Split

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I like these NUnit Assert extensions  http   svn caffeine-it com openrasta trunk src Rasta Testing AssertExtensions cs

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A generic Try   class Program       static void Main string   args                var z   0          var a   0 AsDefaultFor      gt  1   z           Console WriteLine a           Console ReadLine             public static class TryExtensions       public static T AsDefaultFor lt T gt  this T  this  Func lt T gt  operation                try                       return operation                      catch                       return  this                      Put it up on the CodePlex project if you want

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I ve written like a quad zillion extension methods  so here are a few ones I find particulary usefull  Feel free to implement   public static class ControlExtenders            lt summary gt          Advanced version of find control           lt  summary gt           lt typeparam name  T  gt Type of control to find  lt  typeparam gt           lt param name  id  gt Control id to find  lt  param gt           lt returns gt Control of given type  lt  returns gt           lt remarks gt          If the control with the given id is not found         a new control instance of the given type is returned           lt  remarks gt      public static T FindControl lt T gt  this Control control  string id  where T   Control                  User normal FindControl method to get the control         Control  control   control FindControl id               If control was found and is of the correct type we return it         if   control    null  amp  amp   control is T                           Return new control             return  T  control                        Create new control instance          control    T Activator CreateInstance typeof T                Add control to source control so the            next it is found and the value can be            passed on itd  remember to hide it and            set an ID so it can be found next time         if     control is ExtenderControlBase                          control Visible   false                     control ID   id          control Controls Add  control               Use reflection to create a new instance of the control         return  T  control           public static class GenericListExtenders            lt summary gt          Sorts a generic list by items properties           lt  summary gt           lt typeparam name  T  gt Type of collection  lt  typeparam gt           lt param name  list  gt Generic list  lt  param gt           lt param name  fieldName  gt Field to sort data on  lt  param gt           lt param name  sortDirection  gt Sort direction  lt  param gt           lt remarks gt          Use this method when a dinamyc sort field is requiered  If the          sorting field is known manual sorting might improve performance           lt  remarks gt      public static void SortObjects lt T gt  this List lt T gt  list  string fieldName  SortDirection sortDirection                PropertyInfo propInfo   typeof T  GetProperty fieldName           if  propInfo    null                        Comparison lt T gt  compare   delegate T a  T b                                bool asc   sortDirection    SortDirection Ascending                  object valueA   asc   propInfo GetValue a  null    propInfo GetValue b  null                   object valueB   asc   propInfo GetValue b  null    propInfo GetValue a  null                   return valueA is IComparable     IComparable valueA  CompareTo valueB    0                             list Sort compare                             lt summary gt          Creates a pagged collection from generic list           lt  summary gt           lt typeparam name  T  gt Type of collection  lt  typeparam gt           lt param name  list  gt Generic list  lt  param gt           lt param name  sortField  gt Field to sort data on  lt  param gt           lt param name  sortDirection  gt Sort direction  lt  param gt           lt param name  from  gt Page from item index  lt  param gt           lt param name  to  gt Page to item index  lt  param gt           lt param name  copy  gt Creates a copy and returns a new list instead of changing the current one  lt  param gt           lt returns gt Pagged list collection  lt  returns gt      public static List lt T gt  Page lt T gt  this List lt T gt  list  string sortField  bool sortDirection  int from  int to  bool copy                List lt T gt   pageList   new List lt T gt                 Copy list         if  copy                        T    arrList   new T list Count               list CopyTo  arrList                pageList   new List lt T gt   arrList                     else                        pageList   list                        Make sure there are enough items in the list         if  from  gt   pageList Count                        int diff   Math Abs from - to               from    pageList Count - diff                    if  to  gt   pageList Count                        to    pageList Count                        Sort items         if   string IsNullOrEmpty sortField                         SortDirection sortDir   SortDirection Descending              if   sortDirection  sortDir   SortDirection Ascending               pageList SortObjects sortField  sortDir                         Calculate max number of items per page         int count   to - from          if  from   count  gt   pageList Count  count -   from   count  -  pageList Count              Get max number of items per page         T   pagged   new T count            pageList CopyTo from  pagged  0  count               Return pagged items         return new List lt T gt  pagged                   lt summary gt          Shuffle s list items           lt  summary gt           lt typeparam name  T  gt List type  lt  typeparam gt           lt param name  list  gt Generic list  lt  param gt      public static void Shuffle lt T gt  this List lt T gt  list                Random rng   new Random            for  int i   list Count - 1  i  gt  0  i--                        int swapIndex   rng Next i   1               if  swapIndex    i                                T tmp   list swapIndex                   list swapIndex    list i                   list i    tmp                                          lt summary gt          Converts generic List to DataTable           lt  summary gt           lt typeparam name  T  gt Type  lt  typeparam gt           lt param name  list  gt Generic list  lt  param gt           lt param name  columns  gt Name of the columns to copy to the DataTable  lt  param gt           lt returns gt DataTable  lt  returns gt      public static DataTable ToDataTable lt T gt  this List lt T gt  list  string   columns                List lt string gt   columns   new List lt string gt  columns           DataTable dt   new DataTable             foreach  PropertyInfo info in typeof T  GetProperties                          if   columns Contains info Name     columns    null                                dt Columns Add new DataColumn info Name  info PropertyType                                    foreach  T t in list                        DataRow row   dt NewRow                foreach  PropertyInfo info in typeof T  GetProperties                                  if   columns Contains info Name     columns    null                                        row info Name    info GetValue t  null                                               dt Rows Add row                     return dt           public static class DateTimeExtenders            lt summary gt          Returns number of month from a string representation           lt  summary gt           lt returns gt Number of month  lt  returns gt      public static int MonthToNumber this DateTime datetime  string month                month   month ToLower            for  int i   1  i  lt   12  i                          DateTime  dt   DateTime Parse  1     i     2000                string  month   CultureInfo InvariantCulture DateTimeFormat GetMonthName i  ToLower                if   month    month                                return i                                  return 0                  lt summary gt          Returns month name from month number           lt  summary gt           lt returns gt Name of month  lt  returns gt      public static string MonthToName this DateTime datetime  int month                for  int i   1  i  lt   12  i                          if  i    month                                return CultureInfo InvariantCulture DateTimeFormat GetMonthName i                                   return              public static class ObjectExtender       public static object CloneBinary lt T gt  this T originalObject                using  var stream   new System IO MemoryStream                          BinaryFormatter binaryFormatter   new BinaryFormatter                binaryFormatter Serialize stream  originalObject               stream Position   0              return  T binaryFormatter Deserialize stream                        public static object CloneObject this object obj                using  MemoryStream memStream   new MemoryStream                          BinaryFormatter binaryFormatter   new BinaryFormatter null  new StreamingContext StreamingContextStates Clone                binaryFormatter Serialize memStream  obj               memStream Position   0              return binaryFormatter Deserialize memStream                      public static class StringExtenders            lt summary gt          Returns string as unit           lt  summary gt           lt param name  value  gt Value  lt  param gt           lt returns gt Unit lt  returns gt      public static Unit ToUnit this string value                   Return empty unit         if  string IsNullOrEmpty value               return Unit Empty              Trim value         value   value Trim                Return pixel unit         if  value EndsWith  px                             Set unit type             string  int   value Replace  px                        Try parsing to int             double  val   0              if   double TryParse  int  out  val                                    Invalid value                 return Unit Empty                                Return unit             return new Unit  val  UnitType Pixel                         Return percent unit         if  value EndsWith                                Set unit type             string  int   value Replace                           Try parsing to int             double  val   0              if   double TryParse  int  out  val                                    Invalid value                 return Unit Empty                                Return unit             return new Unit  val  UnitType Percentage                         No match found         return new Unit                    lt summary gt          Returns alternative string if current string is null or empty           lt  summary gt           lt param name  str  gt  lt  param gt           lt param name  alternative  gt  lt  param gt           lt returns gt  lt  returns gt      public static string Alternative this string str  string alternative                if  string IsNullOrEmpty str   return alternative          return str                  lt summary gt          Removes all HTML tags from string           lt  summary gt           lt param name  html  gt String containing HTML tags  lt  param gt           lt returns gt String with no HTML tags  lt  returns gt      public static string StripHTML this string html                string nohtml   Regex Replace html    lt     n    gt                 nohtml   nohtml Replace   r n       Replace   n       Replace   amp nbsp        Trim            return nohtml            The first one is my favourite as it enables me to replace   Control c   this FindControl  tbName    if  c    null           Do something with c     customer Name     TextBox c  Text      With this   TextBox c   this FindControl lt TextBox gt   tbName    customer Name   c Text    Settings default string values   string str       if  string IsNullOrEmpty str         str    I m empty        Becomes   str   str Alternative  I m empty

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HTH  These are some of my main ones   using System  using System Collections Generic  using System Globalization  using System Linq  using System Text   namespace Insert Your Namespace Here Helpers       public static class Extensions               public static bool IsNullOrEmpty lt T gt  this IEnumerable lt T gt  iEnumerable                           Cheers to Joel Mueller for the bugfix  Was  Count    now it s  Any               return iEnumerable    null                        iEnumerable Any                       public static IList lt T gt  ToListIfNotNullOrEmpty lt T gt  this IList lt T gt  iList                        return iList IsNullOrEmpty     null   iList                     public static PagedList lt T gt  ToPagedListIfNotNullOrEmpty lt T gt  this PagedList lt T gt  pagedList                        return pagedList IsNullOrEmpty     null   pagedList                     public static string ToPluralString this int value                        return value    1   string Empty    s                      public static string ToReadableTime this DateTime value                        TimeSpan span   DateTime Now Subtract value               const string plural    s                 if  span Days  gt  7                                return value ToShortDateString                               switch  span Days                                case 0                      switch  span Hours                                                case 0                              if  span Minutes    0                                                                return span Seconds  lt   0                                               now                                               string Format   0  second 1  ago                                                              span Seconds                                                             span Seconds    1   plural   string Empty                                                             return string Format   0  minute 1  ago                                                    span Minutes                                                   span Minutes    1   plural   string Empty                           default                              return string Format   0  hour 1  ago                                                    span Hours                                                   span Hours    1   plural   string Empty                                         default                      return string Format   0  day 1  ago                                            span Days                                           span Days    1   plural   string Empty                                    public static string ToShortGuidString this Guid value                        return Convert ToBase64String value ToByteArray                     Replace                            Replace       -                    Substring 0  22                      public static Guid FromShortGuidString this string value                        return new Guid Convert FromBase64String value Replace                                                                     Replace  -                                     public static string ToStringMaximumLength this string value  int maximumLength                        return ToStringMaximumLength value  maximumLength                             public static string ToStringMaximumLength this string value  int maximumLength  string postFixText                        if  string IsNullOrEmpty postFixText                                 throw new ArgumentNullException  postFixText                               return value Length  gt  maximumLength                          string Format CultureInfo InvariantCulture                                           0  1                                           value Substring 0  maximumLength - postFixText Length                                          postFixText                                                      value                     public static string SlugDecode this string value                        return value Replace                               public static string SlugEncode this string value                        return value Replace

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Pythonic methods for Dictionaries        lt summary gt      If a key exists in a dictionary  return its value       otherwise return the default value for that type       lt  summary gt  public static U GetWithDefault lt T  U gt  this Dictionary lt T  U gt  dict  T key        return dict GetWithDefault key  default U            lt summary gt      If a key exists in a dictionary  return its value      otherwise return the provided default value       lt  summary gt  public static U GetWithDefault lt T  U gt  this Dictionary lt T  U gt  dict  T key  U defaultValue        return dict ContainsKey key            dict key            defaultValue      Useful for when you want to append a timestamp to a filename to assure uniqueness        lt summary gt      Format a DateTime as a string that contains no characters      that are banned from filenames  such as           lt  summary gt       lt returns gt YYYY-MM-DD HH MM SS lt  returns gt  public static string ToFilenameString this DateTime dt        return dt ToString  s   Replace           Replace  T

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public static class ComparableExtensions     public static bool Between lt T gt  this T actual  T lower  T upper  where T   IComparable lt T gt          return actual CompareTo lower   gt   0  amp  amp  actual CompareTo upper   lt  0          Example   if  myNumber Between 3 7

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There s a lot of functionality you can get from the Random class   Below are some extension methods I use from time to time  With these  in addition to Next and NextDouble  the Random class gives you NextBool  NextChar  NextDateTime  NextTimeSpan  NextDouble  accepting minValue and maxValue parameters   and my personal favorite  NextString  There are more  NextByte  NextShort  NextLong  etc    but those are mostly for completeness and don t get used as much  So I didn t include them here  this code is long enough as it is        todo  implement additional CharType values  e g   AsciiAny  public enum CharType       AlphabeticLower      AlphabeticUpper      AlphabeticAny      AlphanumericLower      AlphanumericUpper      AlphanumericAny      Numeric    public static class RandomExtensions          10 digits vs  52 alphabetic characters  upper  amp  lower          probability of being numeric  10   62   0 1612903225806452     private const double AlphanumericProbabilityNumericAny   10 0   62 0          10 digits vs  26 alphabetic characters  upper OR lower          probability of being numeric  10   36   0 2777777777777778     private const double AlphanumericProbabilityNumericCased   10 0   36 0       public static bool NextBool this Random random  double probability            return random NextDouble    lt   probability             public static bool NextBool this Random random            return random NextDouble    lt   0 5             public static char NextChar this Random random  CharType mode            switch  mode                case CharType AlphabeticAny                  return random NextAlphabeticChar                case CharType AlphabeticLower                  return random NextAlphabeticChar false               case CharType AlphabeticUpper                  return random NextAlphabeticChar true               case CharType AlphanumericAny                  return random NextAlphanumericChar                case CharType AlphanumericLower                  return random NextAlphanumericChar false               case CharType AlphanumericUpper                  return random NextAlphanumericChar true               case CharType Numeric                  return random NextNumericChar                default                  return random NextAlphanumericChar                         public static char NextChar this Random random            return random NextChar CharType AlphanumericAny              private static char NextAlphanumericChar this Random random  bool uppercase            bool numeric   random NextBool AlphanumericProbabilityNumericCased            if  numeric              return random NextNumericChar            else             return random NextAlphabeticChar uppercase              private static char NextAlphanumericChar this Random random            bool numeric   random NextBool AlphanumericProbabilityNumericAny            if  numeric              return random NextNumericChar            else             return random NextAlphabeticChar random NextBool                private static char NextAlphabeticChar this Random random  bool uppercase            if  uppercase              return  char random Next 65  91           else             return  char random Next 97  123              private static char NextAlphabeticChar this Random random            return random NextAlphabeticChar random NextBool                private static char NextNumericChar this Random random            return  char random Next 48  58              public static DateTime NextDateTime this Random random  DateTime minValue  DateTime maxValue            return DateTime FromOADate              random NextDouble minValue ToOADate    maxValue ToOADate                          public static DateTime NextDateTime this Random random            return random NextDateTime DateTime MinValue  DateTime MaxValue              public static double NextDouble this Random random  double minValue  double maxValue            if  maxValue  lt  minValue              throw new ArgumentException  Minimum value must be less than maximum value              double difference   maxValue - minValue          if   double IsInfinity difference               return minValue    random NextDouble     difference            else                  to avoid evaluating to Double Infinity  we split the range into two halves              double halfDifference    maxValue   0 5  -  minValue   0 5                   50 50 chance of returning a value from the first or second half of the range             if  random NextBool                    return minValue    random NextDouble     halfDifference               else                 return  minValue   halfDifference     random NextDouble     halfDifference                        public static string NextString this Random random  int numChars  CharType mode            char   chars   new char numChars            for  int i   0  i  lt  numChars    i              chars i    random NextChar mode            return new string chars              public static string NextString this Random random  int numChars            return random NextString numChars  CharType AlphanumericAny              public static TimeSpan NextTimeSpan this Random random  TimeSpan minValue  TimeSpan maxValue            return TimeSpan FromMilliseconds              random NextDouble minValue TotalMilliseconds  maxValue TotalMilliseconds                        public static TimeSpan NextTimeSpan this Random random            return random NextTimeSpan TimeSpan MinValue  TimeSpan MaxValue

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I like these methods for dealing with enums that have the Flags attribute set   public static bool AnyOf this object mask  object flags        return   int mask  amp   int flags     0    public static bool AllOf this object mask  object flags        return   int mask  amp   int flags      int flags    public static object SetOn this object mask  object flags        return  int mask    int flags    etc    Example usage   var options   SomeOptions OptionA  options   options SetOn OptionB   options   options SetOn OptionC    if  options AnyOf SomeOptions OptionA   SomeOptions OptionB     etc    The original methods were from this article  http   www codeproject com KB cs masksandflags aspx display Print I just converted them to extension methods   The one problem with them though is that the parameters of object type  which means that all objects end up being extended with these methods  whereas ideally they should only apply to enums   Update As per the comments  you can get around the  signature pollution   at the expense of performance  like this   public static bool AnyOf this Enum mask  object flags        return  Convert ToInt642 mask   amp   int flags     0

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I like these NUnit Assert extensions  http   svn caffeine-it com openrasta trunk src Rasta Testing AssertExtensions cs

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Some extensions for working with lists        lt summary gt      Wrap an object in a list      lt  summary gt  public static IList lt T gt  WrapInList lt T gt  this T item        List lt T gt  result   new List lt T gt         result Add item       return result      use eg   myList   someObject InList      To make an IEnumerable that contains items from one or more sources  in order to make IEnumerable work more like lists  This may not be a good idea for high-performance code but useful for making tests   public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  enumerable  T newItem        foreach  T item in enumerable                yield return item             yield return newItem     public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  enumerable  params T   newItems        foreach  T item in enumerable                yield return item             foreach  T newItem in newItems                yield return newItem            use e g   someEnumeration   someEnumeration Append newItem     Other variations of this are possible - e g   someEnumeration   someEnumeration Append otherEnumeration     If you are cloning items  you may also want to clone lists of them   public static IList lt T gt  Clone lt T gt  this IEnumerable lt T gt  source  where T  ICloneable       List lt T gt  result   new List lt T gt          foreach  T item in source                result Add  T item Clone                return result      When I am working with ObservableCollection lt T gt   I generally extend it with an AddRange method  Other answers here give implementations of this   You may put this code in the Codeplex project if you want

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Another one  this time to make UriBuilder more friendly when dealing with query params            lt summary gt          Adds the specified query parameter to the URI builder           lt  summary gt           lt param name  builder  gt The builder  lt  param gt           lt param name  parameterName  gt Name of the parameter  lt  param gt           lt param name  value  gt The URI escaped value  lt  param gt           lt returns gt The final full query string  lt  returns gt      public static string AddQueryParam this UriBuilder builder  string parameterName  string value                if  parameterName    null              throw new ArgumentNullException  parameterName             if  parameterName Length    0              throw new ArgumentException  The parameter name is empty              if  value    null              throw new ArgumentNullException  value             if  value Length    0              throw new ArgumentException  The value is empty              if  builder Query Length    0                        builder Query   String Concat parameterName       value                     else if              builder Query Contains String Concat   amp    parameterName                       builder Query Contains String Concat      parameterName                               throw new InvalidOperationException String Format  The parameter  0  already exists    parameterName                      else                       builder Query   String Concat builder Query Substring 1     amp    parameterName       value                      return builder Query

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Easily serialize objects into XML   public static string ToXml lt T gt  this T obj  where T   class       XmlSerializer s   new XmlSerializer obj GetType         using  StringWriter writer   new StringWriter                  s Serialize writer  obj           return writer ToString               lt root gt  lt child gt foo lt  child lt  root gt   ToXml lt MyCustomType gt

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The ThrowIfArgumentIsNull is a nice way to do that null check we all should do   public static class Extensions       public static void ThrowIfArgumentIsNull lt T gt  this T obj  string parameterName  where T   class               if  obj    null  throw new ArgumentNullException parameterName     not allowed to be null              Below is the way to use it and it works on all classes in your namespace or wherever you use the namespace its within   internal class Test       public Test string input1                input1 ThrowIfArgumentIsNull  input1              It s ok to use this code on the CodePlex project

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Find more examples here  www extensionmethod net

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The WhereIf   Method  var query   dc Reviewer       Where r   gt  r FacilityID    facilityID        WhereIf CheckBoxActive Checked  r   gt  r IsActive     public static IEnumerable lt TSource gt  WhereIf lt TSource gt       this IEnumerable lt TSource gt  source      bool condition  Func lt TSource  bool gt  predicate          if  condition           return source Where predicate        else          return source      public static IQueryable lt TSource gt  WhereIf lt TSource gt       this IQueryable lt TSource gt  source      bool condition  Expression lt Func lt TSource  bool gt  gt  predicate          if  condition           return source Where predicate        else          return source       I also added overloads for the index predicate in the Where   extension method  For more fun  add a flavor that includes an additional  else  predicate

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I ve been looking for a way to contribute back to the community some of the things I ve developed     Here s some FileInfo extensions that I find quite useful        lt summary gt      Open with default  open  program      lt  summary gt       lt param name  value  gt  lt  param gt  public static Process Open this FileInfo value        if   value Exists          throw new FileNotFoundException  File doesn t exist        Process p   new Process        p StartInfo FileName   value FullName      p StartInfo Verb    Open       p Start        return p          lt summary gt      Print the file      lt  summary gt       lt param name  value  gt  lt  param gt  public static void Print this FileInfo value        if   value Exists          throw new FileNotFoundException  File doesn t exist        Process p   new Process        p StartInfo FileName   value FullName      p StartInfo Verb    Print       p Start            lt summary gt      Send this file to the Recycle Bin      lt  summary gt       lt exception cref  File doesn t exist    gt       lt param name  value  gt  lt  param gt  public static void Recycle this FileInfo value                value Recycle false           lt summary gt      Send this file to the Recycle Bin     On show  if person refuses to send file to the recycle bin       exception is thrown or otherwise delete fails      lt  summary gt       lt exception cref  File doesn t exist    gt       lt exception cref  On show  if user refuses  throws exception  The operation was canceled      gt       lt param name  value  gt File being recycled lt  param gt       lt param name  showDialog  gt true to show pop-up lt  param gt  public static void Recycle this FileInfo value  bool showDialog        if   value Exists              throw new FileNotFoundException  File doesn t exist        if  showDialog           FileSystem DeleteFile              value FullName  UIOption AllDialogs               RecycleOption SendToRecycleBin       else         FileSystem DeleteFile              value FullName  UIOption OnlyErrorDialogs               RecycleOption SendToRecycleBin       Open any file in the user s favorite editor   new FileInfo  C  image jpg   Open      Print any file that the operating system knows how to print   new FileInfo  C  image jpg   Print      Send any file to the recycle bin    You have to include the Microsoft VisualBasic reference use the using Microsoft VisualBasic FileIO    Example   new FileInfo  C  image jpg   Recycle      Or     let user have a chance to cancel send to recycle bin  new FileInfo  C  image jpg   Recycle true

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A couple of useful extensions if you work with Fiscal Years       lt summary gt      Returns the fiscal year for the passed in date      lt  summary gt       lt param name  value  gt the date lt  param gt       lt returns gt the fiscal year lt  returns gt  public static int FiscalYear this DateTime value      int ret   value Year    if  value Month  gt   7  ret      return ret          lt summary gt      Returns the fiscal year for the passed in date      lt  summary gt       lt param name  value  gt the date lt  param gt       lt returns gt the fiscal year lt  returns gt  public static string FiscalYearString this DateTime value      int fy   FiscalYear value     return   0   1   Format fy - 1  fy

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Some extensions for working with lists        lt summary gt      Wrap an object in a list      lt  summary gt  public static IList lt T gt  WrapInList lt T gt  this T item        List lt T gt  result   new List lt T gt         result Add item       return result      use eg   myList   someObject InList      To make an IEnumerable that contains items from one or more sources  in order to make IEnumerable work more like lists  This may not be a good idea for high-performance code but useful for making tests   public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  enumerable  T newItem        foreach  T item in enumerable                yield return item             yield return newItem     public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  enumerable  params T   newItems        foreach  T item in enumerable                yield return item             foreach  T newItem in newItems                yield return newItem            use e g   someEnumeration   someEnumeration Append newItem     Other variations of this are possible - e g   someEnumeration   someEnumeration Append otherEnumeration     If you are cloning items  you may also want to clone lists of them   public static IList lt T gt  Clone lt T gt  this IEnumerable lt T gt  source  where T  ICloneable       List lt T gt  result   new List lt T gt          foreach  T item in source                result Add  T item Clone                return result      When I am working with ObservableCollection lt T gt   I generally extend it with an AddRange method  Other answers here give implementations of this   You may put this code in the Codeplex project if you want

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String As lt T gt   which can be used to convert a string value as some type  intended to be used primarily with primitives and types that support IConvertable  Works great with Nullable types and even Enums   public static partial class StringExtensions            lt summary gt          Converts the string to the specified type  using the default value configured for the type           lt  summary gt           lt typeparam name  T  gt Type the string will be converted to  The type must implement IConvertable  lt  typeparam gt           lt param name  original  gt The original string  lt  param gt           lt returns gt The converted value  lt  returns gt      public static T As lt T gt  this String original                return As original  CultureInfo CurrentCulture                    default T                    lt summary gt          Converts the string to the specified type  using the default value configured for the type           lt  summary gt           lt typeparam name  T  gt Type the string will be converted to  lt  typeparam gt           lt param name  original  gt The original string  lt  param gt           lt param name  defaultValue  gt The default value to use in case the original string is null or empty  or can t be converted  lt  param gt           lt returns gt The converted value  lt  returns gt      public static T As lt T gt  this String original  T defaultValue                return As original  CultureInfo CurrentCulture  defaultValue                   lt summary gt          Converts the string to the specified type  using the default value configured for the type           lt  summary gt           lt typeparam name  T  gt Type the string will be converted to  lt  typeparam gt           lt param name  original  gt The original string  lt  param gt           lt param name  provider  gt Format provider used during the type conversion  lt  param gt           lt returns gt The converted value  lt  returns gt      public static T As lt T gt  this String original  IFormatProvider provider                return As original  provider  default T                    lt summary gt          Converts the string to the specified type           lt  summary gt           lt typeparam name  T  gt Type the string will be converted to  lt  typeparam gt           lt param name  original  gt The original string  lt  param gt           lt param name  provider  gt Format provider used during the type conversion  lt  param gt           lt param name  defaultValue  gt The default value to use in case the original string is null or empty  or can t be converted  lt  param gt           lt returns gt The converted value  lt  returns gt           lt remarks gt          If an error occurs while converting the specified value to the requested type  the exception is caught and the default is returned  It is strongly recommended you         do NOT use this method if it is important that conversion failures are not swallowed up                  This method is intended to be used to convert string values to primatives  not for parsing  converting  or deserializing complex types           lt  remarks gt      public static T As lt T gt  this String original  IFormatProvider provider                            T defaultValue                T result          Type type   typeof  T            if  String IsNullOrEmpty original   result   defaultValue          else                          need to get the underlying type if T is Nullable lt  gt                if  type IsNullableType                                  type   Nullable GetUnderlyingType type                              try                                  ChangeType doesn t work properly on Enums                 result   type IsEnum                                 T  Enum Parse type  original  true                                  T  Convert ChangeType original  type  provider                             catch    HACK  what can we do to minimize or avoid raising exceptions as part of normal operation  custom string parsing  regex   for well-known types  it would be best to know if you can convert to the desired type before you attempt to do so                                result   defaultValue                                   return result            This relies on another simple extension for Type        lt summary gt      Extension methods for  lt see cref  Type   gt        lt  summary gt  public static class TypeExtensions            lt summary gt          Returns whether or not the specified type is  lt see cref  Nullable T    gt            lt  summary gt           lt param name  type  gt A  lt see cref  Type   gt   lt  param gt           lt returns gt True if the specified type is  lt see cref  Nullable T    gt   otherwise  false  lt  returns gt           lt remarks gt Use  lt see cref  Nullable GetUnderlyingType   gt  to access the underlying type  lt  remarks gt      public static bool IsNullableType this Type type                if  type    null  throw new ArgumentNullException  type             return type IsGenericType  amp  amp  type GetGenericTypeDefinition   Equals typeof  Nullable lt  gt               Usage   var someInt    1  As lt int gt     var someIntDefault    bad value  As 1       bad value  won t convert  so the default value 1 is returned  var someEnum    Sunday  As lt DayOfWeek gt     someEnum    0  As lt DayOfWeek gt        returns Sunday var someNullableEnum      As lt DayOfWeek  gt  null      returns a null value since    can t be converted

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public static class DictionaryExtensions       public static Nullable lt TValue gt  GetValueOrNull lt TKey  TValue gt  this Dictionary lt TKey  TValue gt  dictionary  TKey key          where TValue   struct               TValue result          if  dictionary TryGetValue key  out result               return result          else             return null            Free to use  just mention my name  Janko R  bisch  in the code

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When using a dictionary where the key is a string  return existing key using a case-insensitive search  Our use case for this was for file paths        lt summary gt      Gets the key using  lt paramref name  caseInsensitiveKey   gt  from  lt paramref name  dictionary   gt        lt  summary gt       lt typeparam name  T  gt The dictionary value  lt  typeparam gt       lt param name  dictionary  gt The dictionary  lt  param gt       lt param name  caseInsensitiveKey  gt The case insensitive key  lt  param gt       lt returns gt      An existing key  or  lt see cref  string Empty   gt  if not found       lt  returns gt  public static string GetKeyIgnoringCase lt T gt  this IDictionary lt string  T gt  dictionary  string caseInsensitiveKey        if  string IsNullOrEmpty caseInsensitiveKey   return string Empty      foreach  string key in dictionary Keys                if  key Equals caseInsensitiveKey  StringComparison InvariantCultureIgnoreCase                         return key                      return string Empty

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This one is for MVC it adds the ability to generate a  lt label   gt  tag to the Html variable that is available in every ViewPage  Hopefully it will be of use to others trying to develop similar extensions   Use    lt    Html Label  LabelId    ForId    Text    gt    Output    lt label id  LabelId  for  ForId  gt Text lt  label gt    Code   public static class HtmlHelperExtensions       public static string Label this HtmlHelper Html  string  for  string text                return Html Label null   for  text              public static string Label this HtmlHelper Html  string  for  string text  object htmlAttributes                return Html Label null   for  text  htmlAttributes              public static string Label this HtmlHelper Html  string  for  string text  IDictionary lt string  object gt  htmlAttributes                return Html Label null   for  text  htmlAttributes              public static string Label this HtmlHelper Html  string id  string  for  string text                return Html Label id   for  text  null              public static string Label this HtmlHelper Html  string id  string  for  string text  object htmlAttributes                return Html Label id   for  text  new RouteValueDictionary htmlAttributes               public static string Label this HtmlHelper Html  string id  string  for  string text  IDictionary lt string  object gt  htmlAttributes                TagBuilder tag   new TagBuilder  label             tag MergeAttributes htmlAttributes            if   string IsNullOrEmpty id               tag MergeAttribute  id   Html AttributeEncode id             tag MergeAttribute  for   Html AttributeEncode  for             tag SetInnerText Html Encode text             return tag ToString TagRenderMode Normal

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While working with MVC and having lots of if statements where i only care about either true or false  and printing null  or string Empty in the other case  I came up with   public static TResult WhenTrue lt TResult gt  this Boolean value  Func lt TResult gt  expression        return value   expression     default TResult      public static TResult WhenTrue lt TResult gt  this Boolean value  TResult content        return value   content   default TResult      public static TResult WhenFalse lt TResult gt  this Boolean value  Func lt TResult gt  expression        return  value   expression     default TResult      public static TResult WhenFalse lt TResult gt  this Boolean value  TResult content        return  value   content   default TResult       It allows me to change  lt     someBool     print y    string Empty   gt  into  lt    someBool WhenTrue  print y     gt     I only use it in my Views where I mix code and HTML  in code files writing the  longer  version is more clear IMHO

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These extension methods are pretty useful for me when parsing form input before putting into the database  public static int  ToInt this string input         int val      if  int TryParse input  out val           return val      return null     public static DateTime  ToDate this string input        DateTime val      if  DateTime TryParse input  out val           return val      return null     public static decimal  ToDecimal this string input        decimal val      if  decimal TryParse input  out val           return val      return null

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A generic Try   class Program       static void Main string   args                var z   0          var a   0 AsDefaultFor      gt  1   z           Console WriteLine a           Console ReadLine             public static class TryExtensions       public static T AsDefaultFor lt T gt  this T  this  Func lt T gt  operation                try                       return operation                      catch                       return  this                      Put it up on the CodePlex project if you want

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ASP NET HTML Encode - short and sweet   public static string ToHtmlEncodedString this string s        if  String IsNullOrEmpty s           return s      return HttpUtility HtmlEncode s

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Here is one I use frequently for presentation formatting   public static string ToTitleCase this string mText        if  mText    null  return mText       System Globalization CultureInfo cultureInfo   System Threading Thread CurrentThread CurrentCulture      System Globalization TextInfo textInfo   cultureInfo TextInfo          TextInfo ToTitleCase only operates on the string if is all lower case  otherwise it returns the string unchanged      return textInfo ToTitleCase mText ToLower

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public static class StringExtensions             lt summary gt          Parses a string into an Enum          lt  summary gt           lt typeparam name  T  gt The type of the Enum lt  typeparam gt           lt param name  value  gt String value to parse lt  param gt           lt returns gt The Enum corresponding to the stringExtensions lt  returns gt      public static T EnumParse lt T gt  this string value            return StringExtensions EnumParse lt T gt  value  false              public static T EnumParse lt T gt  this string value  bool ignorecase             if  value    null                throw new ArgumentNullException  value                       value   value Trim             if  value Length    0                throw new ArgumentException  Must specify valid information for parsing in the string     value                       Type t   typeof T            if   t IsEnum                throw new ArgumentException  Type provided must be an Enum     T                       return  T Enum Parse t  value  ignorecase             Useful to parse a string into an Enum   public enum TestEnum       Bar      Test    public class Test       public void Test                 TestEnum foo    Test  EnumParse lt TestEnum gt                Credit goes to Scott Dorman  --- Edit for Codeplex project ---  I have asked Scott Dorman if he would mind us publishing his code in the Codeplex project  This is the reply I got from him      Thanks for the heads-up on both the SO post and the CodePlex project  I have upvoted your answer on the question  Yes  the code is effectively in the public domain currently under the CodeProject Open License  http   www codeproject com info cpol10 aspx        I have no problems with this being included in the CodePlex project  and if you want to add me to the project  username is sdorman  I will add that method plus some additional enum helper methods

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I have an extension method for logging exceptions   public static void Log this Exception obj        your logging logic here     And it is used like this   try         Your stuff here   catch Exception ex        ex Log         sorry for posting twice  the 2nd one is better designed  -

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public static class StringExtensions             lt summary gt          Parses a string into an Enum          lt  summary gt           lt typeparam name  T  gt The type of the Enum lt  typeparam gt           lt param name  value  gt String value to parse lt  param gt           lt returns gt The Enum corresponding to the stringExtensions lt  returns gt      public static T EnumParse lt T gt  this string value            return StringExtensions EnumParse lt T gt  value  false              public static T EnumParse lt T gt  this string value  bool ignorecase             if  value    null                throw new ArgumentNullException  value                       value   value Trim             if  value Length    0                throw new ArgumentException  Must specify valid information for parsing in the string     value                       Type t   typeof T            if   t IsEnum                throw new ArgumentException  Type provided must be an Enum     T                       return  T Enum Parse t  value  ignorecase             Useful to parse a string into an Enum   public enum TestEnum       Bar      Test    public class Test       public void Test                 TestEnum foo    Test  EnumParse lt TestEnum gt                Credit goes to Scott Dorman  --- Edit for Codeplex project ---  I have asked Scott Dorman if he would mind us publishing his code in the Codeplex project  This is the reply I got from him      Thanks for the heads-up on both the SO post and the CodePlex project  I have upvoted your answer on the question  Yes  the code is effectively in the public domain currently under the CodeProject Open License  http   www codeproject com info cpol10 aspx        I have no problems with this being included in the CodePlex project  and if you want to add me to the project  username is sdorman  I will add that method plus some additional enum helper methods

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I m using this one quite a lot     Original code   if  guid    Guid Empty  return guid  else return Guid NewGuid      New code   return guid NewGuidIfEmpty      Extension method   public static Guid NewGuidIfEmpty this Guid uuid        return  uuid    Guid Empty   uuid   Guid NewGuid

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public static class StringExtensions             lt summary gt          Parses a string into an Enum          lt  summary gt           lt typeparam name  T  gt The type of the Enum lt  typeparam gt           lt param name  value  gt String value to parse lt  param gt           lt returns gt The Enum corresponding to the stringExtensions lt  returns gt      public static T EnumParse lt T gt  this string value            return StringExtensions EnumParse lt T gt  value  false              public static T EnumParse lt T gt  this string value  bool ignorecase             if  value    null                throw new ArgumentNullException  value                       value   value Trim             if  value Length    0                throw new ArgumentException  Must specify valid information for parsing in the string     value                       Type t   typeof T            if   t IsEnum                throw new ArgumentException  Type provided must be an Enum     T                       return  T Enum Parse t  value  ignorecase             Useful to parse a string into an Enum   public enum TestEnum       Bar      Test    public class Test       public void Test                 TestEnum foo    Test  EnumParse lt TestEnum gt                Credit goes to Scott Dorman  --- Edit for Codeplex project ---  I have asked Scott Dorman if he would mind us publishing his code in the Codeplex project  This is the reply I got from him      Thanks for the heads-up on both the SO post and the CodePlex project  I have upvoted your answer on the question  Yes  the code is effectively in the public domain currently under the CodeProject Open License  http   www codeproject com info cpol10 aspx        I have no problems with this being included in the CodePlex project  and if you want to add me to the project  username is sdorman  I will add that method plus some additional enum helper methods

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I found this one helpful  public static IEnumerable lt T gt  EmptyIfNull lt T gt  this IEnumerable lt T gt  pSeq        return pSeq    Enumerable Empty lt T gt         It removes the null check in the calling code  You could now do  MyList EmptyIfNull   Where

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A generic Try   class Program       static void Main string   args                var z   0          var a   0 AsDefaultFor      gt  1   z           Console WriteLine a           Console ReadLine             public static class TryExtensions       public static T AsDefaultFor lt T gt  this T  this  Func lt T gt  operation                try                       return operation                      catch                       return  this                      Put it up on the CodePlex project if you want

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Are these any use   public static bool CoinToss this Random rng        return rng Next 2     0     public static T OneOf lt T gt  this Random rng  params T   things        return things rng Next things Length       Random rand  bool luckyDay   rand CoinToss    string babyName   rand OneOf  John    George    Radio XBR74 ROCKS

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I didn t want to add anything that was already said  so here are some that I use that haven t been mentioned   Sorry if this is too lengthy    public static class MyExtensions       public static bool IsInteger this string input                int temp           return int TryParse input  out temp              public static bool IsDecimal this string input                decimal temp           return decimal TryParse input  out temp              public static int ToInteger this string input  int defaultValue                int temp           return  int TryParse input  out temp     temp   defaultValue             public static decimal ToDecimal this string input  decimal defaultValue                decimal temp           return  decimal TryParse input  out temp     temp   defaultValue             public static DateTime ToFirstOfTheMonth this DateTime input                return input Date AddDays -1   input Day   1                 Intentionally returns 0 if the target date is before the input date      public static int MonthsUntil this DateTime input  DateTime targetDate                input   input ToFirstOfTheMonth             targetDate   targetDate ToFirstOfTheMonth             int result   0           while  input  lt  targetDate                    input   input AddMonths 1               result                       return result                Used for backwards compatibility in a system built before my time      public static DataTable ToDataTable this IEnumerable input                   too much code to show here right now

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I m using this one quite a lot     Original code   if  guid    Guid Empty  return guid  else return Guid NewGuid      New code   return guid NewGuidIfEmpty      Extension method   public static Guid NewGuidIfEmpty this Guid uuid        return  uuid    Guid Empty   uuid   Guid NewGuid

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While working with MVC and having lots of if statements where i only care about either true or false  and printing null  or string Empty in the other case  I came up with   public static TResult WhenTrue lt TResult gt  this Boolean value  Func lt TResult gt  expression        return value   expression     default TResult      public static TResult WhenTrue lt TResult gt  this Boolean value  TResult content        return value   content   default TResult      public static TResult WhenFalse lt TResult gt  this Boolean value  Func lt TResult gt  expression        return  value   expression     default TResult      public static TResult WhenFalse lt TResult gt  this Boolean value  TResult content        return  value   content   default TResult       It allows me to change  lt     someBool     print y    string Empty   gt  into  lt    someBool WhenTrue  print y     gt     I only use it in my Views where I mix code and HTML  in code files writing the  longer  version is more clear IMHO

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Sql server has a limit of  2000 parameters  which is a pain if you have 10k Ids and want the records connected with them   I wrote these methods which accept batched lists of ids and are called like this   List lt Order gt  orders   dataContext Orders FetchByIds    orderIdChunks    list   gt  row   gt  list Contains row OrderId      List lt Customer gt  customers   dataContext Orders FetchByIds    orderIdChunks    list   gt  row   gt  list Contains row OrderId     row   gt  row Customer     public static List lt ResultType gt  FetchByIds lt RecordType  ResultType gt       this IQueryable lt RecordType gt  querySource      List lt List lt int gt  gt  IdChunks      Func lt List lt int gt   Expression lt Func lt RecordType  bool gt  gt  gt  filterExpressionGenerator      Expression lt Func lt RecordType  ResultType gt  gt  projectionExpression       where RecordType   class       List lt ResultType gt  result   new List lt ResultType gt         foreach  List lt int gt  chunk in IdChunks                Expression lt Func lt RecordType  bool gt  gt  filterExpression               filterExpressionGenerator chunk            IQueryable lt ResultType gt  query   querySource              Where filterExpression               Select projectionExpression            List lt ResultType gt  rows   query ToList            result AddRange rows              return result     public static List lt RecordType gt  FetchByIds lt RecordType gt       this IQueryable lt RecordType gt  querySource      List lt List lt int gt  gt  IdChunks      Func lt List lt int gt   Expression lt Func lt RecordType  bool gt  gt  gt  filterExpressionGenerator       where RecordType   class       Expression lt Func lt RecordType  RecordType gt  gt  identity   r   gt  r       return FetchByIds          querySource          IdChunks          filterExpressionGenerator          identity

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Here s a to-and-from for Roman numerals   Not often used  but could be handy   Usage   if   IV  IsValidRomanNumeral            Do useful stuff with the number 4     Console WriteLine  MMMDCCCLXXXVIII  ParseRomanNumeral     Console WriteLine 3888 ToRomanNumeralString       The source       public static class RomanNumeralExtensions               private const int NumberOfRomanNumeralMaps   13           private static readonly Dictionary lt string  int gt  romanNumerals               new Dictionary lt string  int gt  NumberOfRomanNumeralMaps                                   M   1000                        CM   900                        D   500                        CD   400                        C   100                        XC   90                        L   50                        XL   40                        X   10                        IX   9                        V   5                        IV   4                        I   1                           private static readonly Regex validRomanNumeral   new Regex                  i     MDCLXVI    M 0 3    C DM    D C 0 3                       X LC    L XX 0 2   L    I VX    V  II 0 2    V                     RegexOptions Compiled            public static bool IsValidRomanNumeral this string value                        return validRomanNumeral IsMatch value                      public static int ParseRomanNumeral this string value                        if  value    null                                throw new ArgumentNullException  value                               value   value ToUpperInvariant   Trim                 var length   value Length               if   length    0      value IsValidRomanNumeral                                  throw new ArgumentException  Empty or invalid Roman numeral string     value                               var total   0              var i   length               while  i  gt  0                                var digit   romanNumerals value --i  ToString                      if  i  gt  0                                        var previousDigit   romanNumerals value i - 1  ToString                          if  previousDigit  lt  digit                                                digit -  previousDigit                          i--                                                           total    digit                             return total                     public static string ToRomanNumeralString this int value                        const int MinValue   1              const int MaxValue   3999               if   value  lt  MinValue      value  gt  MaxValue                                 throw new ArgumentOutOfRangeException  value   value   Argument out of Roman numeral range                                const int MaxRomanNumeralLength   15              var sb   new StringBuilder MaxRomanNumeralLength                foreach  var pair in romanNumerals                                while  value   pair Value  gt  0                                        sb Append pair Key                       value -  pair Value                                               return sb ToString

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I like these methods for dealing with enums that have the Flags attribute set   public static bool AnyOf this object mask  object flags        return   int mask  amp   int flags     0    public static bool AllOf this object mask  object flags        return   int mask  amp   int flags      int flags    public static object SetOn this object mask  object flags        return  int mask    int flags    etc    Example usage   var options   SomeOptions OptionA  options   options SetOn OptionB   options   options SetOn OptionC    if  options AnyOf SomeOptions OptionA   SomeOptions OptionB     etc    The original methods were from this article  http   www codeproject com KB cs masksandflags aspx display Print I just converted them to extension methods   The one problem with them though is that the parameters of object type  which means that all objects end up being extended with these methods  whereas ideally they should only apply to enums   Update As per the comments  you can get around the  signature pollution   at the expense of performance  like this   public static bool AnyOf this Enum mask  object flags        return  Convert ToInt642 mask   amp   int flags     0

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This one is not fully baked as we just came up with it this morning  It will generate a full class definition for a Type  Useful for situations where you have a a large class and want to create a subset or full definition but don t have access to it  For example  to store the object in a database etc   public static class TypeExtensions       public static string GenerateClassDefinition this Type type                var properties   type GetFields            var sb   new StringBuilder            var classtext     private class  name                     props             foreach  var p in GetTypeInfo type                         sb AppendFormat    public  0   1     p Item2  p Item1  AppendLine     get  set                          return classtext Replace   name   type Name  Replace   props   sb ToString                 region Private Methods     private static List lt Tuple lt string  string gt  gt  GetTypeInfo Type type                var ret   new List lt Tuple lt string  string gt  gt             var fields   type GetFields            var props   type GetProperties             foreach var p in props  ret Add new Tuple lt string  string gt  p Name  TranslateType p PropertyType                 foreach var f in fields  ret Add new Tuple lt string  string gt  f Name  TranslateType f FieldType              return ret              private static string TranslateType Type input                string ret           if  Nullable GetUnderlyingType input     null                        ret   string Format   0     TranslateType Nullable GetUnderlyingType input                       else                       switch  input Name                                case  Int32   ret    int   break                  case  Int64   ret    long   break                  case  IntPtr   ret    long   break                  case  Boolean   ret    bool   break                  case  String                   case  Char                   case  Decimal                       ret   input Name ToLower    break                  default  ret   input Name  break                                   return ret             endregion     Example usage   Process GetProcesses   First   GetType   GenerateClassDefinition      Becomes even more handy if using linqpad   Process GetProcesses   First   GetType   GenerateClassDefinition   Dump

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It irritated me that LINQ gives me an OrderBy that takes a class implementing IComparer as an argument  but does not support passing in a simple anonymous comparer function  I rectified that   This class creates an IComparer from your comparer function          lt summary gt          Creates an  lt see cref  IComparer T    gt  instance for the given         delegate function       lt  summary gt  internal class ComparerFactory lt T gt    IComparer lt T gt        public static IComparer lt T gt  Create Func lt T  T  int gt  comparison                return new ComparerFactory lt T gt  comparison              private readonly Func lt T  T  int gt   comparison       private ComparerFactory Func lt T  T  int gt  comparison                 comparison   comparison              region IComparer lt T gt  Members      public int Compare T x  T y                return  comparison x  y               endregion        and these extension methods expose my new OrderBy overloads on enumerables  I doubt this works for LINQ to SQL  but it s great for LINQ to Objects   public static class EnumerableExtensions            lt summary gt          Sorts the elements of a sequence in ascending order by using a specified comparison delegate           lt  summary gt      public static IOrderedEnumerable lt TSource gt  OrderBy lt TSource  TKey gt  this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector                                                                       Func lt TKey  TKey  int gt  comparison                var comparer   ComparerFactory lt TKey gt  Create comparison           return source OrderBy keySelector  comparer                   lt summary gt          Sorts the elements of a sequence in descending order by using a specified comparison delegate           lt  summary gt      public static IOrderedEnumerable lt TSource gt  OrderByDescending lt TSource  TKey gt  this IEnumerable lt TSource gt  source  Func lt TSource  TKey gt  keySelector                                                                                 Func lt TKey  TKey  int gt  comparison                var comparer   ComparerFactory lt TKey gt  Create comparison           return source OrderByDescending keySelector  comparer             You re welcome to put this on codeplex if you like

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Here is the only extension that I wrote that I use regularly   It makes sending email with System Net Mail a bit easier   public static class MailExtension          GetEmailCreditial out strServer  gets credentials from an XML file     public static void Send this MailMessage email                string strServer   String Empty          NetworkCredential credentials   GetEmailCreditial out strServer           SmtpClient client   new SmtpClient strServer    Credentials   credentials            client Send email              public static void Send this IEnumerable lt MailMessage gt  emails                string strServer   String Empty          NetworkCredential credentials   GetEmailCreditial out strServer           SmtpClient client   new SmtpClient strServer    Credentials   credentials            foreach  MailMessage email in emails              client Send email               Example of use   new MailMessage  info myDomain com   you gmail com   This is an important Subject    Body goes here   Send      Assume email1 email2 email3 are MailMessage objects new List lt MailMessage gt    email1  email2  email  Send

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I have various extension methods in my MiscUtil project  full source is available there - I m not going to repeat it here   My favourites  some of which involve other classes  such as ranges    Date and time stuff - mostly for unit tests  Not sure I d use them in production     var birthday   19 June 1976   var workingDay   7 Hours     30 Minutes      Ranges and stepping - massive thanks to Marc Gravell for his operator stuff to make this possible   var evenNaturals   2 To int MaxValue  Step 2   var daysSinceBirth   birthday To DateTime Today  Step 1 Days       Comparisons   var myComparer   ProjectionComparer Create Person p   gt  p Name   var next   myComparer ThenBy p   gt  p Age   var reversed   myComparer Reverse      Argument checking   x ThrowIfNull  x      LINQ to XML applied to anonymous types  or other types with appropriate properties        lt Name gt Jon lt  Name gt  lt Age gt 32 lt  Age gt  new   Name  Jon   Age 32  ToXElements       Name  Jon  Age  32   as XAttributes  obviously  new   Name  Jon   Age 32  ToXAttributes     Push LINQ - would take too long to explain here  but search for it

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My most used extension is one which can format byte arrays        lt summary gt      Returns a string representation of a byte array       lt  summary gt       lt param name  bytearray  gt The byte array to represent  lt  param gt       lt param name  subdivision  gt The number of elements per group      or 0 to not restrict it  The default is 0  lt  param gt       lt param name  subsubdivision  gt The number of elements per line      or 0 to not restrict it  The default is 0  lt  param gt       lt param name  divider  gt The string dividing the individual bytes  The default is      lt  param gt       lt param name  subdivider  gt The string dividing the groups  The default is       lt  param gt       lt param name  subsubdivider  gt The string dividing the lines  The default is   r n   lt  param gt       lt param name  uppercase  gt Whether the representation is in uppercase hexadecimal      The default is  lt see langword  true   gt   lt  param gt       lt param name  prebyte  gt The string to put before each byte  The default is an empty string  lt  param gt       lt param name  postbyte  gt The string to put after each byte  The default is an empty string  lt  param gt       lt returns gt The string representation  lt  returns gt       lt exception cref  ArgumentNullException  gt       lt paramref name  bytearray   gt  is  lt see langword  null   gt        lt  exception gt  public static string ToArrayString this byte   bytearray      int subdivision   0      int subsubdivision   0      string divider            string subdivider             string subsubdivider     r n       bool uppercase   true      string prebyte           string postbyte              region Contract     if  bytearray    null          throw new ArgumentNullException  bytearray         endregion      StringBuilder sb   new StringBuilder          bytearray Length    2   divider Length   prebyte Length   postbyte Length             subdivision  gt  0    bytearray Length   subdivision    subdivider Length   0             subsubdivision  gt  0    bytearray Length   subsubdivision    subsubdivider Length   0        int groupElements    subdivision  gt  0   subdivision - 1   -1       int lineElements    subsubdivision  gt  0   subsubdivision - 1   -1       for  long i   0  i  lt  bytearray LongLength - 1  i                  sb Append prebyte           sb Append String Format CultureInfo InvariantCulture   uppercase     0 X2       0 x2     bytearray i             sb Append postbyte            if  lineElements    0                        sb Append subsubdivider               groupElements   subdivision              lineElements   subsubdivision                    else if  groupElements    0                        sb Append subdivider               groupElements   subdivision                    else             sb Append divider            lineElements--          groupElements--            sb Append prebyte       sb Append String Format CultureInfo InvariantCulture   uppercase     0 X2       0 x2     bytearray bytearray LongLength - 1         sb Append postbyte        return sb ToString        By default ToArrayString   just prints the byte array as a long string of individual bytes  However  ToArrayString 4  16  groups the bytes in groups of four  with 16 bytes on a line  just as it is in your favorite hex editor  And the following nicely formats the byte array for usage in C  code   byte   bytearray   new byte           Console Write bytearray ToArrayString 4  16                   r n   true   0x       It was written by me  so you may put it on Codeplex

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My suggestion   public static bool IsNullOrEmpty this ICollection obj      return  obj    null    obj Count    0       Works with collections and arrays   bool isNullOrEmpty   array IsNullOrEmpty     instead of  bool isNullOrEmpty   array    null    array Length    0

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public static class EnumerableExtensions        Pure      public static U MapReduce lt T  U gt  this IEnumerable lt T gt  enumerable  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways enumerable    null           CodeContract RequiresAlways enumerable Skip 1  Any             CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null           return enumerable AsParallel   Select map  Aggregate reduce              Pure      public static U MapReduce lt T  U gt  this IList lt T gt  list  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways list    null           CodeContract RequiresAlways list Count  gt   2           CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null           U result   map list 0            for  int i   1  i  lt  list Count  i                          result   reduce result map list i                       return result               Parallel version  creates garbage      Pure      public static U MapReduce lt T  U gt  this IList lt T gt  list  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways list    null           CodeContract RequiresAlways list Skip 1  Any             CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null            U   mapped   new U list Count           Parallel For 0  mapped Length  i   gt                                mapped i    map list i                            U result   mapped 0           for  int i   1  i  lt  list Count  i                          result   reduce result  mapped i                      return result

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Convert any string to type Int32     Calls the underlying int TryParse method to convert a string    representation of a number to its 32-bit signed integer equivalent     Returns Zero if conversion fails   public static int ToInt32 this string s        int retInt      int TryParse s  out retInt       return retInt      SAMPLE USE  string s    999   int i   s ToInt32

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http   www codeplex com linqext

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Perhaps the most useful extension methods I ve written and used are here   http   www codeproject com KB cs fun-with-cs-extensions aspx msg 2838918 xx2838918xx

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I use this extension method usually with anonymous types to get a dictionary ala ruby  public static Dictionary lt string  object gt  ToDictionary this object o        var dictionary   new Dictionary lt string  object gt          foreach  var propertyInfo in o GetType   GetProperties                  if  propertyInfo GetIndexParameters   Length    0                        dictionary Add propertyInfo Name  propertyInfo GetValue o  null                         return dictionary      You can use it   var dummy   new   color     000000   width    100    id    myid     Dictionary lt string  object gt  dict   dummy ToDictionary      And with an extended method as   public static void ForEach lt T gt  this IEnumerable lt T gt  source  Action lt T gt  action        foreach  T item in source                action item             You can do it  dummy ToDictionary   ForEach  p    gt  Console Write   0    1      p Key  p Value      Output   color   000000  width  100   id  myid

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I got tired of tedious null-checking while pulling values from MySqlDataReader  so   public static DateTime  GetNullableDateTime this MySqlDataReader dr  string fieldName        DateTime  nullDate   null      return dr IsDBNull dr GetOrdinal fieldName     nullDate   dr GetDateTime fieldName      public static string GetNullableString this MySqlDataReader dr  string fieldName        return dr IsDBNull dr GetOrdinal fieldName     String Empty   dr GetString fieldName      public static char  GetNullableChar this MySqlDataReader dr  string fieldName        char  nullChar   null      return dr IsDBNull dr GetOrdinal fieldName     nullChar   dr GetChar fieldName       Of course this could be used with any SqlDataReader     Both hangy and Joe had some good comments on how to do this  and I have since had an opportunity to implement something similar in a different context  so here is another version   public static int  GetNullableInt32 this IDataRecord dr  int ordinal        int  nullInt   null      return dr IsDBNull ordinal    nullInt   dr GetInt32 ordinal      public static int  GetNullableInt32 this IDataRecord dr  string fieldname        int ordinal   dr GetOrdinal fieldname       return dr GetNullableInt32 ordinal      public static bool  GetNullableBoolean this IDataRecord dr  int ordinal        bool  nullBool   null      return dr IsDBNull ordinal    nullBool   dr GetBoolean ordinal      public static bool  GetNullableBoolean this IDataRecord dr  string fieldname        int ordinal   dr GetOrdinal fieldname       return dr GetNullableBoolean ordinal

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You all probably already know that an interesting usage for extension methods is as a kind of mixin  Some extension methods  like the XmlSerializable  pollute almost every class  and it doesn t make sense to most of them  like Thread and SqlConnection   Some functionality should be explicitly mixed in to the classes that want to have it  I propose a new notation to this kind of type  with the M prefix   The XmlSerializable then  is this   public interface MXmlSerializable     public static class XmlSerializable     public static string ToXml this MXmlSerializable self        if  self    null  throw new ArgumentNullException        var serializer   new XmlSerializer self GetType         using  var writer   new StringWriter            serializer Serialize writer  self         return writer GetStringBuilder   ToString                public static T FromXml lt T gt  string xml  where T   MXmlSerializable       var serializer   new XmlSerializer typeof T        return  T serializer Deserialize new StringReader xml            A class then mixes it in   public class Customer   MXmlSerializable     public string Name   get  set      public bool Preferred   get  set        And the usage is simply   var customer   new Customer      Name    Guybrush Threepwood      Preferred   true    var xml   customer ToXml      If you like the idea  you can create a new namespace for useful mixins in the project  What do you think   Oh  and by the way  I think most extension methods should explicitly test for null

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Convert a double to string formatted using the specified culture   public static class ExtensionMethods      public static string ToCurrency this double value  string cultureName          CultureInfo currentCulture   new CultureInfo cultureName       return  string Format currentCulture    0 C    value            Example   double test   154 20  string testString   test ToCurrency  en-US        154 20

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Smalltalk style if else in c    Feel free to put this on codeplex under whatever license you are using  using System  namespace SmalltalkBooleanExtensionMethods        public static class BooleanExtension               public static T ifTrue lt T gt   this bool aBoolean  Func lt T gt  method                    if  aBoolean              return  T method            else             return default T                  public static void ifTrue  this bool aBoolean  Action method                        if  aBoolean                  method                        public static T ifFalse lt T gt   this bool aBoolean  Func lt T gt  method                        if   aBoolean                  return  T method                else                 return default T                      public static void ifFalse  this bool aBoolean  Action method                        if   aBoolean                  method                        public static T ifTrueifFalse lt T gt   this Boolean aBoolean  Func lt T gt  methodA  Func lt T gt  methodB                        if  aBoolean                  return  T methodA                else                 return  T methodB                       public static void ifTrueifFalse  this Boolean aBoolean  Action methodA  Action methodB                        if  aBoolean                  methodA                else                 methodB                           You probably already have a timesRepeat method but its in there   using System   namespace SmalltalkBooleanExtensionMethods       public static class IntExtension               public static int timesRepeat lt T gt  this int x  Func lt T gt  method                        for  int i   x  i  gt  0  i--                                method                               return x                     public static int timesRepeat this int x  Action method                        for  int i   x  i  gt  0  i--                                method                               return x                      Nunit Tests  using System  using SmalltalkBooleanExtensionMethods  using NUnit Framework   namespace SmalltalkBooleanExtensionMethodsTest        TestFixture      public class SBEMTest               int i          bool itWorks            SetUp          public void Init                          i   0              itWorks   false                      Test            public void TestifTrue                          itWorks    true ifTrue      gt  true                Assert IsTrue itWorks                      Test            public void TestifFalse                         itWorks    false ifFalse      gt  true                Assert IsTrue itWorks                       Test            public void TestifTrueifFalse                         itWorks   false ifTrueifFalse      gt  false       gt  true               Assert IsTrue itWorks               itWorks   false              itWorks   true ifTrueifFalse      gt  true       gt  false               Assert IsTrue itWorks                       Test            public void TestTimesRepeat                          5  timesRepeat      gt  i   i   1               Assert AreEqual i  5                       Test            public void TestVoidMethodIfTrue                          true ifTrue      gt  SetItWorksBooleanToTrue                 Assert IsTrue itWorks                       Test            public void TestVoidMethodIfFalse                          false ifFalse      gt  SetItWorksBooleanToTrue                 Assert IsTrue itWorks                      public void TestVoidMethodIfTrueIfFalse                         true ifTrueifFalse      gt  SetItWorksBooleanToTrue         gt  SetItWorksBooleanToFalse                 false ifTrueifFalse      gt  SetItWorksBooleanToFalse         gt  SetItWorksBooleanToTrue                 Assert IsTrue itWorks                       public void TestVoidMethodTimesRepeat                          5  timesRepeat      gt  AddOneToi                 Assert AreEqual i  5                      public void SetItWorksBooleanToTrue                         itWorks   true                     public void SetItWorksBooleanToFalse                         itWorks   false                     public void AddOneToi                         i   i   1

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DateTimeExtensions  Examples   DateTime firstDayOfMonth   DateTime Now First    DateTime lastdayOfMonth   DateTime Now Last    DateTime lastFridayInMonth   DateTime Now Last DayOfWeek Friday   DateTime nextFriday   DateTime Now Next DayOfWeek Friday   DateTime lunchTime   DateTime Now SetTime 11  30   DateTime noonOnFriday   DateTime Now Next DayOfWeek Friday  Noon    DateTime secondMondayOfMonth   DateTime Now First DayOfWeek Monday  Next DayOfWeek Monday  Midnight

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An easier way to load default settings from a collection  in real life I use it to populate the settings from any source  including the command line  ClickOnce URL parameters etc     public static void LoadFrom this ApplicationSettingsBase settings  NameValueCollection configuration        if  configuration    null          foreach  string key in configuration AllKeys              if   String IsNullOrEmpty key                   try                                       settings key    configuration Get key                                     catch  SettingsPropertyNotFoundException                                         handle bad arguments as you wish                       Example   Settings Default LoadFrom new NameValueCollection        Setting1    Value1        Setting2    Value2

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The Substring method on the string class has always felt inadequate to me  Usually when you do a substring  you know the character s  from where you want to start  and the charachter s  where you want to end  Thus  I ve always felt that have to specify length as the second parameter is stupid  Therefore  I ve written my own extension methods  One that takes a startIndex and an endIndex  And one  that takes a startText  string  and endText  string  so you can just specify the text from where to start the substring  and the text for where to end it   NOTE  I couldn t name the method Substring as in  NET because my first overload takes the same parameter types as one of the  NET overloads  Therefore I named them Subsetstring  Feel free to add to the CodePlex     public static class StringExtensions            lt summary gt          Returns a Subset string starting at the specified start index and ending and the specified end         index           lt  summary gt           lt param name  s  gt The string to retrieve the subset from  lt  param gt           lt param name  startIndex  gt The specified start index for the subset  lt  param gt           lt param name  endIndex  gt The specified end index for the subset  lt  param gt           lt returns gt A Subset string starting at the specified start index and ending and the specified end         index  lt  returns gt      public static string Subsetstring this string s  int startIndex  int endIndex                if  startIndex  gt  endIndex                        throw new InvalidOperationException  End Index must be after Start Index                        if  startIndex  lt  0                        throw new InvalidOperationException  Start Index must be a positive number                        if endIndex  lt 0                        throw new InvalidOperationException  End Index must be a positive number                        return s Substring startIndex   endIndex - startIndex                    lt summary gt          Finds the specified Start Text and the End Text in this string instance  and returns a string         containing all the text starting from startText  to the begining of endText   endText is not         included            lt  summary gt           lt param name  s  gt The string to retrieve the subset from  lt  param gt           lt param name  startText  gt The Start Text to begin the Subset from  lt  param gt           lt param name  endText  gt The End Text to where the Subset goes to  lt  param gt           lt param name  ignoreCase  gt Whether or not to ignore case when comparing startText endText to the string  lt  param gt           lt returns gt A string containing all the text starting from startText  to the begining of endText  lt  returns gt      public static string Subsetstring this string s  string startText  string endText  bool ignoreCase                if  string IsNullOrEmpty startText     string IsNullOrEmpty endText                         throw new ArgumentException  Start Text and End Text cannot be empty                       string temp   s          if  ignoreCase                        temp   s ToUpperInvariant                startText   startText ToUpperInvariant                endText   endText ToUpperInvariant                      int start   temp IndexOf startText           int end   temp IndexOf endText  start           return Subsetstring s  start  end             Usage   string s    This is a tester for my cool extension method            s   s Subsetstring  tester    cool  true     Output   tester for my

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Find more examples here  www extensionmethod net

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If you have persian language and must show the numbers to users in persian language   static public string ToFaString          this string value                           1728   1584             string result                   if  value    null                                char   resChar   value ToCharArray                    for  int i   0  i  lt  resChar Length  i                                          if  resChar i   gt    0   amp  amp  resChar i   lt    9                           result     char  resChar i    1728                       else                         result    resChar i                                               return result

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By all means put this in the codeplex project   Serializing   Deserializing objects to XML        lt summary gt Serializes an object of type T in to an xml string lt  summary gt       lt typeparam name  T  gt Any class type lt  typeparam gt       lt param name  obj  gt Object to serialize lt  param gt       lt returns gt A string that represents Xml  empty otherwise lt  returns gt  public static string XmlSerialize lt T gt  this T obj  where T   class  new         if  obj    null  throw new ArgumentNullException  obj         var serializer   new XmlSerializer typeof T        using  var writer   new StringWriter                  serializer Serialize writer  obj           return writer ToString                  lt summary gt Deserializes an xml string in to an object of Type T lt  summary gt       lt typeparam name  T  gt Any class type lt  typeparam gt       lt param name  xml  gt Xml as string to deserialize from lt  param gt       lt returns gt A new object of type T is successful  null if failed lt  returns gt  public static T XmlDeserialize lt T gt  this string xml  where T   class  new         if  xml    null  throw new ArgumentNullException  xml         var serializer   new XmlSerializer typeof T        using  var reader   new StringReader xml                 try   return  T serializer Deserialize reader             catch   return null       Could not be deserialized to this type

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This is an extension method for the ASP Net MVC action link helper method that allows it to use the controller s authorize attributes to decide if the link should be enabled  disabled or hidden from the current user s view   I saves you from having to enclose your restricted actions in  if  clauses that check for user membership in all the views  Thanks to Maarten Balliauw for the idea and the code bits that showed me the way     using System  using System Collections Generic  using System Linq  using System Web  using System Security Principal  using System Web Routing  using System Web Mvc  using System Collections  using System Reflection  namespace System Web Mvc Html       public static class HtmlHelperExtensions                     lt summary gt              Shows or hides an action link based on the user s membership status             and the controller s authorize attributes              lt  summary gt               lt param name  linkText  gt The link text  lt  param gt               lt param name  action  gt The controller action name  lt  param gt               lt param name  controller  gt The controller name  lt  param gt               lt returns gt  lt  returns gt          public static string SecurityTrimmedActionLink              this HtmlHelper htmlHelper              string linkText              string action              string controller                        return SecurityTrimmedActionLink htmlHelper  linkText  action  controller  false  null                           lt summary gt              Enables  disables or hides an action link based on the user s membership status             and the controller s authorize attributes              lt  summary gt               lt param name  linkText  gt The link text  lt  param gt               lt param name  action  gt The action name  lt  param gt               lt param name  controller  gt The controller name  lt  param gt               lt param name  showDisabled  gt if set to  lt c gt true lt  c gt   show link as disabled -              using a span tag instead of an anchor tag    lt  param gt               lt param name  disabledAttributeText  gt Use this to add attributes to the disabled             span tag  lt  param gt               lt returns gt  lt  returns gt          public static string SecurityTrimmedActionLink              this HtmlHelper htmlHelper               string linkText               string action               string controller               bool showDisabled               string disabledAttributeText                        if  IsAccessibleToUser action  controller  HttpContext Current                                  return htmlHelper ActionLink linkText  action  controller                             else                               return showDisabled                        String Format                            lt span 1  gt  0  lt  span gt                             linkText                           disabledAttributeText  null        disabledAttributeText                                                                  private static IController GetControllerInstance string controllerName                        Assembly assembly   Assembly GetExecutingAssembly                Type controllerType   GetControllerType controllerName               return  IController Activator CreateInstance controllerType                      private static ArrayList GetControllerAttributes string controllerName  HttpContext context                        if  context Cache controllerName     ControllerAttributes      null                                var controller   GetControllerInstance controllerName                    context Cache Add                      controllerName     ControllerAttributes                       new ArrayList controller GetType   GetCustomAttributes typeof AuthorizeAttribute   true                        null                      Caching Cache NoAbsoluteExpiration                      Caching Cache NoSlidingExpiration                      Caching CacheItemPriority Default                      null                              return  ArrayList context Cache controllerName     ControllerAttributes                        private static ArrayList GetMethodAttributes string controllerName  string actionName  HttpContext context                        if  context Cache controllerName         actionName     ActionAttributes      null                                ArrayList actionAttrs   new ArrayList                    var controller   GetControllerInstance controllerName                   MethodInfo   methods   controller GetType   GetMethods                     foreach  MethodInfo method in methods                                        object   attributes   method GetCustomAttributes typeof ActionNameAttribute   true                        if   attributes Length    0  amp  amp  method Name    actionName                                                      attributes Length  gt  0  amp  amp    ActionNameAttribute attributes 0   Name    actionName                                                 actionAttrs AddRange method GetCustomAttributes typeof AuthorizeAttribute   true                                                             context Cache Add                      controllerName         actionName     ActionAttributes                       actionAttrs                      null                      Caching Cache NoAbsoluteExpiration                      Caching Cache NoSlidingExpiration                      Caching CacheItemPriority Default                      null                               return  ArrayList context Cache controllerName         actionName    ActionAttributes                        public static bool IsAccessibleToUser string actionToAuthorize  string controllerToAuthorize  HttpContext context                        IPrincipal principal   context User                 cache the attribute list for both controller class and it s methods              ArrayList controllerAttributes   GetControllerAttributes controllerToAuthorize  context                ArrayList actionAttributes   GetMethodAttributes controllerToAuthorize  actionToAuthorize  context                                        if  controllerAttributes Count    0  amp  amp  actionAttributes Count    0                  return true               string roles                   string users                   if  controllerAttributes Count  gt  0                                AuthorizeAttribute attribute   controllerAttributes 0  as AuthorizeAttribute                  roles    attribute Roles                  users    attribute Users                            if  actionAttributes Count  gt  0                                AuthorizeAttribute attribute   actionAttributes 0  as AuthorizeAttribute                  roles    attribute Roles                  users    attribute Users                             if  string IsNullOrEmpty roles   amp  amp  string IsNullOrEmpty users   amp  amp  principal Identity IsAuthenticated                  return true               string   roleArray   roles Split                   string   usersArray   users Split                   foreach  string role in roleArray                                if  role           principal IsInRole role                       return true                            foreach  string user in usersArray                                if  user         amp  amp   principal Identity Name    user                       return true                            return false                     private static Type GetControllerType string controllerName                        Assembly assembly   Assembly GetExecutingAssembly                foreach  Type type in assembly GetTypes                                  if                       type BaseType  null                       amp  amp  type BaseType Name     Controller                        amp  amp   type Name ToUpper       controllerName ToUpper      Controller  ToUpper                                            return type                                              return null

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Equivalent to Python s Join method        lt summary gt      same as python  join       lt  summary gt       lt typeparam name  T  gt list type lt  typeparam gt       lt param name  separator  gt string separator  lt  param gt       lt param name  list  gt list of objects to be ToString d lt  param gt       lt returns gt a concatenated list interleaved with separators lt  returns gt  static public string Join lt T gt  this string separator  IEnumerable lt T gt  list        var sb   new StringBuilder        bool first   true       foreach  T v in list                if   first              sb Append separator           first   false           if  v    null              sb Append v ToString                return sb ToString

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Simple but nicer than  Enumerable Range   IMHO        lt summary gt      Replace  Enumerable Range n   with  n Range          lt  summary gt       lt param name  n  gt iterations lt  param gt       lt returns gt 0  n-1 lt  returns gt  public static IEnumerable lt int gt  Range this int n        for  int i   0  i  lt  n  i            yield return i

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The ThrowIfArgumentIsNull is a nice way to do that null check we all should do   public static class Extensions       public static void ThrowIfArgumentIsNull lt T gt  this T obj  string parameterName  where T   class               if  obj    null  throw new ArgumentNullException parameterName     not allowed to be null              Below is the way to use it and it works on all classes in your namespace or wherever you use the namespace its within   internal class Test       public Test string input1                input1 ThrowIfArgumentIsNull  input1              It s ok to use this code on the CodePlex project

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Two little ones  some people find them silly  that I put in all my projects are   public static bool IsNull this object o     return o    null      and  public static bool IsNullOrEmpty this string s     return string IsNullOrEmpty s       It makes my code so much more fluent    if  myClassInstance IsNull          do something  if  myString IsNullOrEmpty          do something   I think these would make really nice extension properties  if we ever get those

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Simple but nicer than  Enumerable Range   IMHO        lt summary gt      Replace  Enumerable Range n   with  n Range          lt  summary gt       lt param name  n  gt iterations lt  param gt       lt returns gt 0  n-1 lt  returns gt  public static IEnumerable lt int gt  Range this int n        for  int i   0  i  lt  n  i            yield return i

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And here s the control-invoke extension i m using regularly   public static class InvokeExtensions       public static void InvokeHandler this Control control  MethodInvoker del     Sync  control-invoke extension                if  control InvokeRequired                        control Invoke del               return                     del       run the actual code             public static void AsyncInvokeHandler this Control control  MethodInvoker del     Async  control-invoke extension                if  control InvokeRequired                        control BeginInvoke del               return                     del       run the actual code            sample   this TreeView AsyncInvokeHandler      gt                        this Text    xyz                which allows cross-thread gui-updates

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This file contains extension methods for generic List lt  gt  class to operate on sorted lists     Duplicate values are OK     O ln n   is still much faster then the O n  of LINQ s searches filters  static partial class SortedList          Return the index of the first element with the key greater then provided         If there s no such element within the provided range  it returns iAfterLast      public static int sortedFirstGreaterIndex lt tElt  tKey gt   this IList lt tElt gt  list  Func lt tElt  tKey  int gt  comparer  tKey key  int iFirst  int iAfterLast                 if  iFirst  lt  0    iAfterLast  lt  0    iFirst  gt  list Count    iAfterLast  gt  list Count               throw new IndexOutOfRangeException            if  iFirst  gt  iAfterLast               throw new ArgumentException            if  iFirst    iAfterLast               return iAfterLast           int low   iFirst  high   iAfterLast             The code below is inspired by the following article             http   en wikipedia org wiki Binary search Single comparison per iteration         while  low  lt  high                         int mid     high   low     2                  mid  might be  iFirst  in case  iFirst 1    iAfterLast                   mid  will never be  iAfterLast               if  comparer  list  mid    key    lt   0        lt    since we gonna find the first  greater  element                 low   mid   1              else                 high   mid                    return low                Return the index of the first element with the key greater then the provided key         If there s no such element  returns list Count      public static int sortedFirstGreaterIndex lt tElt  tKey gt   this IList lt tElt gt  list  Func lt tElt  tKey  int gt  comparer  tKey key                 return list sortedFirstGreaterIndex  comparer  key  0  list Count                  Add an element to the sorted array         This could be an expensive operation if frequently adding elements that sort firstly         This is cheap operation when adding elements that sort near the tail of the list      public static int sortedAdd lt tElt gt   this List lt tElt gt  list  Func lt tElt  tElt  int gt  comparer  tElt elt                 if  list Count    0    comparer  list  list Count - 1    elt    lt   0                            either the list is empty  or the item is greater then all elements already in the collection              list Add  elt                return list Count - 1                    int ind   list sortedFirstGreaterIndex  comparer  elt            list Insert  ind  elt            return ind                Find first exactly equal element  return -1 if not found      public static int sortedFindFirstIndex lt tElt  tKey gt   this List lt tElt gt  list  Func lt tElt  tKey  int gt  comparer  tKey elt                 int low   0  high   list Count - 1           while  low  lt  high                         int mid     high   low     2              if  comparer  list  mid    elt    lt  0                   low   mid   1              else                 high   mid     this includes the case when we ve found an element exactly matching the key                   if  high  gt   0  amp  amp  0    comparer  list  high    elt                 return high          return -1                Return the IEnumerable that returns array elements in the reverse order      public static IEnumerable lt tElt gt  sortedReverse lt tElt gt   this List lt tElt gt  list                 for  int i list Count - 1  i  gt   0  i--               yield return list  i

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Sql server has a limit of  2000 parameters  which is a pain if you have 10k Ids and want the records connected with them   I wrote these methods which accept batched lists of ids and are called like this   List lt Order gt  orders   dataContext Orders FetchByIds    orderIdChunks    list   gt  row   gt  list Contains row OrderId      List lt Customer gt  customers   dataContext Orders FetchByIds    orderIdChunks    list   gt  row   gt  list Contains row OrderId     row   gt  row Customer     public static List lt ResultType gt  FetchByIds lt RecordType  ResultType gt       this IQueryable lt RecordType gt  querySource      List lt List lt int gt  gt  IdChunks      Func lt List lt int gt   Expression lt Func lt RecordType  bool gt  gt  gt  filterExpressionGenerator      Expression lt Func lt RecordType  ResultType gt  gt  projectionExpression       where RecordType   class       List lt ResultType gt  result   new List lt ResultType gt         foreach  List lt int gt  chunk in IdChunks                Expression lt Func lt RecordType  bool gt  gt  filterExpression               filterExpressionGenerator chunk            IQueryable lt ResultType gt  query   querySource              Where filterExpression               Select projectionExpression            List lt ResultType gt  rows   query ToList            result AddRange rows              return result     public static List lt RecordType gt  FetchByIds lt RecordType gt       this IQueryable lt RecordType gt  querySource      List lt List lt int gt  gt  IdChunks      Func lt List lt int gt   Expression lt Func lt RecordType  bool gt  gt  gt  filterExpressionGenerator       where RecordType   class       Expression lt Func lt RecordType  RecordType gt  gt  identity   r   gt  r       return FetchByIds          querySource          IdChunks          filterExpressionGenerator          identity

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public static class EnumerableExtensions        Pure      public static U MapReduce lt T  U gt  this IEnumerable lt T gt  enumerable  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways enumerable    null           CodeContract RequiresAlways enumerable Skip 1  Any             CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null           return enumerable AsParallel   Select map  Aggregate reduce              Pure      public static U MapReduce lt T  U gt  this IList lt T gt  list  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways list    null           CodeContract RequiresAlways list Count  gt   2           CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null           U result   map list 0            for  int i   1  i  lt  list Count  i                          result   reduce result map list i                       return result               Parallel version  creates garbage      Pure      public static U MapReduce lt T  U gt  this IList lt T gt  list  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways list    null           CodeContract RequiresAlways list Skip 1  Any             CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null            U   mapped   new U list Count           Parallel For 0  mapped Length  i   gt                                mapped i    map list i                            U result   mapped 0           for  int i   1  i  lt  list Count  i                          result   reduce result  mapped i                      return result

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I didn t want to add anything that was already said  so here are some that I use that haven t been mentioned   Sorry if this is too lengthy    public static class MyExtensions       public static bool IsInteger this string input                int temp           return int TryParse input  out temp              public static bool IsDecimal this string input                decimal temp           return decimal TryParse input  out temp              public static int ToInteger this string input  int defaultValue                int temp           return  int TryParse input  out temp     temp   defaultValue             public static decimal ToDecimal this string input  decimal defaultValue                decimal temp           return  decimal TryParse input  out temp     temp   defaultValue             public static DateTime ToFirstOfTheMonth this DateTime input                return input Date AddDays -1   input Day   1                 Intentionally returns 0 if the target date is before the input date      public static int MonthsUntil this DateTime input  DateTime targetDate                input   input ToFirstOfTheMonth             targetDate   targetDate ToFirstOfTheMonth             int result   0           while  input  lt  targetDate                    input   input AddMonths 1               result                       return result                Used for backwards compatibility in a system built before my time      public static DataTable ToDataTable this IEnumerable input                   too much code to show here right now

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I use these in my web projects  mainly with MVC  I have a handful of these written for the ViewData and TempData       lt summary gt      Checks the Request QueryString for the specified value and returns it  if none      is found then the default value is returned instead      lt  summary gt  public static T QueryValue lt T gt  this HtmlHelper helper  string param  T defaultValue        object value   HttpContext Current Request QueryString param  as object      if  value    null    return defaultValue        try           return  T Convert ChangeType value  typeof T          catch  Exception            return defaultValue            That way I can write something like      lt   if  Html QueryValue  login   false       gt       lt div gt Welcome Back  lt  div gt    lt     else     gt       lt  -- Render the control or something --  gt    lt       gt

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I use these in my web projects  mainly with MVC  I have a handful of these written for the ViewData and TempData       lt summary gt      Checks the Request QueryString for the specified value and returns it  if none      is found then the default value is returned instead      lt  summary gt  public static T QueryValue lt T gt  this HtmlHelper helper  string param  T defaultValue        object value   HttpContext Current Request QueryString param  as object      if  value    null    return defaultValue        try           return  T Convert ChangeType value  typeof T          catch  Exception            return defaultValue            That way I can write something like      lt   if  Html QueryValue  login   false       gt       lt div gt Welcome Back  lt  div gt    lt     else     gt       lt  -- Render the control or something --  gt    lt       gt

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Another one  this time to make UriBuilder more friendly when dealing with query params            lt summary gt          Adds the specified query parameter to the URI builder           lt  summary gt           lt param name  builder  gt The builder  lt  param gt           lt param name  parameterName  gt Name of the parameter  lt  param gt           lt param name  value  gt The URI escaped value  lt  param gt           lt returns gt The final full query string  lt  returns gt      public static string AddQueryParam this UriBuilder builder  string parameterName  string value                if  parameterName    null              throw new ArgumentNullException  parameterName             if  parameterName Length    0              throw new ArgumentException  The parameter name is empty              if  value    null              throw new ArgumentNullException  value             if  value Length    0              throw new ArgumentException  The value is empty              if  builder Query Length    0                        builder Query   String Concat parameterName       value                     else if              builder Query Contains String Concat   amp    parameterName                       builder Query Contains String Concat      parameterName                               throw new InvalidOperationException String Format  The parameter  0  already exists    parameterName                      else                       builder Query   String Concat builder Query Substring 1     amp    parameterName       value                      return builder Query

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A pattern for parsing that avoids out parameters   public static bool TryParseInt32 this string input  Action lt int gt  action        int result      if  Int32 TryParse input  out result                 action result           return true            return false      Usage   if   textBox Text TryParseInt32 number   gt  label Text   SomeMathFunction number        label Text    Please enter a valid integer     This can be put this in the codeplex project  if so desired

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Pythonic methods for Dictionaries        lt summary gt      If a key exists in a dictionary  return its value       otherwise return the default value for that type       lt  summary gt  public static U GetWithDefault lt T  U gt  this Dictionary lt T  U gt  dict  T key        return dict GetWithDefault key  default U            lt summary gt      If a key exists in a dictionary  return its value      otherwise return the provided default value       lt  summary gt  public static U GetWithDefault lt T  U gt  this Dictionary lt T  U gt  dict  T key  U defaultValue        return dict ContainsKey key            dict key            defaultValue      Useful for when you want to append a timestamp to a filename to assure uniqueness        lt summary gt      Format a DateTime as a string that contains no characters      that are banned from filenames  such as           lt  summary gt       lt returns gt YYYY-MM-DD HH MM SS lt  returns gt  public static string ToFilenameString this DateTime dt        return dt ToString  s   Replace           Replace  T

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Are these any use   public static bool CoinToss this Random rng        return rng Next 2     0     public static T OneOf lt T gt  this Random rng  params T   things        return things rng Next things Length       Random rand  bool luckyDay   rand CoinToss    string babyName   rand OneOf  John    George    Radio XBR74 ROCKS

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Would be great to have Unix TimeStamp and ISO 8601 formatted date and times  heavily used in websites and rest services   I use it in my Facebook Library  You can find the source http   github com prabirshrestha FacebookSharp blob master src FacebookSharp Core FacebookUtils DateUtils cs  private static readonly DateTime EPOCH   DateTime SpecifyKind new DateTime 1970  1  1  0  0  0  0  DateTimeKind Utc    public static DateTime FromUnixTimestamp long timestamp        return EPOCH AddSeconds timestamp      public static long ToUnixTimestamp DateTime date        TimeSpan diff   date ToUniversalTime   - EPOCH      return  long diff TotalSeconds     public static DateTime FromIso8601FormattedDateTime string iso8601DateTime       return DateTime ParseExact iso8601DateTime   o   System Globalization CultureInfo InvariantCulture      public static string ToIso8601FormattedDateTime DateTime dateTime        return dateTime ToString  o        Feel free to use in the codeplex project

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string Format shortcut   public static class StringExtensions          Enable quick and more natural string Format calls     public static string F this string s  params object   args                return string Format s  args             Example   var s    The co-ordinate is   0    1    F point X  point Y     For quick copy-and-paste go here   Don t you find it more natural to type  some string  F  param   instead of string Format  some string    param      For a more readable name  try one of these suggestion   s    Hello  0  world  1    Fmt  Stack    Overflow    s    Hello  0  world  1    FormatBy  Stack    Overflow    s    Hello  0  world  1    FormatWith  Stack    Overflow    s    Hello  0  world  1    Display  Stack    Overflow    s    Hello  0  world  1    With  Stack    Overflow

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My conversion extensions which allow you to do   int i   myString To lt int gt       Here it is  as posted on TheSoftwareJedi com  public static T To lt T gt  this IConvertible obj      return  T Convert ChangeType obj  typeof T       public static T ToOrDefault lt T gt                this IConvertible obj        try               return To lt T gt  obj             catch               return default T            public static bool ToOrDefault lt T gt                       this IConvertible obj                       out T newObj        try               newObj   To lt T gt  obj            return true            catch               newObj   default T            return false           public static T ToOrOther lt T gt                          this IConvertible obj                         T other      try           return To lt T gt obj         catch           return other         public static bool ToOrOther lt T gt                            this IConvertible obj                           out T newObj                           T other        try               newObj   To lt T gt  obj           return true            catch               newObj   other          return false           public static T ToOrNull lt T gt                         this IConvertible obj                        where T   class       try               return To lt T gt  obj             catch               return null           public static bool ToOrNull lt T gt                     this IConvertible obj                    out T newObj                    where T   class       try               newObj   To lt T gt  obj           return true            catch               newObj   null          return false            You can ask for default  calls blank constructor or  0  for numerics  on failure  specify a  default  value  I call it  other    or ask for null  where T   class   I ve also provided both silent exception models  and a typical TryParse model that returns a bool indicating the action taken  and an out param holds the new value  So our code can do things like this  int i   myString To lt int gt     string a   myInt ToOrDefault lt string gt       note type inference DateTime d   myString ToOrOther DateTime MAX VALUE   double d    note type inference bool didItGiveDefault   myString ToOrDefault out d   string s   myDateTime ToOrNull lt string gt       I couldn t get Nullable types to roll into the whole thing very cleanly  I tried for about 20 minutes before I threw in the towel

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Equivalent to Python s Join method        lt summary gt      same as python  join       lt  summary gt       lt typeparam name  T  gt list type lt  typeparam gt       lt param name  separator  gt string separator  lt  param gt       lt param name  list  gt list of objects to be ToString d lt  param gt       lt returns gt a concatenated list interleaved with separators lt  returns gt  static public string Join lt T gt  this string separator  IEnumerable lt T gt  list        var sb   new StringBuilder        bool first   true       foreach  T v in list                if   first              sb Append separator           first   false           if  v    null              sb Append v ToString                return sb ToString

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I like these methods for dealing with enums that have the Flags attribute set   public static bool AnyOf this object mask  object flags        return   int mask  amp   int flags     0    public static bool AllOf this object mask  object flags        return   int mask  amp   int flags      int flags    public static object SetOn this object mask  object flags        return  int mask    int flags    etc    Example usage   var options   SomeOptions OptionA  options   options SetOn OptionB   options   options SetOn OptionC    if  options AnyOf SomeOptions OptionA   SomeOptions OptionB     etc    The original methods were from this article  http   www codeproject com KB cs masksandflags aspx display Print I just converted them to extension methods   The one problem with them though is that the parameters of object type  which means that all objects end up being extended with these methods  whereas ideally they should only apply to enums   Update As per the comments  you can get around the  signature pollution   at the expense of performance  like this   public static bool AnyOf this Enum mask  object flags        return  Convert ToInt642 mask   amp   int flags     0

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I use these in my Silverlight projects   public static void Show this UIElement element        element Visibility   Visibility Visible     public static void Hide this UIElement element        element Visibility   Visibility Collapsed

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Didn t check the whole thread  so it may already be here  but   public static class FluentOrderingExtensions     public class FluentOrderer lt T gt    IEnumerable lt T gt                internal List lt Comparison lt T gt  gt  Comparers   new List lt Comparison lt T gt  gt              internal IEnumerable lt T gt  Source           public FluentOrderer IEnumerable lt T gt  source                        Source   source                      region Implementation of IEnumerable          public IEnumerator lt T gt  GetEnumerator                         var workingArray   Source ToArray                Array Sort workingArray  IterativeComparison                foreach var element in workingArray  yield return element                     private int IterativeComparison T a  T b                        foreach  var comparer in Comparers                                var result   comparer a b                   if result    0  return result                            return 0                     IEnumerator IEnumerable GetEnumerator                         return GetEnumerator                        endregion            public static FluentOrderer lt T gt  OrderFluentlyBy lt T TResult gt  this IEnumerable lt T gt  source  Func lt T TResult gt  predicate           where TResult   IComparable lt TResult gt                var result   new FluentOrderer lt T gt  source           result Comparers Add  a b   gt predicate a  CompareTo predicate b             return result             public static FluentOrderer lt T gt  OrderFluentlyByDescending lt T TResult gt  this IEnumerable lt T gt  source  Func lt T TResult gt  predicate           where TResult   IComparable lt TResult gt                var result   new FluentOrderer lt T gt  source           result Comparers Add  a b   gt predicate a  CompareTo predicate b     -1           return result             public static FluentOrderer lt T gt  ThenBy lt T  TResult gt  this FluentOrderer lt T gt  source  Func lt T  TResult gt  predicate          where TResult   IComparable lt TResult gt                source Comparers Add  a  b    gt  predicate a  CompareTo predicate b             return source             public static FluentOrderer lt T gt  ThenByDescending lt T  TResult gt  this FluentOrderer lt T gt  source  Func lt T  TResult gt  predicate          where TResult   IComparable lt TResult gt                source Comparers Add  a  b    gt  predicate a  CompareTo predicate b     -1           return source            Usage   var myFluentlyOrderedList   GetABunchOfComplexObjects        OrderFluentlyBy x  gt x PropertyA       ThenByDescending x  gt x PropertyB       ThenBy x  gt x SomeMethod         ThenBy x  gt SomeOtherMethodAppliedTo x        ToList          assuming of course that all the predicates return types that are IComparable to themselves  It would work better with a stable sort like a MergeSort instead of  NET s built-in QuickSort  but it provides you with readable multi-field ordering ability similar to SQL  as close as a method chain can get  anyway   You can extend this to accomodate members that aren t IComparable  by defining overloads that take a comparison lambda instead of creating it based on a predicate   EDIT  A little explanation  since the commenter got some upticks  this set of methods improves upon the basic OrderBy   functionality by allowing you to sort based on multiple fields in descending order of importance  A real-world example would be sorting a list of invoices by customer  then by invoice number  or invoice date   Other methods of getting the data in this order either wouldn t work  OrderBy   uses an unstable sort  so it cannot be chained  or would be inefficient and not look like it does what you re trying to do

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Pythonic methods for Dictionaries        lt summary gt      If a key exists in a dictionary  return its value       otherwise return the default value for that type       lt  summary gt  public static U GetWithDefault lt T  U gt  this Dictionary lt T  U gt  dict  T key        return dict GetWithDefault key  default U            lt summary gt      If a key exists in a dictionary  return its value      otherwise return the provided default value       lt  summary gt  public static U GetWithDefault lt T  U gt  this Dictionary lt T  U gt  dict  T key  U defaultValue        return dict ContainsKey key            dict key            defaultValue      Useful for when you want to append a timestamp to a filename to assure uniqueness        lt summary gt      Format a DateTime as a string that contains no characters      that are banned from filenames  such as           lt  summary gt       lt returns gt YYYY-MM-DD HH MM SS lt  returns gt  public static string ToFilenameString this DateTime dt        return dt ToString  s   Replace           Replace  T

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The Substring method on the string class has always felt inadequate to me  Usually when you do a substring  you know the character s  from where you want to start  and the charachter s  where you want to end  Thus  I ve always felt that have to specify length as the second parameter is stupid  Therefore  I ve written my own extension methods  One that takes a startIndex and an endIndex  And one  that takes a startText  string  and endText  string  so you can just specify the text from where to start the substring  and the text for where to end it   NOTE  I couldn t name the method Substring as in  NET because my first overload takes the same parameter types as one of the  NET overloads  Therefore I named them Subsetstring  Feel free to add to the CodePlex     public static class StringExtensions            lt summary gt          Returns a Subset string starting at the specified start index and ending and the specified end         index           lt  summary gt           lt param name  s  gt The string to retrieve the subset from  lt  param gt           lt param name  startIndex  gt The specified start index for the subset  lt  param gt           lt param name  endIndex  gt The specified end index for the subset  lt  param gt           lt returns gt A Subset string starting at the specified start index and ending and the specified end         index  lt  returns gt      public static string Subsetstring this string s  int startIndex  int endIndex                if  startIndex  gt  endIndex                        throw new InvalidOperationException  End Index must be after Start Index                        if  startIndex  lt  0                        throw new InvalidOperationException  Start Index must be a positive number                        if endIndex  lt 0                        throw new InvalidOperationException  End Index must be a positive number                        return s Substring startIndex   endIndex - startIndex                    lt summary gt          Finds the specified Start Text and the End Text in this string instance  and returns a string         containing all the text starting from startText  to the begining of endText   endText is not         included            lt  summary gt           lt param name  s  gt The string to retrieve the subset from  lt  param gt           lt param name  startText  gt The Start Text to begin the Subset from  lt  param gt           lt param name  endText  gt The End Text to where the Subset goes to  lt  param gt           lt param name  ignoreCase  gt Whether or not to ignore case when comparing startText endText to the string  lt  param gt           lt returns gt A string containing all the text starting from startText  to the begining of endText  lt  returns gt      public static string Subsetstring this string s  string startText  string endText  bool ignoreCase                if  string IsNullOrEmpty startText     string IsNullOrEmpty endText                         throw new ArgumentException  Start Text and End Text cannot be empty                       string temp   s          if  ignoreCase                        temp   s ToUpperInvariant                startText   startText ToUpperInvariant                endText   endText ToUpperInvariant                      int start   temp IndexOf startText           int end   temp IndexOf endText  start           return Subsetstring s  start  end             Usage   string s    This is a tester for my cool extension method            s   s Subsetstring  tester    cool  true     Output   tester for my

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I use this extension method usually with anonymous types to get a dictionary ala ruby  public static Dictionary lt string  object gt  ToDictionary this object o        var dictionary   new Dictionary lt string  object gt          foreach  var propertyInfo in o GetType   GetProperties                  if  propertyInfo GetIndexParameters   Length    0                        dictionary Add propertyInfo Name  propertyInfo GetValue o  null                         return dictionary      You can use it   var dummy   new   color     000000   width    100    id    myid     Dictionary lt string  object gt  dict   dummy ToDictionary      And with an extended method as   public static void ForEach lt T gt  this IEnumerable lt T gt  source  Action lt T gt  action        foreach  T item in source                action item             You can do it  dummy ToDictionary   ForEach  p    gt  Console Write   0    1      p Key  p Value      Output   color   000000  width  100   id  myid

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Sometimes its handy to write out a string on a selected element in a list with a custom seperator   For instance if you have a List lt Person gt  and want to loop out lastname seperated with a comma you could do this   string result   string Empty  foreach  var person in personList       result    person LastName           result   result Substring 0  result Length - 2   return result    Or you could use this handy extension method  public static string Join lt T gt  this IEnumerable lt T gt  collection  Func lt T  string gt  func  string separator      return String Join separator  collection Select func  ToArray         And use it like this  personList Join x   gt  x LastName           Which produces the same result  in this case a list of lastnames seperated by a comma

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IEnumerable lt  gt  Shuffle  I used the Fisher-Yates the algorithm to implement a shuffle function   By using yield return and breaking the code in two functions  it achieves proper argument validation and deferred execution   thanks  Dan  for pointing this flaw in my first version   static public IEnumerable lt T gt  Shuffle lt T gt  this IEnumerable lt T gt  source        if  source    null  throw new ArgumentNullException  source         return ShuffleIterator source      static private IEnumerable lt T gt  ShuffleIterator lt T gt  this IEnumerable lt T gt  source        T   array   source ToArray        Random rnd   new Random                  for  int n   array Length  n  gt  1                 int k   rnd Next n--      0  lt   k  lt  n            Swap items         if  n    k                        T tmp   array k               array k    array n               array n    tmp                       foreach  var item in array  yield return item

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This is an extension method for the ASP Net MVC action link helper method that allows it to use the controller s authorize attributes to decide if the link should be enabled  disabled or hidden from the current user s view   I saves you from having to enclose your restricted actions in  if  clauses that check for user membership in all the views  Thanks to Maarten Balliauw for the idea and the code bits that showed me the way     using System  using System Collections Generic  using System Linq  using System Web  using System Security Principal  using System Web Routing  using System Web Mvc  using System Collections  using System Reflection  namespace System Web Mvc Html       public static class HtmlHelperExtensions                     lt summary gt              Shows or hides an action link based on the user s membership status             and the controller s authorize attributes              lt  summary gt               lt param name  linkText  gt The link text  lt  param gt               lt param name  action  gt The controller action name  lt  param gt               lt param name  controller  gt The controller name  lt  param gt               lt returns gt  lt  returns gt          public static string SecurityTrimmedActionLink              this HtmlHelper htmlHelper              string linkText              string action              string controller                        return SecurityTrimmedActionLink htmlHelper  linkText  action  controller  false  null                           lt summary gt              Enables  disables or hides an action link based on the user s membership status             and the controller s authorize attributes              lt  summary gt               lt param name  linkText  gt The link text  lt  param gt               lt param name  action  gt The action name  lt  param gt               lt param name  controller  gt The controller name  lt  param gt               lt param name  showDisabled  gt if set to  lt c gt true lt  c gt   show link as disabled -              using a span tag instead of an anchor tag    lt  param gt               lt param name  disabledAttributeText  gt Use this to add attributes to the disabled             span tag  lt  param gt               lt returns gt  lt  returns gt          public static string SecurityTrimmedActionLink              this HtmlHelper htmlHelper               string linkText               string action               string controller               bool showDisabled               string disabledAttributeText                        if  IsAccessibleToUser action  controller  HttpContext Current                                  return htmlHelper ActionLink linkText  action  controller                             else                               return showDisabled                        String Format                            lt span 1  gt  0  lt  span gt                             linkText                           disabledAttributeText  null        disabledAttributeText                                                                  private static IController GetControllerInstance string controllerName                        Assembly assembly   Assembly GetExecutingAssembly                Type controllerType   GetControllerType controllerName               return  IController Activator CreateInstance controllerType                      private static ArrayList GetControllerAttributes string controllerName  HttpContext context                        if  context Cache controllerName     ControllerAttributes      null                                var controller   GetControllerInstance controllerName                    context Cache Add                      controllerName     ControllerAttributes                       new ArrayList controller GetType   GetCustomAttributes typeof AuthorizeAttribute   true                        null                      Caching Cache NoAbsoluteExpiration                      Caching Cache NoSlidingExpiration                      Caching CacheItemPriority Default                      null                              return  ArrayList context Cache controllerName     ControllerAttributes                        private static ArrayList GetMethodAttributes string controllerName  string actionName  HttpContext context                        if  context Cache controllerName         actionName     ActionAttributes      null                                ArrayList actionAttrs   new ArrayList                    var controller   GetControllerInstance controllerName                   MethodInfo   methods   controller GetType   GetMethods                     foreach  MethodInfo method in methods                                        object   attributes   method GetCustomAttributes typeof ActionNameAttribute   true                        if   attributes Length    0  amp  amp  method Name    actionName                                                      attributes Length  gt  0  amp  amp    ActionNameAttribute attributes 0   Name    actionName                                                 actionAttrs AddRange method GetCustomAttributes typeof AuthorizeAttribute   true                                                             context Cache Add                      controllerName         actionName     ActionAttributes                       actionAttrs                      null                      Caching Cache NoAbsoluteExpiration                      Caching Cache NoSlidingExpiration                      Caching CacheItemPriority Default                      null                               return  ArrayList context Cache controllerName         actionName    ActionAttributes                        public static bool IsAccessibleToUser string actionToAuthorize  string controllerToAuthorize  HttpContext context                        IPrincipal principal   context User                 cache the attribute list for both controller class and it s methods              ArrayList controllerAttributes   GetControllerAttributes controllerToAuthorize  context                ArrayList actionAttributes   GetMethodAttributes controllerToAuthorize  actionToAuthorize  context                                        if  controllerAttributes Count    0  amp  amp  actionAttributes Count    0                  return true               string roles                   string users                   if  controllerAttributes Count  gt  0                                AuthorizeAttribute attribute   controllerAttributes 0  as AuthorizeAttribute                  roles    attribute Roles                  users    attribute Users                            if  actionAttributes Count  gt  0                                AuthorizeAttribute attribute   actionAttributes 0  as AuthorizeAttribute                  roles    attribute Roles                  users    attribute Users                             if  string IsNullOrEmpty roles   amp  amp  string IsNullOrEmpty users   amp  amp  principal Identity IsAuthenticated                  return true               string   roleArray   roles Split                   string   usersArray   users Split                   foreach  string role in roleArray                                if  role           principal IsInRole role                       return true                            foreach  string user in usersArray                                if  user         amp  amp   principal Identity Name    user                       return true                            return false                     private static Type GetControllerType string controllerName                        Assembly assembly   Assembly GetExecutingAssembly                foreach  Type type in assembly GetTypes                                  if                       type BaseType  null                       amp  amp  type BaseType Name     Controller                        amp  amp   type Name ToUpper       controllerName ToUpper      Controller  ToUpper                                            return type                                              return null

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IEnumerable lt  gt  Shuffle  I used the Fisher-Yates the algorithm to implement a shuffle function   By using yield return and breaking the code in two functions  it achieves proper argument validation and deferred execution   thanks  Dan  for pointing this flaw in my first version   static public IEnumerable lt T gt  Shuffle lt T gt  this IEnumerable lt T gt  source        if  source    null  throw new ArgumentNullException  source         return ShuffleIterator source      static private IEnumerable lt T gt  ShuffleIterator lt T gt  this IEnumerable lt T gt  source        T   array   source ToArray        Random rnd   new Random                  for  int n   array Length  n  gt  1                 int k   rnd Next n--      0  lt   k  lt  n            Swap items         if  n    k                        T tmp   array k               array k    array n               array n    tmp                       foreach  var item in array  yield return item

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HTH  These are some of my main ones   using System  using System Collections Generic  using System Globalization  using System Linq  using System Text   namespace Insert Your Namespace Here Helpers       public static class Extensions               public static bool IsNullOrEmpty lt T gt  this IEnumerable lt T gt  iEnumerable                           Cheers to Joel Mueller for the bugfix  Was  Count    now it s  Any               return iEnumerable    null                        iEnumerable Any                       public static IList lt T gt  ToListIfNotNullOrEmpty lt T gt  this IList lt T gt  iList                        return iList IsNullOrEmpty     null   iList                     public static PagedList lt T gt  ToPagedListIfNotNullOrEmpty lt T gt  this PagedList lt T gt  pagedList                        return pagedList IsNullOrEmpty     null   pagedList                     public static string ToPluralString this int value                        return value    1   string Empty    s                      public static string ToReadableTime this DateTime value                        TimeSpan span   DateTime Now Subtract value               const string plural    s                 if  span Days  gt  7                                return value ToShortDateString                               switch  span Days                                case 0                      switch  span Hours                                                case 0                              if  span Minutes    0                                                                return span Seconds  lt   0                                               now                                               string Format   0  second 1  ago                                                              span Seconds                                                             span Seconds    1   plural   string Empty                                                             return string Format   0  minute 1  ago                                                    span Minutes                                                   span Minutes    1   plural   string Empty                           default                              return string Format   0  hour 1  ago                                                    span Hours                                                   span Hours    1   plural   string Empty                                         default                      return string Format   0  day 1  ago                                            span Days                                           span Days    1   plural   string Empty                                    public static string ToShortGuidString this Guid value                        return Convert ToBase64String value ToByteArray                     Replace                            Replace       -                    Substring 0  22                      public static Guid FromShortGuidString this string value                        return new Guid Convert FromBase64String value Replace                                                                     Replace  -                                     public static string ToStringMaximumLength this string value  int maximumLength                        return ToStringMaximumLength value  maximumLength                             public static string ToStringMaximumLength this string value  int maximumLength  string postFixText                        if  string IsNullOrEmpty postFixText                                 throw new ArgumentNullException  postFixText                               return value Length  gt  maximumLength                          string Format CultureInfo InvariantCulture                                           0  1                                           value Substring 0  maximumLength - postFixText Length                                          postFixText                                                      value                     public static string SlugDecode this string value                        return value Replace                               public static string SlugEncode this string value                        return value Replace

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Inline Conversions  I like this little pattern  Completed it for Boolean  Double and DateTime  Designed to follow the C  is and as operators   public static Int32  AsInt32 this string s        Int32 value      if  Int32 TryParse s  out value           return value       return null     public static bool IsInt32 this string s        return s AsInt32   HasValue     public static Int32 ToInt32 this string s        return Int32 Parse s

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My suggestion   public static bool IsNullOrEmpty this ICollection obj      return  obj    null    obj Count    0       Works with collections and arrays   bool isNullOrEmpty   array IsNullOrEmpty     instead of  bool isNullOrEmpty   array    null    array Length    0

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Another useful one for me        lt summary gt      Converts any type in to an Int32      lt  summary gt       lt typeparam name  T  gt Any Object lt  typeparam gt       lt param name  value  gt Value to convert lt  param gt       lt returns gt The integer  0 if unsuccessful lt  returns gt  public static int ToInt32 lt T gt  this T value      int result    if  int TryParse value ToString    out result           return result        return 0          lt summary gt      Converts any type in to an Int32 but if null then returns the default      lt  summary gt       lt param name  value  gt Value to convert lt  param gt       lt typeparam name  T  gt Any Object lt  typeparam gt       lt param name  defaultValue  gt Default to use lt  param gt       lt returns gt The defaultValue if unsuccessful lt  returns gt  public static int ToInt32 lt T gt  this T value  int defaultValue      int result    if  int TryParse value ToString    out result           return result        return defaultValue      Example   int number    123  ToInt32      or   int badNumber    a  ToInt32 100      Returns 100 since a is nan

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Here s one I just created today      requires  NET 4  public static TReturn NullOr lt TIn  TReturn gt  this TIn obj  Func lt TIn  TReturn gt  func          TReturn elseValue   default TReturn   where TIn   class       return obj    null   func obj    elseValue        versions for CLR 2  which doesn t support optional params  public static TReturn NullOr lt TIn  TReturn gt  this TIn obj  Func lt TIn  TReturn gt  func          TReturn elseValue  where TIn   class       return obj    null   func obj    elseValue    public static TReturn NullOr lt TIn  TReturn gt  this TIn obj  Func lt TIn  TReturn gt  func          where TIn   class       return obj    null   func obj    default TReturn       It lets you do this   var lname   thingy NullOr t   gt  t Name  NullOr n   gt  n ToLower       which is more fluent and  IMO  easier to read than this   var lname    thingy    null   thingy Name   null     null       thingy Name ToLower     null

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Use reflection to find the TryParse method and invoke it upon string target  The optional parameter specifies what should be returned if the conversion fails  I find this method quite useful  most of the time  Well aware of the Convert ChangeType option  but I find this more useful what with the default result handy and whatnot  Note that the found methods are kept in a dictionary  although I do suspect that boxing ultimately slows this down a bit    This method is my favorite  because it legitimately uses a lot of language features   private static readonly Dictionary lt Type  MethodInfo gt  Parsers   new Dictionary lt Type  MethodInfo gt      public static T Parse lt T gt  this string value  T defaultValue   default T         if  string IsNullOrEmpty value   return defaultValue       if   Parsers ContainsKey typeof T            Parsers typeof  T     typeof  T  GetMethods BindingFlags Public   BindingFlags Static               Where mi   gt  mi Name     TryParse                Single mi   gt                                                        var parameters   mi GetParameters                                if  parameters Length    2  return false                              return parameters 0  ParameterType    typeof  string   amp  amp                                     parameters 1  ParameterType    typeof  T  MakeByRefType                                     var  params   new object    value  default T        return  bool  Parsers typeof  T   Invoke null   params             T   params 1    defaultValue      Usage   var hundredTwentyThree    123  Parse 0   var badnumber    test  Parse -1   var date    01 01 01  Parse lt DateTime gt

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An easier way to load default settings from a collection  in real life I use it to populate the settings from any source  including the command line  ClickOnce URL parameters etc     public static void LoadFrom this ApplicationSettingsBase settings  NameValueCollection configuration        if  configuration    null          foreach  string key in configuration AllKeys              if   String IsNullOrEmpty key                   try                                       settings key    configuration Get key                                     catch  SettingsPropertyNotFoundException                                         handle bad arguments as you wish                       Example   Settings Default LoadFrom new NameValueCollection        Setting1    Value1        Setting2    Value2

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Some of my best method extensions  I have a lot      public static T ToEnum lt T gt  this string str  where T   struct       return  T Enum Parse typeof T   str        DayOfWeek sunday     Sunday  ToEnum lt DayOfWeek gt      public static string ToString lt T gt  this IEnumerable lt T gt  collection  string separator        return ToString collection  t   gt  t ToString    separator      public static string ToString lt T gt  this IEnumerable lt T gt  collection  Func lt T  string gt  stringElement  string separator        StringBuilder sb   new StringBuilder        foreach  var item in collection                sb Append stringElement item            sb Append separator             return sb ToString 0  Math Max 0  sb Length - separator Length        quita el ultimo separador      new    1 2 3  ToString i  gt i 2         -- gt   2  4  6    Also  the next ones are meant to be able to continue in the same line in almost any situation  not declaring new variables and then removing state   public static R Map lt T  R gt  this T t  Func lt T  R gt  func        return func t      ExpensiveFindWally   Map wally  gt wally FirstName         wally LastName   public static R TryCC lt T  R gt  this T t  Func lt T  R gt  func      where T   class     where R   class       if  t    null  return null      return func t      public static R  TryCS lt T  R gt  this T t  Func lt T  R gt  func      where T   class     where R   struct       if  t    null  return null      return func t      public static R  TryCS lt T  R gt  this T t  Func lt T  R  gt  func      where T   class     where R   struct       if  t    null  return null      return func t      public static R TrySC lt T  R gt  this T  t  Func lt T  R gt  func      where T   struct     where R   class       if  t    null  return null      return func t Value      public static R  TrySS lt T  R gt  this T  t  Func lt T  R gt  func      where T   struct     where R   struct       if  t    null  return null      return func t Value      public static R  TrySS lt T  R gt  this T  t  Func lt T  R  gt  func      where T   struct     where R   struct       if  t    null  return null      return func t Value        int  bossNameLength    Departament Boss TryCC b  gt b Name  TryCS s  gt s Length     public static T ThrowIfNullS lt T gt  this T  t  string mensaje      where T   struct       if  t    null          throw new NullReferenceException mensaje       return t Value     public static T ThrowIfNullC lt T gt  this T t  string mensaje      where T   class       if  t    null          throw new NullReferenceException mensaje       return t     public static T Do lt T gt  this T t  Action lt T gt  action        action t       return t       Button b   new Button Content    Click   Do b  gt Canvas SetColumn b 2     public static T TryDo lt T gt  this T t  Action lt T gt  action  where T   class       if  t    null          action t       return t     public static T  TryDoS lt T gt  this T  t  Action lt T gt  action  where T   struct       if  t    null          action t Value       return t      Hope it doesn t look like coming from Mars

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Some extensions for working with lists        lt summary gt      Wrap an object in a list      lt  summary gt  public static IList lt T gt  WrapInList lt T gt  this T item        List lt T gt  result   new List lt T gt         result Add item       return result      use eg   myList   someObject InList      To make an IEnumerable that contains items from one or more sources  in order to make IEnumerable work more like lists  This may not be a good idea for high-performance code but useful for making tests   public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  enumerable  T newItem        foreach  T item in enumerable                yield return item             yield return newItem     public static IEnumerable lt T gt  Append lt T gt  this IEnumerable lt T gt  enumerable  params T   newItems        foreach  T item in enumerable                yield return item             foreach  T newItem in newItems                yield return newItem            use e g   someEnumeration   someEnumeration Append newItem     Other variations of this are possible - e g   someEnumeration   someEnumeration Append otherEnumeration     If you are cloning items  you may also want to clone lists of them   public static IList lt T gt  Clone lt T gt  this IEnumerable lt T gt  source  where T  ICloneable       List lt T gt  result   new List lt T gt          foreach  T item in source                result Add  T item Clone                return result      When I am working with ObservableCollection lt T gt   I generally extend it with an AddRange method  Other answers here give implementations of this   You may put this code in the Codeplex project if you want

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Whitespace normalization is rather useful  especially when dealing with user input   namespace Extensions String       using System Text RegularExpressions       public static class Extensions                    lt summary gt              Normalizes whitespace in a string              Leading Trailing whitespace is eliminated and             all sequences of internal whitespace are reduced to             a single SP  ASCII 0x20  character               lt  summary gt               lt param name  s  gt The string whose whitespace is to be normalized lt  param gt               lt returns gt a normalized string lt  returns gt          public static string NormalizeWS  this string  this                         string src           this                     string normalized   rxWS Replace  src   m   gt                     bool isLeadingTrailingWS     m Index    0    m Index m Length    src Length   true   false                       string p                     isLeadingTrailingWS                                  return p                                     return normalized                      private static Regex rxWS   new Regex     s

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Whitespace normalization is rather useful  especially when dealing with user input   namespace Extensions String       using System Text RegularExpressions       public static class Extensions                    lt summary gt              Normalizes whitespace in a string              Leading Trailing whitespace is eliminated and             all sequences of internal whitespace are reduced to             a single SP  ASCII 0x20  character               lt  summary gt               lt param name  s  gt The string whose whitespace is to be normalized lt  param gt               lt returns gt a normalized string lt  returns gt          public static string NormalizeWS  this string  this                         string src           this                     string normalized   rxWS Replace  src   m   gt                     bool isLeadingTrailingWS     m Index    0    m Index m Length    src Length   true   false                       string p                     isLeadingTrailingWS                                  return p                                     return normalized                      private static Regex rxWS   new Regex     s

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This one can be quite useful        public static IEnumerable lt TResult gt  Zip lt TFirst  TSecond  TResult gt  this IEnumerable lt TFirst gt  first  IEnumerable lt TSecond gt  second  Func lt TFirst  TSecond  TResult gt  selector                if  first    null              throw new ArgumentNullException  first            if  second    null              throw new ArgumentNullException  second            if  selector    null              throw new ArgumentNullException  selector             using  var enum1   first GetEnumerator            using  var enum2   second GetEnumerator                          while  enum1 MoveNext    amp  amp  enum2 MoveNext                                  yield return selector enum1 Current  enum2 Current                                   It has been added to the Enumerable class in  NET 4 0  but it s handy to have it in 3 5   Example    var names   new      Joe    Jane    Jack    John     var ages   new     42  22  18  33     var persons   names Zip ages   n  a    gt  new   Name   n  Age   a      foreach  var p in persons        Console WriteLine   0  is  1  years old   p Name  p Age

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Takes a camelCaseWord or PascalCaseWord and  wordifies  it  ie camelCaseWord    camel Case Word  public static string Wordify  this string camelCaseWord            if the word is all upper  just return it     if   Regex IsMatch  camelCaseWord    a-z               return camelCaseWord       return string Join       Regex Split  camelCaseWord       lt        A-Z             I often use it in conjuction with Capitalize  public static string Capitalize  this string word         return word 0  ToString    ToUpper      word Substring  1        Example usage  SomeEntityObject entity   DataAccessObject GetSomeEntityObject  id    List lt PropertyInfo gt  properties   entity GetType   GetPublicNonCollectionProperties         wordify the property names to act as column headers for an html table or something List lt string gt  columns   properties Select  p   gt  p Name Capitalize    Wordify      ToList       Free to use in codeplex project

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Here s one I just created today      requires  NET 4  public static TReturn NullOr lt TIn  TReturn gt  this TIn obj  Func lt TIn  TReturn gt  func          TReturn elseValue   default TReturn   where TIn   class       return obj    null   func obj    elseValue        versions for CLR 2  which doesn t support optional params  public static TReturn NullOr lt TIn  TReturn gt  this TIn obj  Func lt TIn  TReturn gt  func          TReturn elseValue  where TIn   class       return obj    null   func obj    elseValue    public static TReturn NullOr lt TIn  TReturn gt  this TIn obj  Func lt TIn  TReturn gt  func          where TIn   class       return obj    null   func obj    default TReturn       It lets you do this   var lname   thingy NullOr t   gt  t Name  NullOr n   gt  n ToLower       which is more fluent and  IMO  easier to read than this   var lname    thingy    null   thingy Name   null     null       thingy Name ToLower     null

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public static class StringHelper       public static String F this String str  params object   args                return String Format str  args             Using like    Say  0   F  Hello

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I m always using format that wants a new line with StringBuilder so the very simple extension below saves a few lines of code   public static class Extensions       public static void AppendLine this StringBuilder builder string format  params object   args                builder AppendLine string Format format  args              The alternative is AppendFormat in StringBuilder with a  n or Environment NewLine

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public static class ComparableExtensions     public static bool Between lt T gt  this T actual  T lower  T upper  where T   IComparable lt T gt          return actual CompareTo lower   gt   0  amp  amp  actual CompareTo upper   lt  0          Example   if  myNumber Between 3 7

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I ve been looking for a way to contribute back to the community some of the things I ve developed     Here s some FileInfo extensions that I find quite useful        lt summary gt      Open with default  open  program      lt  summary gt       lt param name  value  gt  lt  param gt  public static Process Open this FileInfo value        if   value Exists          throw new FileNotFoundException  File doesn t exist        Process p   new Process        p StartInfo FileName   value FullName      p StartInfo Verb    Open       p Start        return p          lt summary gt      Print the file      lt  summary gt       lt param name  value  gt  lt  param gt  public static void Print this FileInfo value        if   value Exists          throw new FileNotFoundException  File doesn t exist        Process p   new Process        p StartInfo FileName   value FullName      p StartInfo Verb    Print       p Start            lt summary gt      Send this file to the Recycle Bin      lt  summary gt       lt exception cref  File doesn t exist    gt       lt param name  value  gt  lt  param gt  public static void Recycle this FileInfo value                value Recycle false           lt summary gt      Send this file to the Recycle Bin     On show  if person refuses to send file to the recycle bin       exception is thrown or otherwise delete fails      lt  summary gt       lt exception cref  File doesn t exist    gt       lt exception cref  On show  if user refuses  throws exception  The operation was canceled      gt       lt param name  value  gt File being recycled lt  param gt       lt param name  showDialog  gt true to show pop-up lt  param gt  public static void Recycle this FileInfo value  bool showDialog        if   value Exists              throw new FileNotFoundException  File doesn t exist        if  showDialog           FileSystem DeleteFile              value FullName  UIOption AllDialogs               RecycleOption SendToRecycleBin       else         FileSystem DeleteFile              value FullName  UIOption OnlyErrorDialogs               RecycleOption SendToRecycleBin       Open any file in the user s favorite editor   new FileInfo  C  image jpg   Open      Print any file that the operating system knows how to print   new FileInfo  C  image jpg   Print      Send any file to the recycle bin    You have to include the Microsoft VisualBasic reference use the using Microsoft VisualBasic FileIO    Example   new FileInfo  C  image jpg   Recycle      Or     let user have a chance to cancel send to recycle bin  new FileInfo  C  image jpg   Recycle true

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Similar to the string As and Is above  but global to all objects   It s quite simple  but I use these a lot to alleviate parens explosion with boxing   public static class ExtensionMethods Object        DebuggerStepThrough        public static bool Is lt T gt  this object item  where T   class               return item is T              DebuggerStepThrough        public static bool IsNot lt T gt  this object item  where T   class               return   item Is lt T gt                  DebuggerStepThrough        public static T As lt T gt  this object item  where T   class               return item as T            I am happy for this code to be used at codeplex  indeed it already is

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Use reflection to find the TryParse method and invoke it upon string target  The optional parameter specifies what should be returned if the conversion fails  I find this method quite useful  most of the time  Well aware of the Convert ChangeType option  but I find this more useful what with the default result handy and whatnot  Note that the found methods are kept in a dictionary  although I do suspect that boxing ultimately slows this down a bit    This method is my favorite  because it legitimately uses a lot of language features   private static readonly Dictionary lt Type  MethodInfo gt  Parsers   new Dictionary lt Type  MethodInfo gt      public static T Parse lt T gt  this string value  T defaultValue   default T         if  string IsNullOrEmpty value   return defaultValue       if   Parsers ContainsKey typeof T            Parsers typeof  T     typeof  T  GetMethods BindingFlags Public   BindingFlags Static               Where mi   gt  mi Name     TryParse                Single mi   gt                                                        var parameters   mi GetParameters                                if  parameters Length    2  return false                              return parameters 0  ParameterType    typeof  string   amp  amp                                     parameters 1  ParameterType    typeof  T  MakeByRefType                                     var  params   new object    value  default T        return  bool  Parsers typeof  T   Invoke null   params             T   params 1    defaultValue      Usage   var hundredTwentyThree    123  Parse 0   var badnumber    test  Parse -1   var date    01 01 01  Parse lt DateTime gt

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HTH  These are some of my main ones   using System  using System Collections Generic  using System Globalization  using System Linq  using System Text   namespace Insert Your Namespace Here Helpers       public static class Extensions               public static bool IsNullOrEmpty lt T gt  this IEnumerable lt T gt  iEnumerable                           Cheers to Joel Mueller for the bugfix  Was  Count    now it s  Any               return iEnumerable    null                        iEnumerable Any                       public static IList lt T gt  ToListIfNotNullOrEmpty lt T gt  this IList lt T gt  iList                        return iList IsNullOrEmpty     null   iList                     public static PagedList lt T gt  ToPagedListIfNotNullOrEmpty lt T gt  this PagedList lt T gt  pagedList                        return pagedList IsNullOrEmpty     null   pagedList                     public static string ToPluralString this int value                        return value    1   string Empty    s                      public static string ToReadableTime this DateTime value                        TimeSpan span   DateTime Now Subtract value               const string plural    s                 if  span Days  gt  7                                return value ToShortDateString                               switch  span Days                                case 0                      switch  span Hours                                                case 0                              if  span Minutes    0                                                                return span Seconds  lt   0                                               now                                               string Format   0  second 1  ago                                                              span Seconds                                                             span Seconds    1   plural   string Empty                                                             return string Format   0  minute 1  ago                                                    span Minutes                                                   span Minutes    1   plural   string Empty                           default                              return string Format   0  hour 1  ago                                                    span Hours                                                   span Hours    1   plural   string Empty                                         default                      return string Format   0  day 1  ago                                            span Days                                           span Days    1   plural   string Empty                                    public static string ToShortGuidString this Guid value                        return Convert ToBase64String value ToByteArray                     Replace                            Replace       -                    Substring 0  22                      public static Guid FromShortGuidString this string value                        return new Guid Convert FromBase64String value Replace                                                                     Replace  -                                     public static string ToStringMaximumLength this string value  int maximumLength                        return ToStringMaximumLength value  maximumLength                             public static string ToStringMaximumLength this string value  int maximumLength  string postFixText                        if  string IsNullOrEmpty postFixText                                 throw new ArgumentNullException  postFixText                               return value Length  gt  maximumLength                          string Format CultureInfo InvariantCulture                                           0  1                                           value Substring 0  maximumLength - postFixText Length                                          postFixText                                                      value                     public static string SlugDecode this string value                        return value Replace                               public static string SlugEncode this string value                        return value Replace

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Takes a camelCaseWord or PascalCaseWord and  wordifies  it  ie camelCaseWord    camel Case Word  public static string Wordify  this string camelCaseWord            if the word is all upper  just return it     if   Regex IsMatch  camelCaseWord    a-z               return camelCaseWord       return string Join       Regex Split  camelCaseWord       lt        A-Z             I often use it in conjuction with Capitalize  public static string Capitalize  this string word         return word 0  ToString    ToUpper      word Substring  1        Example usage  SomeEntityObject entity   DataAccessObject GetSomeEntityObject  id    List lt PropertyInfo gt  properties   entity GetType   GetPublicNonCollectionProperties         wordify the property names to act as column headers for an html table or something List lt string gt  columns   properties Select  p   gt  p Name Capitalize    Wordify      ToList       Free to use in codeplex project

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Here is another ThrowIfNull implementation    ThreadStatic  private static string lastMethodName   null    ThreadStatic  private static int lastParamIndex   0    MethodImpl MethodImplOptions NoInlining   public static void ThrowIfNull lt T gt  this T parameter        var currentStackFrame   new StackFrame 1       var props   currentStackFrame GetMethod   GetParameters         if   String IsNullOrEmpty lastMethodName             if  currentStackFrame GetMethod   Name    lastMethodName                lastParamIndex   0            else if  lastParamIndex  gt   props Length - 1                lastParamIndex   0            else               lastParamIndex                    else           lastParamIndex   0             if   typeof T  IsValueType            for  int i   lastParamIndex  i  amp lt  props Length  i                  if  props i  ParameterType IsValueType                    lastParamIndex                  else                   break                                     if  parameter    null            string paramName   props lastParamIndex  Name          throw new ArgumentNullException paramName              lastMethodName   currentStackFrame GetMethod   Name      It s not as efficient as the other impementations  but has cleaner usage   public void Foo         Bar 1  2   Hello    World      no exception     Bar 1  2   Hello   null     exception     Bar 1  2  null   World      exception    public void Bar int x  int y  string someString1  string someString2          will also work with comments removed       x ThrowIfNull          y ThrowIfNull        someString1 ThrowIfNull        someString2 ThrowIfNull           Do something incredibly useful here      Changing the parameters to int  will also work   -bill

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Binary search    public static T BinarySearch lt T  TKey gt  this IList lt T gt  list  Func lt T  TKey gt  keySelector  TKey key          where TKey   IComparable lt TKey gt        int min   0      int max   list Count      int index   0      while  min  lt  max                int mid    max   min    2          T midItem   list mid           TKey midKey   keySelector midItem           int comp   midKey CompareTo key           if  comp  lt  0                        min   mid   1                    else if  comp  gt  0                        max   mid - 1                    else                       return midItem                      if  min    max  amp  amp          keySelector list min   CompareTo key     0                return list min             throw new InvalidOperationException  Item not found        Usage  assuming that the list is sorted by Id     var item   list BinarySearch i   gt  i Id  42     The fact that it throws an InvalidOperationException may seem strange  but that s what Enumerable First does when there s no matching item

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Easily serialize objects into XML   public static string ToXml lt T gt  this T obj  where T   class       XmlSerializer s   new XmlSerializer obj GetType         using  StringWriter writer   new StringWriter                  s Serialize writer  obj           return writer ToString               lt root gt  lt child gt foo lt  child lt  root gt   ToXml lt MyCustomType gt

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The Substring method on the string class has always felt inadequate to me  Usually when you do a substring  you know the character s  from where you want to start  and the charachter s  where you want to end  Thus  I ve always felt that have to specify length as the second parameter is stupid  Therefore  I ve written my own extension methods  One that takes a startIndex and an endIndex  And one  that takes a startText  string  and endText  string  so you can just specify the text from where to start the substring  and the text for where to end it   NOTE  I couldn t name the method Substring as in  NET because my first overload takes the same parameter types as one of the  NET overloads  Therefore I named them Subsetstring  Feel free to add to the CodePlex     public static class StringExtensions            lt summary gt          Returns a Subset string starting at the specified start index and ending and the specified end         index           lt  summary gt           lt param name  s  gt The string to retrieve the subset from  lt  param gt           lt param name  startIndex  gt The specified start index for the subset  lt  param gt           lt param name  endIndex  gt The specified end index for the subset  lt  param gt           lt returns gt A Subset string starting at the specified start index and ending and the specified end         index  lt  returns gt      public static string Subsetstring this string s  int startIndex  int endIndex                if  startIndex  gt  endIndex                        throw new InvalidOperationException  End Index must be after Start Index                        if  startIndex  lt  0                        throw new InvalidOperationException  Start Index must be a positive number                        if endIndex  lt 0                        throw new InvalidOperationException  End Index must be a positive number                        return s Substring startIndex   endIndex - startIndex                    lt summary gt          Finds the specified Start Text and the End Text in this string instance  and returns a string         containing all the text starting from startText  to the begining of endText   endText is not         included            lt  summary gt           lt param name  s  gt The string to retrieve the subset from  lt  param gt           lt param name  startText  gt The Start Text to begin the Subset from  lt  param gt           lt param name  endText  gt The End Text to where the Subset goes to  lt  param gt           lt param name  ignoreCase  gt Whether or not to ignore case when comparing startText endText to the string  lt  param gt           lt returns gt A string containing all the text starting from startText  to the begining of endText  lt  returns gt      public static string Subsetstring this string s  string startText  string endText  bool ignoreCase                if  string IsNullOrEmpty startText     string IsNullOrEmpty endText                         throw new ArgumentException  Start Text and End Text cannot be empty                       string temp   s          if  ignoreCase                        temp   s ToUpperInvariant                startText   startText ToUpperInvariant                endText   endText ToUpperInvariant                      int start   temp IndexOf startText           int end   temp IndexOf endText  start           return Subsetstring s  start  end             Usage   string s    This is a tester for my cool extension method            s   s Subsetstring  tester    cool  true     Output   tester for my

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Wraps a string every n chars   public static string WrapAt this string str  int WrapPos        if  string IsNullOrEmpty str           throw new ArgumentNullException  str    Cannot wrap a null string        str   str Replace   r       Replace   n             if  str Length  lt   WrapPos          return str       for  int i   str Length  i  gt   0  i--          if  i   WrapPos    0  amp  amp  i  gt  0  amp  amp  i    str Length              str   str Insert i    r n        return str

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public static class EnumerableExtensions        Pure      public static U MapReduce lt T  U gt  this IEnumerable lt T gt  enumerable  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways enumerable    null           CodeContract RequiresAlways enumerable Skip 1  Any             CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null           return enumerable AsParallel   Select map  Aggregate reduce              Pure      public static U MapReduce lt T  U gt  this IList lt T gt  list  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways list    null           CodeContract RequiresAlways list Count  gt   2           CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null           U result   map list 0            for  int i   1  i  lt  list Count  i                          result   reduce result map list i                       return result               Parallel version  creates garbage      Pure      public static U MapReduce lt T  U gt  this IList lt T gt  list  Func lt T  U gt  map  Func lt U  U  U gt  reduce                CodeContract RequiresAlways list    null           CodeContract RequiresAlways list Skip 1  Any             CodeContract RequiresAlways map    null           CodeContract RequiresAlways reduce    null            U   mapped   new U list Count           Parallel For 0  mapped Length  i   gt                                mapped i    map list i                            U result   mapped 0           for  int i   1  i  lt  list Count  i                          result   reduce result  mapped i                      return result

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I actually just blogged this today  It s a strongly typed reactive wrapper around a INotifyPropertyChanged property   GetPropertyValues  returns an IObservable lt T gt  of the values as they change  starting with the current value  If ignore the current value  you can just call Skip 1  on the result   Usage is like so   IObservable lt int gt  values   viewModel GetPropertyValues x   gt  x IntProperty     Implementation   public static class NotifyPropertyChangeReactiveExtensions          Returns the values of property  an Expression  as they change          starting with the current value     public static IObservable lt TValue gt  GetPropertyValues lt TSource  TValue gt           this TSource source  Expression lt Func lt TSource  TValue gt  gt  property          where TSource   INotifyPropertyChanged               MemberExpression memberExpression   property Body as MemberExpression           if  memberExpression    null                        throw new ArgumentException                   property must directly access a property of the source                       string propertyName   memberExpression Member Name           Func lt TSource  TValue gt  accessor   property Compile             return source GetPropertyChangedEvents                Where x   gt  x EventArgs PropertyName    propertyName               Select x   gt  accessor source                StartWith accessor source                  This is a wrapper around FromEvent PropertyChanged      public static IObservable lt IEvent lt PropertyChangedEventArgs gt  gt          GetPropertyChangedEvents this INotifyPropertyChanged source                return Observable FromEvent lt PropertyChangedEventHandler               PropertyChangedEventArgs gt               h   gt  new PropertyChangedEventHandler h               h   gt  source PropertyChanged    h              h   gt  source PropertyChanged -  h

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Converts a List to a datatable  public static class DataTableConverter            lt summary gt          Convert a List T  to a DataTable           lt  summary gt      public static DataTable ToDataTable lt T gt  this IList lt T gt  items                var tb   new DataTable typeof T  Name            PropertyInfo   props   typeof T  GetProperties BindingFlags Public   BindingFlags Instance            foreach  PropertyInfo prop in props                        Type t   GetCoreType prop PropertyType               tb Columns Add prop Name  t                      foreach  T item in items                        var values   new object props Length                for  int i   0  i  lt  props Length  i                                  values i    props i  GetValue item  null                              tb Rows Add values                      return tb                  lt summary gt          Determine of specified type is nullable          lt  summary gt      public static bool IsNullable Type t                return  t IsValueType     t IsGenericType  amp  amp  t GetGenericTypeDefinition      typeof Nullable lt  gt                     lt summary gt          Return underlying type if type is Nullable otherwise return the type          lt  summary gt      public static Type GetCoreType Type t                if  t    null  amp  amp  IsNullable t                         if   t IsValueType                                return t                            else                               return Nullable GetUnderlyingType t                                   else                       return t                      Usage       IList lt MyClass gt  myClassList   new List lt MyClass gt         DataTable myClassDataTable   myClassList ToDataTable

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gitorious org cadenza is a full library of some of the most useful extension methods I ve seen

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Timespan-related extensions like   public static TimeSpan Seconds this int seconds      return TimeSpan FromSeconds seconds      public static TimeSpan Minutes this int minutes      return TimeSpan FromMinutes minutes       That allow to use   1 Seconds   20 Minutes     Lock extensions like   public static IDisposable GetReadLock this ReaderWriterLockSlim slimLock      slimLock EnterReadLock      return new DisposableAction slimLock ExitReadLock      public static IDisposable GetWriteLock this ReaderWriterLockSlim slimLock      slimLock EnterWriteLock      return new DisposableAction slimLock ExitWriteLock      public static IDisposable GetUpgradeableReadLock this ReaderWriterLockSlim slimLock      slimLock EnterUpgradeableReadLock      return new DisposableAction slimLock ExitUpgradeableReadLock       That allow to use locks like   using  lock GetUpgradeableReadLock           try read   using  lock GetWriteLock              do write         And many other from the Lokad Shared Libraries

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I use these in my web projects  mainly with MVC  I have a handful of these written for the ViewData and TempData       lt summary gt      Checks the Request QueryString for the specified value and returns it  if none      is found then the default value is returned instead      lt  summary gt  public static T QueryValue lt T gt  this HtmlHelper helper  string param  T defaultValue        object value   HttpContext Current Request QueryString param  as object      if  value    null    return defaultValue        try           return  T Convert ChangeType value  typeof T          catch  Exception            return defaultValue            That way I can write something like      lt   if  Html QueryValue  login   false       gt       lt div gt Welcome Back  lt  div gt    lt     else     gt       lt  -- Render the control or something --  gt    lt       gt

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Here are a couple of methods that I use to make extracting single attributes a little less painful   public static T GetAttribute lt T gt  this ICustomAttributeProvider provider  bool inherit   false  int index   0  where T   Attribute       return provider GetAttribute typeof T   inherit  index  as T     public static Attribute GetAttribute this ICustomAttributeProvider provider  Type type  bool inherit   false  int index   0        bool exists   provider IsDefined type  inherit       if   exists                return null             object   attributes   provider GetCustomAttributes type  inherit       if  attributes    null  amp  amp  attributes Length    0                return attributes index  as Attribute            else               return null            Usage  implementation of this enum description hack    public static string GetDescription this Enum value        var fieldInfo   value GetType   GetField value ToString         var attribute   fieldInfo GetAttribute lt DescriptionAttribute gt         return attribute    null   attribute Description   null      Feel free to include this in the CodePlex project

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This is an extension method for the ASP Net MVC action link helper method that allows it to use the controller s authorize attributes to decide if the link should be enabled  disabled or hidden from the current user s view   I saves you from having to enclose your restricted actions in  if  clauses that check for user membership in all the views  Thanks to Maarten Balliauw for the idea and the code bits that showed me the way     using System  using System Collections Generic  using System Linq  using System Web  using System Security Principal  using System Web Routing  using System Web Mvc  using System Collections  using System Reflection  namespace System Web Mvc Html       public static class HtmlHelperExtensions                     lt summary gt              Shows or hides an action link based on the user s membership status             and the controller s authorize attributes              lt  summary gt               lt param name  linkText  gt The link text  lt  param gt               lt param name  action  gt The controller action name  lt  param gt               lt param name  controller  gt The controller name  lt  param gt               lt returns gt  lt  returns gt          public static string SecurityTrimmedActionLink              this HtmlHelper htmlHelper              string linkText              string action              string controller                        return SecurityTrimmedActionLink htmlHelper  linkText  action  controller  false  null                           lt summary gt              Enables  disables or hides an action link based on the user s membership status             and the controller s authorize attributes              lt  summary gt               lt param name  linkText  gt The link text  lt  param gt               lt param name  action  gt The action name  lt  param gt               lt param name  controller  gt The controller name  lt  param gt               lt param name  showDisabled  gt if set to  lt c gt true lt  c gt   show link as disabled -              using a span tag instead of an anchor tag    lt  param gt               lt param name  disabledAttributeText  gt Use this to add attributes to the disabled             span tag  lt  param gt               lt returns gt  lt  returns gt          public static string SecurityTrimmedActionLink              this HtmlHelper htmlHelper               string linkText               string action               string controller               bool showDisabled               string disabledAttributeText                        if  IsAccessibleToUser action  controller  HttpContext Current                                  return htmlHelper ActionLink linkText  action  controller                             else                               return showDisabled                        String Format                            lt span 1  gt  0  lt  span gt                             linkText                           disabledAttributeText  null        disabledAttributeText                                                                  private static IController GetControllerInstance string controllerName                        Assembly assembly   Assembly GetExecutingAssembly                Type controllerType   GetControllerType controllerName               return  IController Activator CreateInstance controllerType                      private static ArrayList GetControllerAttributes string controllerName  HttpContext context                        if  context Cache controllerName     ControllerAttributes      null                                var controller   GetControllerInstance controllerName                    context Cache Add                      controllerName     ControllerAttributes                       new ArrayList controller GetType   GetCustomAttributes typeof AuthorizeAttribute   true                        null                      Caching Cache NoAbsoluteExpiration                      Caching Cache NoSlidingExpiration                      Caching CacheItemPriority Default                      null                              return  ArrayList context Cache controllerName     ControllerAttributes                        private static ArrayList GetMethodAttributes string controllerName  string actionName  HttpContext context                        if  context Cache controllerName         actionName     ActionAttributes      null                                ArrayList actionAttrs   new ArrayList                    var controller   GetControllerInstance controllerName                   MethodInfo   methods   controller GetType   GetMethods                     foreach  MethodInfo method in methods                                        object   attributes   method GetCustomAttributes typeof ActionNameAttribute   true                        if   attributes Length    0  amp  amp  method Name    actionName                                                      attributes Length  gt  0  amp  amp    ActionNameAttribute attributes 0   Name    actionName                                                 actionAttrs AddRange method GetCustomAttributes typeof AuthorizeAttribute   true                                                             context Cache Add                      controllerName         actionName     ActionAttributes                       actionAttrs                      null                      Caching Cache NoAbsoluteExpiration                      Caching Cache NoSlidingExpiration                      Caching CacheItemPriority Default                      null                               return  ArrayList context Cache controllerName         actionName    ActionAttributes                        public static bool IsAccessibleToUser string actionToAuthorize  string controllerToAuthorize  HttpContext context                        IPrincipal principal   context User                 cache the attribute list for both controller class and it s methods              ArrayList controllerAttributes   GetControllerAttributes controllerToAuthorize  context                ArrayList actionAttributes   GetMethodAttributes controllerToAuthorize  actionToAuthorize  context                                        if  controllerAttributes Count    0  amp  amp  actionAttributes Count    0                  return true               string roles                   string users                   if  controllerAttributes Count  gt  0                                AuthorizeAttribute attribute   controllerAttributes 0  as AuthorizeAttribute                  roles    attribute Roles                  users    attribute Users                            if  actionAttributes Count  gt  0                                AuthorizeAttribute attribute   actionAttributes 0  as AuthorizeAttribute                  roles    attribute Roles                  users    attribute Users                             if  string IsNullOrEmpty roles   amp  amp  string IsNullOrEmpty users   amp  amp  principal Identity IsAuthenticated                  return true               string   roleArray   roles Split                   string   usersArray   users Split                   foreach  string role in roleArray                                if  role           principal IsInRole role                       return true                            foreach  string user in usersArray                                if  user         amp  amp   principal Identity Name    user                       return true                            return false                     private static Type GetControllerType string controllerName                        Assembly assembly   Assembly GetExecutingAssembly                foreach  Type type in assembly GetTypes                                  if                       type BaseType  null                       amp  amp  type BaseType Name     Controller                        amp  amp   type Name ToUpper       controllerName ToUpper      Controller  ToUpper                                            return type                                              return null

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Convert any string to type Int32     Calls the underlying int TryParse method to convert a string    representation of a number to its 32-bit signed integer equivalent     Returns Zero if conversion fails   public static int ToInt32 this string s        int retInt      int TryParse s  out retInt       return retInt      SAMPLE USE  string s    999   int i   s ToInt32

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I find this one pretty useful   public static class PaulaBean       private static String paula    Brillant       public static String GetPaula lt T gt  this T obj            return paula            You may use it on CodePlex

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Checks for an empty collection  and sends the value set in the default constructor for the desired field public static TResult MinGuarded lt T  TResult gt  this IEnumerable lt T gt  items  Func lt T  TResult gt  expression  where T   new         if items IsEmpty              return  new List lt T gt    new T      Min expression             return items Min expression         Checks for an empty collection  and sends the value set in the default constructor for the desired field public static TResult MaxGuarded lt T  TResult gt  this IEnumerable lt T gt  items  Func lt T  TResult gt  expression  where T   new         if items IsEmpty              return  new List lt T gt    new T      Max expression             return items Max expression       I am not sure if there is a better way to do this  but this extension is very helpful whenever I want to have control over the default values of fields in my object  For instance  if I want to control the value of a DateTime and want to be set as per my business logic  then I can do so in the default constructor  Otherwise  it comes out to be DateTime MinDate

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Timespan-related extensions like   public static TimeSpan Seconds this int seconds      return TimeSpan FromSeconds seconds      public static TimeSpan Minutes this int minutes      return TimeSpan FromMinutes minutes       That allow to use   1 Seconds   20 Minutes     Lock extensions like   public static IDisposable GetReadLock this ReaderWriterLockSlim slimLock      slimLock EnterReadLock      return new DisposableAction slimLock ExitReadLock      public static IDisposable GetWriteLock this ReaderWriterLockSlim slimLock      slimLock EnterWriteLock      return new DisposableAction slimLock ExitWriteLock      public static IDisposable GetUpgradeableReadLock this ReaderWriterLockSlim slimLock      slimLock EnterUpgradeableReadLock      return new DisposableAction slimLock ExitUpgradeableReadLock       That allow to use locks like   using  lock GetUpgradeableReadLock           try read   using  lock GetWriteLock              do write         And many other from the Lokad Shared Libraries

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I use this one all the time   public static void DelimitedAppend this StringBuilder sb  string value  string delimiter        if  sb Length  gt  0          sb Append delimiter       sb Append value       This just ensures that the delimiter is not inserted when the string is empty  For example  to create a comma-seperated list of words   var farmAnimals   new     new   Species    Dog   IsTasty   false    new   Species    Cat   IsTasty   false    new   Species    Chicken   IsTasty   true       var soupIngredients   new StringBuilder    foreach  var edible in farmAnimals Where farmAnimal   gt  farmAnimal IsTasty       soupIngredients DelimitedAppend edible Species

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Some of my best method extensions  I have a lot      public static T ToEnum lt T gt  this string str  where T   struct       return  T Enum Parse typeof T   str        DayOfWeek sunday     Sunday  ToEnum lt DayOfWeek gt      public static string ToString lt T gt  this IEnumerable lt T gt  collection  string separator        return ToString collection  t   gt  t ToString    separator      public static string ToString lt T gt  this IEnumerable lt T gt  collection  Func lt T  string gt  stringElement  string separator        StringBuilder sb   new StringBuilder        foreach  var item in collection                sb Append stringElement item            sb Append separator             return sb ToString 0  Math Max 0  sb Length - separator Length        quita el ultimo separador      new    1 2 3  ToString i  gt i 2         -- gt   2  4  6    Also  the next ones are meant to be able to continue in the same line in almost any situation  not declaring new variables and then removing state   public static R Map lt T  R gt  this T t  Func lt T  R gt  func        return func t      ExpensiveFindWally   Map wally  gt wally FirstName         wally LastName   public static R TryCC lt T  R gt  this T t  Func lt T  R gt  func      where T   class     where R   class       if  t    null  return null      return func t      public static R  TryCS lt T  R gt  this T t  Func lt T  R gt  func      where T   class     where R   struct       if  t    null  return null      return func t      public static R  TryCS lt T  R gt  this T t  Func lt T  R  gt  func      where T   class     where R   struct       if  t    null  return null      return func t      public static R TrySC lt T  R gt  this T  t  Func lt T  R gt  func      where T   struct     where R   class       if  t    null  return null      return func t Value      public static R  TrySS lt T  R gt  this T  t  Func lt T  R gt  func      where T   struct     where R   struct       if  t    null  return null      return func t Value      public static R  TrySS lt T  R gt  this T  t  Func lt T  R  gt  func      where T   struct     where R   struct       if  t    null  return null      return func t Value        int  bossNameLength    Departament Boss TryCC b  gt b Name  TryCS s  gt s Length     public static T ThrowIfNullS lt T gt  this T  t  string mensaje      where T   struct       if  t    null          throw new NullReferenceException mensaje       return t Value     public static T ThrowIfNullC lt T gt  this T t  string mensaje      where T   class       if  t    null          throw new NullReferenceException mensaje       return t     public static T Do lt T gt  this T t  Action lt T gt  action        action t       return t       Button b   new Button Content    Click   Do b  gt Canvas SetColumn b 2     public static T TryDo lt T gt  this T t  Action lt T gt  action  where T   class       if  t    null          action t       return t     public static T  TryDoS lt T gt  this T  t  Action lt T gt  action  where T   struct       if  t    null          action t Value       return t      Hope it doesn t look like coming from Mars

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http   www codeplex com linqext

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An easier way to load default settings from a collection  in real life I use it to populate the settings from any source  including the command line  ClickOnce URL parameters etc     public static void LoadFrom this ApplicationSettingsBase settings  NameValueCollection configuration        if  configuration    null          foreach  string key in configuration AllKeys              if   String IsNullOrEmpty key                   try                                       settings key    configuration Get key                                     catch  SettingsPropertyNotFoundException                                         handle bad arguments as you wish                       Example   Settings Default LoadFrom new NameValueCollection        Setting1    Value1        Setting2    Value2

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Another useful one for me        lt summary gt      Converts any type in to an Int32      lt  summary gt       lt typeparam name  T  gt Any Object lt  typeparam gt       lt param name  value  gt Value to convert lt  param gt       lt returns gt The integer  0 if unsuccessful lt  returns gt  public static int ToInt32 lt T gt  this T value      int result    if  int TryParse value ToString    out result           return result        return 0          lt summary gt      Converts any type in to an Int32 but if null then returns the default      lt  summary gt       lt param name  value  gt Value to convert lt  param gt       lt typeparam name  T  gt Any Object lt  typeparam gt       lt param name  defaultValue  gt Default to use lt  param gt       lt returns gt The defaultValue if unsuccessful lt  returns gt  public static int ToInt32 lt T gt  this T value  int defaultValue      int result    if  int TryParse value ToString    out result           return result        return defaultValue      Example   int number    123  ToInt32      or   int badNumber    a  ToInt32 100      Returns 100 since a is nan

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Please mark your answers with an acceptance to put the code in the Codeplex project    Why  All the Stuff on this site under CC-by-sa-2 5  so just put your Extension overflow Project under the same license and you can freely use it   Anyway  here is a String Reverse function  based on this question        lt summary gt      Reverse a String      lt  summary gt       lt param name  input  gt The string to Reverse lt  param gt       lt returns gt The reversed String lt  returns gt  public static string Reverse this string input        char   array   input ToCharArray        Array Reverse array       return new string array

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I haven t seen any answer with this one yet     public static string   Split this string value  string regexPattern        return value Split regexPattern  RegexOptions None      public static string   Split this string value  string regexPattern       RegexOptions options        return Regex Split value  regexPattern  options       Usage   var obj    test1 test2 test3   string   arrays   obj Split

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Inspired by String IsNullOrEmpty  To validate the given List is null or empty  public static bool IsNullOrEmpty lt TSource gt  this List lt TSource gt  src                    return  src    null    src Count    0       And this one is to validate given 2 files and properties  public static bool Compare this FileInfo f1  FileInfo f2  string propertyName        try               PropertyInfo p1   f1 GetType   GetProperty propertyName           PropertyInfo p2   f2 GetType   GetProperty propertyName            if  p1 GetValue f1  null     p2 GetValue f1  null               return true            catch  Exception ex                return false            return false      And use it like this  FileInfo fo   new FileInfo  c   netlog txt    FileInfo f1   new FileInfo  c   regkey txt     fo compare f1   CreationTime

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You all probably already know that an interesting usage for extension methods is as a kind of mixin  Some extension methods  like the XmlSerializable  pollute almost every class  and it doesn t make sense to most of them  like Thread and SqlConnection   Some functionality should be explicitly mixed in to the classes that want to have it  I propose a new notation to this kind of type  with the M prefix   The XmlSerializable then  is this   public interface MXmlSerializable     public static class XmlSerializable     public static string ToXml this MXmlSerializable self        if  self    null  throw new ArgumentNullException        var serializer   new XmlSerializer self GetType         using  var writer   new StringWriter            serializer Serialize writer  self         return writer GetStringBuilder   ToString                public static T FromXml lt T gt  string xml  where T   MXmlSerializable       var serializer   new XmlSerializer typeof T        return  T serializer Deserialize new StringReader xml            A class then mixes it in   public class Customer   MXmlSerializable     public string Name   get  set      public bool Preferred   get  set        And the usage is simply   var customer   new Customer      Name    Guybrush Threepwood      Preferred   true    var xml   customer ToXml      If you like the idea  you can create a new namespace for useful mixins in the project  What do you think   Oh  and by the way  I think most extension methods should explicitly test for null

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I m using this one quite a lot     Original code   if  guid    Guid Empty  return guid  else return Guid NewGuid      New code   return guid NewGuidIfEmpty      Extension method   public static Guid NewGuidIfEmpty this Guid uuid        return  uuid    Guid Empty   uuid   Guid NewGuid

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For adding multiple elements to a collection that doesn t have AddRange  e g   collection Add item1  item2  itemN    static void Add lt T gt  this ICollection lt T gt  coll  params T   items     foreach  var item in items  coll Add item       The following is like string Format   but with custom string representation of arguments  e g     0   1   2   Format lt Custom gt  c  gt c Name  string  new object   new Custom    results in  string  System Object  Custom1Name   static string Format lt T gt    this string format                           Func lt T object gt  select                           params object   args     for int i 0  i  lt  args Length    i        var x   args i  as T        if  x    null  args i    select x            return string Format format  args

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http   www codeplex com linqext

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I created a nice Each extension that has the same behavior as jQuery s each function   It allows something like below  where you can get the index of the current value and break out of the loop by returning false   new      first    second    third    Each  value  index    gt        if  value Contains  d            return false      Console Write value       return true        Here s the code       lt summary gt      Generic iterator function that is useful to replace a foreach loop with at your discretion   A provided action is performed on each element       lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  source  gt  lt  param gt       lt param name  action  gt Function that takes in the current value in the sequence        lt returns gt  lt  returns gt  public static IEnumerable lt T gt  Each lt T gt  this IEnumerable lt T gt  source  Action lt T gt  action        return source Each  value  index    gt                action value           return true                   lt summary gt      Generic iterator function that is useful to replace a foreach loop with at your discretion   A provided action is performed on each element       lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  source  gt  lt  param gt       lt param name  action  gt Function that takes in the current value and its index in the sequence         lt returns gt  lt  returns gt  public static IEnumerable lt T gt  Each lt T gt  this IEnumerable lt T gt  source  Action lt T  int gt  action        return source Each  value  index    gt                action value  index           return true                  lt summary gt      Generic iterator function that is useful to replace a foreach loop with at your discretion   A provided action is performed on each element       lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  source  gt  lt  param gt       lt param name  action  gt Function that takes in the current value in the sequence   Returns a value indicating whether the iteration should continue   So return false if you don t want to iterate anymore  lt  param gt       lt returns gt  lt  returns gt  public static IEnumerable lt T gt  Each lt T gt  this IEnumerable lt T gt  source  Func lt T  bool gt  action        return source Each  value  index    gt                return action value                   lt summary gt      Generic iterator function that is useful to replace a foreach loop with at your discretion   A provided action is performed on each element       lt  summary gt       lt typeparam name  T  gt  lt  typeparam gt       lt param name  source  gt  lt  param gt       lt param name  action  gt Function that takes in the current value and its index in the sequence   Returns a value indicating whether the iteration should continue   So return false if you don t want to iterate anymore  lt  param gt       lt returns gt  lt  returns gt  public static IEnumerable lt T gt  Each lt T gt  this IEnumerable lt T gt  source  Func lt T  int  bool gt  action        if  source    null          return source       int index   0      foreach  var sourceItem in source                if   action sourceItem  index               break          index              return source

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A pattern for parsing that avoids out parameters   public static bool TryParseInt32 this string input  Action lt int gt  action        int result      if  Int32 TryParse input  out result                 action result           return true            return false      Usage   if   textBox Text TryParseInt32 number   gt  label Text   SomeMathFunction number        label Text    Please enter a valid integer     This can be put this in the codeplex project  if so desired

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Aww why not  Here s an extension to IList  can t be IEnumerable because i use list specific features  for insertion sort   internal static class SortingHelpers            lt summary gt          Performs an insertion sort on this list           lt  summary gt           lt typeparam name  T  gt The type of the list supplied  lt  typeparam gt           lt param name  list  gt the list to sort  lt  param gt           lt param name  comparison  gt the method for comparison of two elements  lt  param gt           lt returns gt  lt  returns gt      public static void InsertionSort lt T gt  this IList lt T gt  list  Comparison lt T gt  comparison                for  int i   2  i  lt  list Count  i                          for  int j   i  j  gt  1  amp  amp  comparison list j   list j - 1    lt  0  j--                                T tempItem   list j                   list RemoveAt j                   list Insert j - 1  tempItem                                     An example   List lt int gt  list1     3  5  1  2  9  4  6    list1 InsertionSort  a b    gt  a - b     list is now in order of 1 2 3 4 5 6 9

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I use these in my web projects  mainly with MVC  I have a handful of these written for the ViewData and TempData       lt summary gt      Checks the Request QueryString for the specified value and returns it  if none      is found then the default value is returned instead      lt  summary gt  public static T QueryValue lt T gt  this HtmlHelper helper  string param  T defaultValue        object value   HttpContext Current Request QueryString param  as object      if  value    null    return defaultValue        try           return  T Convert ChangeType value  typeof T          catch  Exception            return defaultValue            That way I can write something like      lt   if  Html QueryValue  login   false       gt       lt div gt Welcome Back  lt  div gt    lt     else     gt       lt  -- Render the control or something --  gt    lt       gt

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Please mark your answers with an acceptance to put the code in the Codeplex project    Why  All the Stuff on this site under CC-by-sa-2 5  so just put your Extension overflow Project under the same license and you can freely use it   Anyway  here is a String Reverse function  based on this question        lt summary gt      Reverse a String      lt  summary gt       lt param name  input  gt The string to Reverse lt  param gt       lt returns gt The reversed String lt  returns gt  public static string Reverse this string input        char   array   input ToCharArray        Array Reverse array       return new string array

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By all means put this in the codeplex project   Serializing   Deserializing objects to XML        lt summary gt Serializes an object of type T in to an xml string lt  summary gt       lt typeparam name  T  gt Any class type lt  typeparam gt       lt param name  obj  gt Object to serialize lt  param gt       lt returns gt A string that represents Xml  empty otherwise lt  returns gt  public static string XmlSerialize lt T gt  this T obj  where T   class  new         if  obj    null  throw new ArgumentNullException  obj         var serializer   new XmlSerializer typeof T        using  var writer   new StringWriter                  serializer Serialize writer  obj           return writer ToString                  lt summary gt Deserializes an xml string in to an object of Type T lt  summary gt       lt typeparam name  T  gt Any class type lt  typeparam gt       lt param name  xml  gt Xml as string to deserialize from lt  param gt       lt returns gt A new object of type T is successful  null if failed lt  returns gt  public static T XmlDeserialize lt T gt  this string xml  where T   class  new         if  xml    null  throw new ArgumentNullException  xml         var serializer   new XmlSerializer typeof T        using  var reader   new StringReader xml                 try   return  T serializer Deserialize reader             catch   return null       Could not be deserialized to this type

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Hate this kind of code   CloneableClass cc1   new CloneableClass     CloneableClass cc2   null  CloneableClass cc3   null   cc3    CloneableClass  cc1 Clone        this is ok cc3   cc2 Clone        this throws null ref exception    code to handle both cases cc3   cc1    null    CloneableClass  cc1 Clone      null    It s a bit clunky  so I replace it with this extension  which I call CloneOrNull -   public static T CloneOrNull lt T gt   this T self  where T   class  ICloneable       if  self    null  return null      return  T  self Clone         Usage is like    CloneableClass cc1   new CloneableClass     CloneableClass cc2   null  CloneableClass cc3   null   cc3   cc1 CloneOrNull        clone of cc1 cc3   cc2 CloneOrNull        null    look mom  no casts    Please feel free to use this anywhere

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I miss the Visual Basic s With statement when moving to C   so here it goes   public static void With lt T gt  this T obj  Action lt T gt  act    act obj       And here s how to use it in C    someVeryVeryLonggggVariableName With x   gt        x Int   123      x Str    Hello       x Str2     World          Saves a lot of typing   Compare this to   someVeryVeryLonggggVariableName Int   123  someVeryVeryLonggggVariableName Str    Hello   someVeryVeryLonggggVariableName Str2     World      put in codeplex project

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Here s a to-and-from for Roman numerals   Not often used  but could be handy   Usage   if   IV  IsValidRomanNumeral            Do useful stuff with the number 4     Console WriteLine  MMMDCCCLXXXVIII  ParseRomanNumeral     Console WriteLine 3888 ToRomanNumeralString       The source       public static class RomanNumeralExtensions               private const int NumberOfRomanNumeralMaps   13           private static readonly Dictionary lt string  int gt  romanNumerals               new Dictionary lt string  int gt  NumberOfRomanNumeralMaps                                   M   1000                        CM   900                        D   500                        CD   400                        C   100                        XC   90                        L   50                        XL   40                        X   10                        IX   9                        V   5                        IV   4                        I   1                           private static readonly Regex validRomanNumeral   new Regex                  i     MDCLXVI    M 0 3    C DM    D C 0 3                       X LC    L XX 0 2   L    I VX    V  II 0 2    V                     RegexOptions Compiled            public static bool IsValidRomanNumeral this string value                        return validRomanNumeral IsMatch value                      public static int ParseRomanNumeral this string value                        if  value    null                                throw new ArgumentNullException  value                               value   value ToUpperInvariant   Trim                 var length   value Length               if   length    0      value IsValidRomanNumeral                                  throw new ArgumentException  Empty or invalid Roman numeral string     value                               var total   0              var i   length               while  i  gt  0                                var digit   romanNumerals value --i  ToString                      if  i  gt  0                                        var previousDigit   romanNumerals value i - 1  ToString                          if  previousDigit  lt  digit                                                digit -  previousDigit                          i--                                                           total    digit                             return total                     public static string ToRomanNumeralString this int value                        const int MinValue   1              const int MaxValue   3999               if   value  lt  MinValue      value  gt  MaxValue                                 throw new ArgumentOutOfRangeException  value   value   Argument out of Roman numeral range                                const int MaxRomanNumeralLength   15              var sb   new StringBuilder MaxRomanNumeralLength                foreach  var pair in romanNumerals                                while  value   pair Value  gt  0                                        sb Append pair Key                       value -  pair Value                                               return sb ToString

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Function to compare Files Directories through the OS File System Info  This is useful to compare shares with local files     Usage   DirectoryInfo dir   new DirectoryInfo   C  test myShareDir    Console WriteLine dir IsSameFileAs     myMachineName myShareDir      FileInfo file   new FileInfo   C  test myShareDir file txt    Console WriteLine file IsSameFileAs     myMachineName myShareDir file txt       Code   public static class FileExtensions       struct BY HANDLE FILE INFORMATION               public uint FileAttributes          public System Runtime InteropServices ComTypes FILETIME CreationTime          public System Runtime InteropServices ComTypes FILETIME LastAccessTime          public System Runtime InteropServices ComTypes FILETIME LastWriteTime          public uint VolumeSerialNumber          public uint FileSizeHigh          public uint FileSizeLow          public uint NumberOfLinks          public uint FileIndexHigh          public uint FileIndexLow                       CreateFile constants            const uint FILE SHARE READ   0x00000001      const uint OPEN EXISTING   3      const uint GENERIC READ    0x80000000       const uint FILE FLAG BACKUP SEMANTICS   0x02000000         DllImport  kernel32 dll   SetLastError   true       static extern IntPtr CreateFile          string lpFileName          uint dwDesiredAccess          uint dwShareMode          IntPtr lpSecurityAttributes          uint dwCreationDisposition          uint dwFlagsAndAttributes          IntPtr hTemplateFile         DllImport  kernel32 dll   SetLastError   true       static extern bool GetFileInformationByHandle IntPtr hFile  out BY HANDLE FILE INFORMATION lpFileInformation        public static bool IsSameFileAs this FileSystemInfo file  string path                BY HANDLE FILE INFORMATION fileInfo1  fileInfo2          IntPtr ptr1   CreateFile file FullName  GENERIC READ  FILE SHARE READ  IntPtr Zero  OPEN EXISTING  FILE FLAG BACKUP SEMANTICS  IntPtr Zero           if   int ptr1    -1                        System ComponentModel Win32Exception e   new System ComponentModel Win32Exception Marshal GetLastWin32Error                 throw e                    IntPtr ptr2   CreateFile path  GENERIC READ  FILE SHARE READ  IntPtr Zero  OPEN EXISTING  FILE FLAG BACKUP SEMANTICS  IntPtr Zero           if   int ptr2    -1                        System ComponentModel Win32Exception e   new System ComponentModel Win32Exception Marshal GetLastWin32Error                 throw e                    GetFileInformationByHandle ptr1  out fileInfo1           GetFileInformationByHandle ptr2  out fileInfo2            return   fileInfo1 FileIndexHigh    fileInfo2 FileIndexHigh   amp  amp               fileInfo1 FileIndexLow    fileInfo2 FileIndexLow

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Convert a double to string formatted using the specified culture   public static class ExtensionMethods      public static string ToCurrency this double value  string cultureName          CultureInfo currentCulture   new CultureInfo cultureName       return  string Format currentCulture    0 C    value            Example   double test   154 20  string testString   test ToCurrency  en-US        154 20

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This one can be quite useful        public static IEnumerable lt TResult gt  Zip lt TFirst  TSecond  TResult gt  this IEnumerable lt TFirst gt  first  IEnumerable lt TSecond gt  second  Func lt TFirst  TSecond  TResult gt  selector                if  first    null              throw new ArgumentNullException  first            if  second    null              throw new ArgumentNullException  second            if  selector    null              throw new ArgumentNullException  selector             using  var enum1   first GetEnumerator            using  var enum2   second GetEnumerator                          while  enum1 MoveNext    amp  amp  enum2 MoveNext                                  yield return selector enum1 Current  enum2 Current                                   It has been added to the Enumerable class in  NET 4 0  but it s handy to have it in 3 5   Example    var names   new      Joe    Jane    Jack    John     var ages   new     42  22  18  33     var persons   names Zip ages   n  a    gt  new   Name   n  Age   a      foreach  var p in persons        Console WriteLine   0  is  1  years old   p Name  p Age

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And here s the control-invoke extension i m using regularly   public static class InvokeExtensions       public static void InvokeHandler this Control control  MethodInvoker del     Sync  control-invoke extension                if  control InvokeRequired                        control Invoke del               return                     del       run the actual code             public static void AsyncInvokeHandler this Control control  MethodInvoker del     Async  control-invoke extension                if  control InvokeRequired                        control BeginInvoke del               return                     del       run the actual code            sample   this TreeView AsyncInvokeHandler      gt                        this Text    xyz                which allows cross-thread gui-updates

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A convenient way to deal with sizes    public static class Extensions       public static int K this int value            return value   1024            public static int M this int value            return value   1024   1024           public class Program       public void Main             WSHttpContextBinding serviceMultipleTokenBinding   new WSHttpContextBinding                 MaxBufferPoolSize   2 M       instead of 2097152             MaxReceivedMessageSize   64 K       instead of 65536

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Inline Conversions  I like this little pattern  Completed it for Boolean  Double and DateTime  Designed to follow the C  is and as operators   public static Int32  AsInt32 this string s        Int32 value      if  Int32 TryParse s  out value           return value       return null     public static bool IsInt32 this string s        return s AsInt32   HasValue     public static Int32 ToInt32 this string s        return Int32 Parse s

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I m disappointed that the  NET Framework prefers that files and directories be represented as strings rather than objects  and that the FileInfo and DirectoryInfo types aren t as powerful as I d wish   So  I started to write fluent extension methods as I needed them  e g    public static FileInfo SetExtension this FileInfo fileInfo  string extension        return new FileInfo Path ChangeExtension fileInfo FullName  extension       public static FileInfo SetDirectory this FileInfo fileInfo  string directory        return new FileInfo Path Combine directory  fileInfo Name        Yes  you can put this in the codeplex

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I like these methods for dealing with enums that have the Flags attribute set   public static bool AnyOf this object mask  object flags        return   int mask  amp   int flags     0    public static bool AllOf this object mask  object flags        return   int mask  amp   int flags      int flags    public static object SetOn this object mask  object flags        return  int mask    int flags    etc    Example usage   var options   SomeOptions OptionA  options   options SetOn OptionB   options   options SetOn OptionC    if  options AnyOf SomeOptions OptionA   SomeOptions OptionB     etc    The original methods were from this article  http   www codeproject com KB cs masksandflags aspx display Print I just converted them to extension methods   The one problem with them though is that the parameters of object type  which means that all objects end up being extended with these methods  whereas ideally they should only apply to enums   Update As per the comments  you can get around the  signature pollution   at the expense of performance  like this   public static bool AnyOf this Enum mask  object flags        return  Convert ToInt642 mask   amp   int flags     0

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A generic Try   class Program       static void Main string   args                var z   0          var a   0 AsDefaultFor      gt  1   z           Console WriteLine a           Console ReadLine             public static class TryExtensions       public static T AsDefaultFor lt T gt  this T  this  Func lt T gt  operation                try                       return operation                      catch                       return  this                      Put it up on the CodePlex project if you want

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Used in winforms to fill a comboBox   List lt MyObject gt  myObjects   new List lt MyObject gt           new MyObject    Name    a   Id   0        new MyObject    Name    b   Id   1        new MyObject    Name    c   Id   2    comboBox FillDataSource lt MyObject gt  myObjects  x   gt  x Name     The extension method        lt summary gt Fills the System Windows Forms ComboBox object DataSource with a     list of T objects  lt  summary gt      lt param name  values  gt The list of T objects  lt  param gt      lt param name  displayedValue  gt A function to apply to each element to get the     display value  lt  param gt      public static void FillDataSource lt T gt  this ComboBox comboBox  List lt T gt  values      Func lt T  String gt  displayedValue            Create dataTable     DataTable data   new DataTable        data Columns Add  ValueMember   typeof T        data Columns Add  DisplayMember         for  int i   0  i  lt  values Count  i                 For each value displayed value             Create new row with value  amp  displayed value         DataRow dr   data NewRow            dr  ValueMember     values i           dr  DisplayMember     displayedValue values i                     Add row to the dataTable         data Rows Add dr                 Bind datasource to the comboBox     comboBox DataSource   data      comboBox ValueMember    ValueMember       comboBox DisplayMember    DisplayMember

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Some tools to IEnumerable  ToString Format   ToString Function  and Join Separator    For example   var names   new      Wagner    Francine    Arthur    Bernardo      names ToString  Name   0  n       Name  Wagner    Name  Francine    Name  Arthur    Name  Bernardo  names ToString name   gt  name Length  gt  6   String Format   0     name    String Empty      Francine Bernardo  names Join   -        Wagner - Francine - Arthur - Bernardo   Extensions   public static string ToString lt T gt  this IEnumerable lt T gt  self  string format        return self ToString i   gt  String Format format  i       public static string ToString lt T gt  this IEnumerable lt T gt  self  Func lt T  object gt  function        var result   new StringBuilder         foreach  var item in self  result Append function item         return result ToString       public static string Join lt T gt  this IEnumerable lt T gt  self  string separator        return String Join separator  values  self ToArray

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Equivalent to Python s Join method        lt summary gt      same as python  join       lt  summary gt       lt typeparam name  T  gt list type lt  typeparam gt       lt param name  separator  gt string separator  lt  param gt       lt param name  list  gt list of objects to be ToString d lt  param gt       lt returns gt a concatenated list interleaved with separators lt  returns gt  static public string Join lt T gt  this string separator  IEnumerable lt T gt  list        var sb   new StringBuilder        bool first   true       foreach  T v in list                if   first              sb Append separator           first   false           if  v    null              sb Append v ToString                return sb ToString

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on serializing and configs there is better using long as DateTime  so       public static readonly DateTime Epoch   new DateTime 1970  1  1  0  0  0        public static long ToUnixTimestamp this DateTime dateTime                return  long   dateTime - Epoch  TotalSeconds             public static long ToUnixUltraTimestamp this DateTime dateTime                return  long   dateTime - Epoch  TotalMilliseconds          and backwards      public static DateTime ToDateTime this long unixDateTime                return Epoch AddSeconds unixDateTime              public static DateTime ToDateTimeUltra this long unixUltraDateTime                return Epoch AddMilliseconds unixUltraDateTime

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Here is the only extension that I wrote that I use regularly   It makes sending email with System Net Mail a bit easier   public static class MailExtension          GetEmailCreditial out strServer  gets credentials from an XML file     public static void Send this MailMessage email                string strServer   String Empty          NetworkCredential credentials   GetEmailCreditial out strServer           SmtpClient client   new SmtpClient strServer    Credentials   credentials            client Send email              public static void Send this IEnumerable lt MailMessage gt  emails                string strServer   String Empty          NetworkCredential credentials   GetEmailCreditial out strServer           SmtpClient client   new SmtpClient strServer    Credentials   credentials            foreach  MailMessage email in emails              client Send email               Example of use   new MailMessage  info myDomain com   you gmail com   This is an important Subject    Body goes here   Send      Assume email1 email2 email3 are MailMessage objects new List lt MailMessage gt    email1  email2  email  Send

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Used in winforms to fill a comboBox   List lt MyObject gt  myObjects   new List lt MyObject gt           new MyObject    Name    a   Id   0        new MyObject    Name    b   Id   1        new MyObject    Name    c   Id   2    comboBox FillDataSource lt MyObject gt  myObjects  x   gt  x Name     The extension method        lt summary gt Fills the System Windows Forms ComboBox object DataSource with a     list of T objects  lt  summary gt      lt param name  values  gt The list of T objects  lt  param gt      lt param name  displayedValue  gt A function to apply to each element to get the     display value  lt  param gt      public static void FillDataSource lt T gt  this ComboBox comboBox  List lt T gt  values      Func lt T  String gt  displayedValue            Create dataTable     DataTable data   new DataTable        data Columns Add  ValueMember   typeof T        data Columns Add  DisplayMember         for  int i   0  i  lt  values Count  i                 For each value displayed value             Create new row with value  amp  displayed value         DataRow dr   data NewRow            dr  ValueMember     values i           dr  DisplayMember     displayedValue values i                     Add row to the dataTable         data Rows Add dr                 Bind datasource to the comboBox     comboBox DataSource   data      comboBox ValueMember    ValueMember       comboBox DisplayMember    DisplayMember

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Some DataSet DataRow extensions to make working with db results a little simpler  Just use  Field  fieldname   on the DataRow and it will cast it if it can  optional default can be included   Also  HasRows   on the DataSet so you don t need to check for the existence of a table and rows   Example   using  DataSet ds   yourcall         if  ds HasRows             foreach  DataRow dr in ds Tables 0  Rows                 int id   dr Field lt int gt   ID            string name   dr Field lt string gt   Name            string Action   dr Field lt string gt   Action    N A                   Code   using System  using System Data   public static class DataSetExtensions       public static T Field lt T gt  this DataRow row  string columnName  T defaultValue                try                       return row Field lt T gt  columnName                     catch                       return defaultValue                       public static T Field lt T gt  this DataRow row  string columnName                if  row columnName     null              throw new NullReferenceException columnName     does not exist in DataRow             string value   row columnName  ToString             if  typeof T        GetType                          return  T Convert ChangeType value  typeof T                      else if  typeof T     0 GetType                          return  T Convert ChangeType int Parse value   typeof T                      else if  typeof T     false GetType                          return  T Convert ChangeType bool Parse value   typeof T                      else if  typeof T     DateTime Now GetType                          return  T Convert ChangeType DateTime Parse value   typeof T                      else if  typeof T     new byte   GetType                          return  T Convert ChangeType byte Parse value   typeof T                      else if  typeof T     new float   GetType                          return  T Convert ChangeType float Parse value   typeof T                      else                       throw new ArgumentException string Format  Cannot cast   0   to   1      value  typeof T  ToString                           public static bool HasRows this DataSet dataSet                 return  dataSet Tables Count  gt  0  amp  amp  dataSet Tables 0  Rows Count  gt  0

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Here are a couple of methods that I use to make extracting single attributes a little less painful   public static T GetAttribute lt T gt  this ICustomAttributeProvider provider  bool inherit   false  int index   0  where T   Attribute       return provider GetAttribute typeof T   inherit  index  as T     public static Attribute GetAttribute this ICustomAttributeProvider provider  Type type  bool inherit   false  int index   0        bool exists   provider IsDefined type  inherit       if   exists                return null             object   attributes   provider GetCustomAttributes type  inherit       if  attributes    null  amp  amp  attributes Length    0                return attributes index  as Attribute            else               return null            Usage  implementation of this enum description hack    public static string GetDescription this Enum value        var fieldInfo   value GetType   GetField value ToString         var attribute   fieldInfo GetAttribute lt DescriptionAttribute gt         return attribute    null   attribute Description   null      Feel free to include this in the CodePlex project

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For adding multiple elements to a collection that doesn t have AddRange  e g   collection Add item1  item2  itemN    static void Add lt T gt  this ICollection lt T gt  coll  params T   items     foreach  var item in items  coll Add item       The following is like string Format   but with custom string representation of arguments  e g     0   1   2   Format lt Custom gt  c  gt c Name  string  new object   new Custom    results in  string  System Object  Custom1Name   static string Format lt T gt    this string format                           Func lt T object gt  select                           params object   args     for int i 0  i  lt  args Length    i        var x   args i  as T        if  x    null  args i    select x            return string Format format  args

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My conversion extensions which allow you to do   int i   myString To lt int gt       Here it is  as posted on TheSoftwareJedi com  public static T To lt T gt  this IConvertible obj      return  T Convert ChangeType obj  typeof T       public static T ToOrDefault lt T gt                this IConvertible obj        try               return To lt T gt  obj             catch               return default T            public static bool ToOrDefault lt T gt                       this IConvertible obj                       out T newObj        try               newObj   To lt T gt  obj            return true            catch               newObj   default T            return false           public static T ToOrOther lt T gt                          this IConvertible obj                         T other      try           return To lt T gt obj         catch           return other         public static bool ToOrOther lt T gt                            this IConvertible obj                           out T newObj                           T other        try               newObj   To lt T gt  obj           return true            catch               newObj   other          return false           public static T ToOrNull lt T gt                         this IConvertible obj                        where T   class       try               return To lt T gt  obj             catch               return null           public static bool ToOrNull lt T gt                     this IConvertible obj                    out T newObj                    where T   class       try               newObj   To lt T gt  obj           return true            catch               newObj   null          return false            You can ask for default  calls blank constructor or  0  for numerics  on failure  specify a  default  value  I call it  other    or ask for null  where T   class   I ve also provided both silent exception models  and a typical TryParse model that returns a bool indicating the action taken  and an out param holds the new value  So our code can do things like this  int i   myString To lt int gt     string a   myInt ToOrDefault lt string gt       note type inference DateTime d   myString ToOrOther DateTime MAX VALUE   double d    note type inference bool didItGiveDefault   myString ToOrDefault out d   string s   myDateTime ToOrNull lt string gt       I couldn t get Nullable types to roll into the whole thing very cleanly  I tried for about 20 minutes before I threw in the towel

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Takes a camelCaseWord or PascalCaseWord and  wordifies  it  ie camelCaseWord    camel Case Word  public static string Wordify  this string camelCaseWord            if the word is all upper  just return it     if   Regex IsMatch  camelCaseWord    a-z               return camelCaseWord       return string Join       Regex Split  camelCaseWord       lt        A-Z             I often use it in conjuction with Capitalize  public static string Capitalize  this string word         return word 0  ToString    ToUpper      word Substring  1        Example usage  SomeEntityObject entity   DataAccessObject GetSomeEntityObject  id    List lt PropertyInfo gt  properties   entity GetType   GetPublicNonCollectionProperties         wordify the property names to act as column headers for an html table or something List lt string gt  columns   properties Select  p   gt  p Name Capitalize    Wordify      ToList       Free to use in codeplex project

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Binary search    public static T BinarySearch lt T  TKey gt  this IList lt T gt  list  Func lt T  TKey gt  keySelector  TKey key          where TKey   IComparable lt TKey gt        int min   0      int max   list Count      int index   0      while  min  lt  max                int mid    max   min    2          T midItem   list mid           TKey midKey   keySelector midItem           int comp   midKey CompareTo key           if  comp  lt  0                        min   mid   1                    else if  comp  gt  0                        max   mid - 1                    else                       return midItem                      if  min    max  amp  amp          keySelector list min   CompareTo key     0                return list min             throw new InvalidOperationException  Item not found        Usage  assuming that the list is sorted by Id     var item   list BinarySearch i   gt  i Id  42     The fact that it throws an InvalidOperationException may seem strange  but that s what Enumerable First does when there s no matching item

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Several times I found myself wanting something like  I think  Groovy s  Safe navigation    From http   groovy codehaus org Statements      If you are walking a complex object   graph and don t want to have   NullPointerExceptions thrown you can   use the    operator rather than   to   perform your navigation       def foo   null def bar     foo  something  myMethod   assert bar      null   So  do you think is a good idea adding an extension method for it  Something like   obj SafelyNavigate x   gt  x SomeProperty MaybeAMethod   AnotherProperty     I think it would be nice even if it can also bring some trouble   If you think it s a good idea    What would you think it should happen for value types    return default  throw   disable it by generic constraint   Swallowing NullReferenceException to implement it would be too risky   What do you propose    Walking the expression tree executing every call or member access seems difficult and kind of overkill  if at all possible  doesn t it     Maybe it s just a bad idea  D  but I see it like something that can be useful if done right  If there s nothing like it and you think it holds some value  I may give it a shot and edit the answer afterwards

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This one is for MVC it adds the ability to generate a  lt label   gt  tag to the Html variable that is available in every ViewPage  Hopefully it will be of use to others trying to develop similar extensions   Use    lt    Html Label  LabelId    ForId    Text    gt    Output    lt label id  LabelId  for  ForId  gt Text lt  label gt    Code   public static class HtmlHelperExtensions       public static string Label this HtmlHelper Html  string  for  string text                return Html Label null   for  text              public static string Label this HtmlHelper Html  string  for  string text  object htmlAttributes                return Html Label null   for  text  htmlAttributes              public static string Label this HtmlHelper Html  string  for  string text  IDictionary lt string  object gt  htmlAttributes                return Html Label null   for  text  htmlAttributes              public static string Label this HtmlHelper Html  string id  string  for  string text                return Html Label id   for  text  null              public static string Label this HtmlHelper Html  string id  string  for  string text  object htmlAttributes                return Html Label id   for  text  new RouteValueDictionary htmlAttributes               public static string Label this HtmlHelper Html  string id  string  for  string text  IDictionary lt string  object gt  htmlAttributes                TagBuilder tag   new TagBuilder  label             tag MergeAttributes htmlAttributes            if   string IsNullOrEmpty id               tag MergeAttribute  id   Html AttributeEncode id             tag MergeAttribute  for   Html AttributeEncode  for             tag SetInnerText Html Encode text             return tag ToString TagRenderMode Normal

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Pythonic methods for Dictionaries        lt summary gt      If a key exists in a dictionary  return its value       otherwise return the default value for that type       lt  summary gt  public static U GetWithDefault lt T  U gt  this Dictionary lt T  U gt  dict  T key        return dict GetWithDefault key  default U            lt summary gt      If a key exists in a dictionary  return its value      otherwise return the provided default value       lt  summary gt  public static U GetWithDefault lt T  U gt  this Dictionary lt T  U gt  dict  T key  U defaultValue        return dict ContainsKey key            dict key            defaultValue      Useful for when you want to append a timestamp to a filename to assure uniqueness        lt summary gt      Format a DateTime as a string that contains no characters      that are banned from filenames  such as           lt  summary gt       lt returns gt YYYY-MM-DD HH MM SS lt  returns gt  public static string ToFilenameString this DateTime dt        return dt ToString  s   Replace           Replace  T

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DateTimeExtensions  Examples   DateTime firstDayOfMonth   DateTime Now First    DateTime lastdayOfMonth   DateTime Now Last    DateTime lastFridayInMonth   DateTime Now Last DayOfWeek Friday   DateTime nextFriday   DateTime Now Next DayOfWeek Friday   DateTime lunchTime   DateTime Now SetTime 11  30   DateTime noonOnFriday   DateTime Now Next DayOfWeek Friday  Noon    DateTime secondMondayOfMonth   DateTime Now First DayOfWeek Monday  Next DayOfWeek Monday  Midnight

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I find the following extension method quite useful   public static T GetService lt T gt  this IServiceProvider provider        return  T provider GetService typeof T        It makes it much easier to use the IServiceProvider interface  Compare   IProvideValueTarget target    IProvideValueTarget serviceProvider typeof IProvideValueTarget      and  var target   serviceProvider GetService lt IProvideValueTarget gt

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In the recent searches section on my blog stats page  I had removed all duplicates  but needed a way to remove nearly-duplicate lines  I d get tons of similar but not quite the same Google queries    I ended up using an anonymous type instead of a dictionary  but wanted a way to create a List of that anonymous type  You can t do that  but you can create a List lt dynamic gt  in  NET 4 0     Mostly I like it because I effectively get a List lt AnonymousType 1 gt           lt summary gt Remove extraneous entries for common word permutations lt  summary gt       lt param name  input  gt Incoming series of words to be filtered lt  param gt       lt param name  MaxIgnoreLength  gt Words this long or shorter will not count as duplicates lt  param gt       lt param name  words2  gt Instance list from BuildInstanceList   lt  param gt       lt returns gt Filtered list of lines from input  based on filter info in words2 lt  returns gt  private static List lt string gt  FilterNearDuplicates List lt string gt  input  int MaxIgnoreLength  List lt dynamic gt  words2        List lt string gt  output   new List lt string gt         foreach  string line in input                int Dupes   0          foreach  string word in line Split new char                                            r     n                       Where p   gt  p Length  gt  MaxIgnoreLength               Distinct                          int Instances   0              foreach  dynamic dyn in words2              if  word    dyn Word                                Instances   dyn Instances                  if  Instances  gt  1                      Dupes                    break                                  if  Dupes    0              output Add line             return output         lt summary gt Builds a list of words and how many times they occur in the overall list lt  summary gt       lt param name  input  gt Incoming series of words to be counted lt  param gt       lt returns gt  lt  returns gt  private static List lt dynamic gt  BuildInstanceList List lt string gt  input        List lt dynamic gt  words2   new List lt object gt         foreach  string line in input      foreach  string word in line Split new char                                            r     n                         if  string IsNullOrEmpty word               continue          else if  ExistsInList word  words2               for  int i   words2 Count - 1  i  gt   0  i--                                if  words2 i  Word    word                      words2 i    new   Word   words2 i  Word  Instances   words2 i  Instances   1                          else             words2 Add new   Word   word  Instances   1                return words2         lt summary gt Determines whether a dynamic Word object exists in a List of this dynamic type  lt  summary gt       lt param name  word  gt Word to look for lt  param gt       lt param name  words  gt Word dynamics to search through lt  param gt       lt returns gt Indicator of whether the word exists in the list of words lt  returns gt  private static bool ExistsInList string word  List lt dynamic gt  words        foreach  dynamic dyn in words          if  dyn Word    word              return true      return false

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There s a lot of functionality you can get from the Random class   Below are some extension methods I use from time to time  With these  in addition to Next and NextDouble  the Random class gives you NextBool  NextChar  NextDateTime  NextTimeSpan  NextDouble  accepting minValue and maxValue parameters   and my personal favorite  NextString  There are more  NextByte  NextShort  NextLong  etc    but those are mostly for completeness and don t get used as much  So I didn t include them here  this code is long enough as it is        todo  implement additional CharType values  e g   AsciiAny  public enum CharType       AlphabeticLower      AlphabeticUpper      AlphabeticAny      AlphanumericLower      AlphanumericUpper      AlphanumericAny      Numeric    public static class RandomExtensions          10 digits vs  52 alphabetic characters  upper  amp  lower          probability of being numeric  10   62   0 1612903225806452     private const double AlphanumericProbabilityNumericAny   10 0   62 0          10 digits vs  26 alphabetic characters  upper OR lower          probability of being numeric  10   36   0 2777777777777778     private const double AlphanumericProbabilityNumericCased   10 0   36 0       public static bool NextBool this Random random  double probability            return random NextDouble    lt   probability             public static bool NextBool this Random random            return random NextDouble    lt   0 5             public static char NextChar this Random random  CharType mode            switch  mode                case CharType AlphabeticAny                  return random NextAlphabeticChar                case CharType AlphabeticLower                  return random NextAlphabeticChar false               case CharType AlphabeticUpper                  return random NextAlphabeticChar true               case CharType AlphanumericAny                  return random NextAlphanumericChar                case CharType AlphanumericLower                  return random NextAlphanumericChar false               case CharType AlphanumericUpper                  return random NextAlphanumericChar true               case CharType Numeric                  return random NextNumericChar                default                  return random NextAlphanumericChar                         public static char NextChar this Random random            return random NextChar CharType AlphanumericAny              private static char NextAlphanumericChar this Random random  bool uppercase            bool numeric   random NextBool AlphanumericProbabilityNumericCased            if  numeric              return random NextNumericChar            else             return random NextAlphabeticChar uppercase              private static char NextAlphanumericChar this Random random            bool numeric   random NextBool AlphanumericProbabilityNumericAny            if  numeric              return random NextNumericChar            else             return random NextAlphabeticChar random NextBool                private static char NextAlphabeticChar this Random random  bool uppercase            if  uppercase              return  char random Next 65  91           else             return  char random Next 97  123              private static char NextAlphabeticChar this Random random            return random NextAlphabeticChar random NextBool                private static char NextNumericChar this Random random            return  char random Next 48  58              public static DateTime NextDateTime this Random random  DateTime minValue  DateTime maxValue            return DateTime FromOADate              random NextDouble minValue ToOADate    maxValue ToOADate                          public static DateTime NextDateTime this Random random            return random NextDateTime DateTime MinValue  DateTime MaxValue              public static double NextDouble this Random random  double minValue  double maxValue            if  maxValue  lt  minValue              throw new ArgumentException  Minimum value must be less than maximum value              double difference   maxValue - minValue          if   double IsInfinity difference               return minValue    random NextDouble     difference            else                  to avoid evaluating to Double Infinity  we split the range into two halves              double halfDifference    maxValue   0 5  -  minValue   0 5                   50 50 chance of returning a value from the first or second half of the range             if  random NextBool                    return minValue    random NextDouble     halfDifference               else                 return  minValue   halfDifference     random NextDouble     halfDifference                        public static string NextString this Random random  int numChars  CharType mode            char   chars   new char numChars            for  int i   0  i  lt  numChars    i              chars i    random NextChar mode            return new string chars              public static string NextString this Random random  int numChars            return random NextString numChars  CharType AlphanumericAny              public static TimeSpan NextTimeSpan this Random random  TimeSpan minValue  TimeSpan maxValue            return TimeSpan FromMilliseconds              random NextDouble minValue TotalMilliseconds  maxValue TotalMilliseconds                        public static TimeSpan NextTimeSpan this Random random            return random NextTimeSpan TimeSpan MinValue  TimeSpan MaxValue

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I like these NUnit Assert extensions  http   svn caffeine-it com openrasta trunk src Rasta Testing AssertExtensions cs

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Ive created a extension method to select an item in a dropdown in ASP NET   Below is the code   public static class Utilities       public enum DropDownListSelectionType               ByValue          ByText            public static void SelectItem this  System Web UI WebControls DropDownList drp  string selectedValue  DropDownListSelectionType type                drp ClearSelection            System Web UI WebControls ListItem li          if  type    DropDownListSelectionType ByValue              li   drp Items FindByValue selectedValue Trim             else             li   drp Items FindByText selectedValue Trim             if  li    null              li Selected   true           This method can be called by the following lines of code to either select by text  DropDownList1 SelectItem  ABCD   Utilities DropDownListSelectionType ByText     or select by value  DropDownList1 SelectItem  11   Utilities DropDownListSelectionType ByValue     The above code doesnt select anything if it cant find the text value passed in

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I am sure this has been done before  but I find myself using this method  and simpler derivatives  often   public static bool CompareEx this string strA  string strB  CultureInfo culture  bool ignoreCase        return string Compare strA  strB  ignoreCase  culture     0      You can write it in a number of ways  but I like it because it very quickly unifies my approach to comparing strings while saving me lines of code  or characters of code

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ForEach for IEnumerables  public static class FrameworkExtensions          a map function     public static void ForEach lt T gt  this IEnumerable lt T gt   enum  Action lt T gt  mapFunction                foreach  var item in  enum  mapFunction item             Naive example   var buttons   GetListOfButtons   as IEnumerable lt Button gt       click all buttons buttons ForEach b   gt  b Click       Cool example      no need to type the same assignment 3 times  just    new   up an array and use foreach   lambda    everything is properly inferred by csc  -  new   itemA  itemB  itemC        ForEach item   gt            item Number   1          item Str    Hello World              Note   This is not like Select because Select expects your function to return something as for transforming into another list   ForEach simply allows you to execute something for each of the items without any transformations data manipulation   I made this so I can program in a more functional style and I was surprised that List has a ForEach while IEnumerable does not   Put this in the codeplex project

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public static bool In lt T gt  this T source  params T   list      if null  source  throw new ArgumentNullException  source      return list Contains source       Allows me to replace   if reallyLongIntegerVariableName    1         reallyLongIntegerVariableName    6         reallyLongIntegerVariableName    9         reallyLongIntegerVariableName    11         do something        and  if reallyLongStringVariableName     string1          reallyLongStringVariableName     string2          reallyLongStringVariableName     string3          do something        and  if reallyLongMethodParameterName    SomeEnum Value1         reallyLongMethodParameterName    SomeEnum Value2         reallyLongMethodParameterName    SomeEnum Value3         reallyLongMethodParameterName    SomeEnum Value4         do something         With   if reallyLongIntegerVariableName In 1 6 9 11              do something        and  if reallyLongStringVariableName In  string1   string2   string3               do something        and  if reallyLongMethodParameterName In SomeEnum Value1  SomeEnum Value2  SomeEnum Value3  SomeEnum Value4         do something

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I found this one helpful  public static IEnumerable lt T gt  EmptyIfNull lt T gt  this IEnumerable lt T gt  pSeq        return pSeq    Enumerable Empty lt T gt         It removes the null check in the calling code  You could now do  MyList EmptyIfNull   Where

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I use this a lot with nullable numbers  I helps catch those division by 0  NaN  Infinity     public static bool IsNullOrDefault lt T gt  this T  o       where T   struct           return o    null    o Value Equals default T

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String As lt T gt   which can be used to convert a string value as some type  intended to be used primarily with primitives and types that support IConvertable  Works great with Nullable types and even Enums   public static partial class StringExtensions            lt summary gt          Converts the string to the specified type  using the default value configured for the type           lt  summary gt           lt typeparam name  T  gt Type the string will be converted to  The type must implement IConvertable  lt  typeparam gt           lt param name  original  gt The original string  lt  param gt           lt returns gt The converted value  lt  returns gt      public static T As lt T gt  this String original                return As original  CultureInfo CurrentCulture                    default T                    lt summary gt          Converts the string to the specified type  using the default value configured for the type           lt  summary gt           lt typeparam name  T  gt Type the string will be converted to  lt  typeparam gt           lt param name  original  gt The original string  lt  param gt           lt param name  defaultValue  gt The default value to use in case the original string is null or empty  or can t be converted  lt  param gt           lt returns gt The converted value  lt  returns gt      public static T As lt T gt  this String original  T defaultValue                return As original  CultureInfo CurrentCulture  defaultValue                   lt summary gt          Converts the string to the specified type  using the default value configured for the type           lt  summary gt           lt typeparam name  T  gt Type the string will be converted to  lt  typeparam gt           lt param name  original  gt The original string  lt  param gt           lt param name  provider  gt Format provider used during the type conversion  lt  param gt           lt returns gt The converted value  lt  returns gt      public static T As lt T gt  this String original  IFormatProvider provider                return As original  provider  default T                    lt summary gt          Converts the string to the specified type           lt  summary gt           lt typeparam name  T  gt Type the string will be converted to  lt  typeparam gt           lt param name  original  gt The original string  lt  param gt           lt param name  provider  gt Format provider used during the type conversion  lt  param gt           lt param name  defaultValue  gt The default value to use in case the original string is null or empty  or can t be converted  lt  param gt           lt returns gt The converted value  lt  returns gt           lt remarks gt          If an error occurs while converting the specified value to the requested type  the exception is caught and the default is returned  It is strongly recommended you         do NOT use this method if it is important that conversion failures are not swallowed up                  This method is intended to be used to convert string values to primatives  not for parsing  converting  or deserializing complex types           lt  remarks gt      public static T As lt T gt  this String original  IFormatProvider provider                            T defaultValue                T result          Type type   typeof  T            if  String IsNullOrEmpty original   result   defaultValue          else                          need to get the underlying type if T is Nullable lt  gt                if  type IsNullableType                                  type   Nullable GetUnderlyingType type                              try                                  ChangeType doesn t work properly on Enums                 result   type IsEnum                                 T  Enum Parse type  original  true                                  T  Convert ChangeType original  type  provider                             catch    HACK  what can we do to minimize or avoid raising exceptions as part of normal operation  custom string parsing  regex   for well-known types  it would be best to know if you can convert to the desired type before you attempt to do so                                result   defaultValue                                   return result            This relies on another simple extension for Type        lt summary gt      Extension methods for  lt see cref  Type   gt        lt  summary gt  public static class TypeExtensions            lt summary gt          Returns whether or not the specified type is  lt see cref  Nullable T    gt            lt  summary gt           lt param name  type  gt A  lt see cref  Type   gt   lt  param gt           lt returns gt True if the specified type is  lt see cref  Nullable T    gt   otherwise  false  lt  returns gt           lt remarks gt Use  lt see cref  Nullable GetUnderlyingType   gt  to access the underlying type  lt  remarks gt      public static bool IsNullableType this Type type                if  type    null  throw new ArgumentNullException  type             return type IsGenericType  amp  amp  type GetGenericTypeDefinition   Equals typeof  Nullable lt  gt               Usage   var someInt    1  As lt int gt     var someIntDefault    bad value  As 1       bad value  won t convert  so the default value 1 is returned  var someEnum    Sunday  As lt DayOfWeek gt     someEnum    0  As lt DayOfWeek gt        returns Sunday var someNullableEnum      As lt DayOfWeek  gt  null      returns a null value since    can t be converted

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Here is one I use frequently for presentation formatting   public static string ToTitleCase this string mText        if  mText    null  return mText       System Globalization CultureInfo cultureInfo   System Threading Thread CurrentThread CurrentCulture      System Globalization TextInfo textInfo   cultureInfo TextInfo          TextInfo ToTitleCase only operates on the string if is all lower case  otherwise it returns the string unchanged      return textInfo ToTitleCase mText ToLower

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These extension methods invoke an event asynchronously  They were inspired by this StackOverflow answer        lt summary gt      Invoke an event asynchronously  Each subscriber to the event will be invoked on a separate thread       lt  summary gt       lt param name  someEvent  gt The event to be invoked asynchronously  lt  param gt       lt param name  sender  gt The sender of the event  lt  param gt       lt param name  args  gt The args of the event  lt  param gt       lt typeparam name  TEventArgs  gt The type of  lt see cref  EventArgs   gt  to be used with the event  lt  typeparam gt  public static void InvokeAsync lt TEventArgs gt  this EventHandler lt TEventArgs gt  someEvent  object sender  TEventArgs args      where TEventArgs   EventArgs       if  someEvent    null                return             var eventListeners   someEvent GetInvocationList         AsyncCallback endAsyncCallback   delegate IAsyncResult iar                var ar   iar as AsyncResult          if  ar    null                        return                     var invokedMethod   ar AsyncDelegate as EventHandler lt TEventArgs gt           if  invokedMethod    null                        invokedMethod EndInvoke iar                         foreach  EventHandler lt TEventArgs gt  methodToInvoke in eventListeners                methodToInvoke BeginInvoke sender  args  endAsyncCallback  null                 lt summary gt      Invoke an event asynchronously  Each subscriber to the event will be invoked on a separate thread       lt  summary gt       lt param name  someEvent  gt The event to be invoked asynchronously  lt  param gt       lt param name  sender  gt The sender of the event  lt  param gt       lt param name  args  gt The args of the event  lt  param gt  public static void InvokeAsync this EventHandler someEvent  object sender  EventArgs args        if  someEvent    null                return             var eventListeners   someEvent GetInvocationList         AsyncCallback endAsyncCallback   delegate IAsyncResult iar                var ar   iar as AsyncResult          if  ar    null                        return                     var invokedMethod   ar AsyncDelegate as EventHandler          if  invokedMethod    null                        invokedMethod EndInvoke iar                         foreach  EventHandler methodToInvoke in eventListeners                methodToInvoke BeginInvoke sender  args  endAsyncCallback  null             To use   public class Foo       public event EventHandler lt EventArgs gt  Bar       public void OnBar                 Bar InvokeAsync this  EventArgs Empty             Notice the added benefit that you don t have to check for null on the event before invoking it  e g    EventHandler lt EventArgs gt  handler   Bar  if  handler    null           Invoke the event     To test   void Main         EventHandler lt EventArgs gt  handler1       delegate object sender  EventArgs args                   Simulate performing work in handler1         Thread Sleep 100           Console WriteLine  Handled 1                EventHandler lt EventArgs gt  handler2       delegate object sender  EventArgs args                   Simulate performing work in handler2         Thread Sleep 50           Console WriteLine  Handled 2                EventHandler lt EventArgs gt  handler3       delegate object sender  EventArgs args                   Simulate performing work in handler3         Thread Sleep 25           Console WriteLine  Handled 3                var foo   new Foo        foo Bar    handler1      foo Bar    handler2      foo Bar    handler3      foo OnBar         Console WriteLine  Start executing important stuff            Simulate performing some important stuff here  where we don t want to        wait around for the event handlers to finish executing     Thread Sleep 1000        Console WriteLine  Finished executing important stuff        Invoking the event will  usually  yield this output      Start executing important stuff   Handled 3   Handled 2   Handled 1   Finished executing important stuff   If the event were invoked synchronously  it would always yield this output - and delay execution of the  important  stuff      Handled 1   Handled 2   Handled 3   Start executing important stuff   Finished executing important stuff

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GetMemberName allows to get the string with the name of a member with compile time safety   public static string GetMemberName lt T  TResult gt       this T anyObject       Expression lt Func lt T  TResult gt  gt  expression        return   MemberExpression expression Body  Member Name      Usage    blah  GetMemberName x   gt  x Length      returns  Length      It comes together with a non-extension static method if you don t have a instance   public static string GetMemberName lt T  TReturn gt       Expression lt Func lt T  TReturn gt  gt  expression      where T   class       return   MemberExpression expression Body  Member Name      But the call doesn t look as pretty of course   ReflectionUtility GetMemberName  string  s   gt  s Length      returns  Length      You can put it on Codeplex if you want

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Please mark your answers with an acceptance to put the code in the Codeplex project    Why  All the Stuff on this site under CC-by-sa-2 5  so just put your Extension overflow Project under the same license and you can freely use it   Anyway  here is a String Reverse function  based on this question        lt summary gt      Reverse a String      lt  summary gt       lt param name  input  gt The string to Reverse lt  param gt       lt returns gt The reversed String lt  returns gt  public static string Reverse this string input        char   array   input ToCharArray        Array Reverse array       return new string array

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The extension method   public static void AddRange lt T  S gt  this ICollection lt T gt  list  params S   values      where S   T       foreach  S value in values          list Add value       The method applies for all types and lets you add a range of items to a list as parameters   Example   var list   new List lt Int32 gt     list AddRange 5  4  8  4  2

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Two color extensions I use  mostly for control development   public static class ColorExtensions        Gets a color that will be readable on top of a given background color   public static Color GetForegroundColor this Color input             Math taken from one of the replies to        http   stackoverflow com questions 2241447 make-foregroundcolor-black-or-white-depending-on-background     if  Math Sqrt input R   input R    241   input G   input G    691   input B   input B    068   gt  128        return Color Black      else       return Color White            Converts a given Color to gray   public static Color ToGray this Color input          int g    int  input R    299     int  input G    587     int  input B    114       return Color FromArgb input A  g  g  g           Usage   Color foreColor   someBackColor GetForegroundColor    Color grayColor   someBackColor ToGray

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I m always using format that wants a new line with StringBuilder so the very simple extension below saves a few lines of code   public static class Extensions       public static void AppendLine this StringBuilder builder string format  params object   args                builder AppendLine string Format format  args              The alternative is AppendFormat in StringBuilder with a  n or Environment NewLine

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Here s another pair I ve found endless use for   public static T ObjectWithMin lt T  TResult gt  this IEnumerable lt T gt  sequence  Func lt T  TResult gt  predicate      where T   class     where TResult   IComparable       if   sequence Any    return null         get the first object with its predicate value     var seed   sequence Select x   gt  new  Object   x  Value   predicate x    FirstOrDefault          compare against all others  replacing the accumulator with the lesser value       tie goes to first object found     return         sequence Select x   gt  new  Object   x  Value   predicate x                 Aggregate seed  acc  x    gt  acc Value CompareTo x Value   lt   0   acc   x  Object     public static T ObjectWithMax lt T  TResult gt  this IEnumerable lt T gt  sequence  Func lt T  TResult gt  predicate      where T   class     where TResult   IComparable       if   sequence Any    return null         get the first object with its predicate value     var seed   sequence Select x   gt  new  Object   x  Value   predicate x    FirstOrDefault          compare against all others  replacing the accumulator with the greater value       tie goes to last object found     return         sequence Select x   gt  new  Object   x  Value   predicate x                 Aggregate seed   acc  x    gt  acc Value CompareTo x Value   gt  0   acc   x  Object      Usage   var myObject   myList ObjectWithMin x  gt x PropA     These methods basically replace usages like  var myObject   myList OrderBy x  gt x PropA  FirstOrDefault      O nlog n   and unstable   and  var myObject   myList Where x  gt x PropA    myList Min x  gt x PropA   FirstOrDefault      O N 2  but stable   and  var minValue   myList Min x  gt x PropA   var myObject   myList Where x  gt x PropA    minValue  FirstOrDefault      not a one-liner  and though linear and stable it s slower  evaluates the enumerable twice

[c#] What are your favorite extension methods for C#? (codeplex.com/extensionoverflow)

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