Hidden Features of C

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This came to my mind after I learned the following from this question   where T   struct   We  C  developers  all know the basics of C   I mean declarations  conditionals  loops  operators  etc   Some of us even mastered the stuff like Generics  anonymous types  lambdas  LINQ       But what are the most hidden features or tricks of C  that even C  fans  addicts  experts barely know   Here are the revealed features so far     Keywords   yield by Michael Stum var by Michael Stum using   statement by kokos readonly by kokos as by Mike Stone as   is by Ed Swangren as   is  improved  by Rocketpants default by deathofrats global   by pzycoman using   blocks by AlexCuse volatile by Jakub   turc extern alias by Jakub   turc   Attributes   DefaultValueAttribute by Michael Stum ObsoleteAttribute by DannySmurf DebuggerDisplayAttribute by Stu DebuggerBrowsable and DebuggerStepThrough by bdukes ThreadStaticAttribute by marxidad FlagsAttribute by Martin Clarke ConditionalAttribute by AndrewBurns   Syntax       coalesce nulls  operator by kokos Number flaggings by Nick Berardi where T new by Lars M  hlum Implicit generics by Keith One-parameter lambdas by Keith Auto properties by Keith Namespace aliases by Keith Verbatim string literals with   by Patrick enum values by lfoust  variablenames by marxidad event operators by marxidad Format string brackets by Portman Property accessor accessibility modifiers by xanadont Conditional  ternary  operator      by JasonS checked and unchecked operators by Binoj Antony implicit and explicit operators by Flory   Language Features   Nullable types by Brad Barker Anonymous types by Keith   makeref   reftype   refvalue by Judah Himango Object initializers by lomaxx Format strings by David in Dakota Extension Methods by marxidad partial methods by Jon Erickson Preprocessor directives by John Asbeck DEBUG pre-processor directive by Robert Durgin Operator overloading by SefBkn Type inferrence by chakrit Boolean operators taken to next level by Rob Gough Pass value-type variable as interface without boxing by Roman Boiko Programmatically determine declared variable type by Roman Boiko Static Constructors by Chris Easier-on-the-eyes   condensed ORM-mapping using LINQ by roosteronacid   arglist by Zac Bowling   Visual Studio Features   Select block of text in editor by Himadri Snippets by DannySmurf     Framework   TransactionScope by KiwiBastard DependantTransaction by KiwiBastard Nullable lt T gt  by IainMH Mutex by Diago System IO Path by ageektrapped WeakReference by Juan Manuel   Methods and Properties   String IsNullOrEmpty   method by KiwiBastard List ForEach   method by KiwiBastard BeginInvoke    EndInvoke   methods by Will Dean Nullable lt T gt  HasValue and Nullable lt T gt  Value properties by Rismo GetValueOrDefault method by John Sheehan   Tips  amp  Tricks   Nice method for event handlers by Andreas H R  Nilsson Uppercase comparisons by John Access anonymous types without reflection by dp A quick way to lazily instantiate collection properties by Will JavaScript-like anonymous inline-functions by roosteronacid   Other   netmodules by kokos   LINQBridge by Duncan Smart   Parallel Extensions by Joel Coehoorn

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If you have the search textbox in your Visual Studio toolbar  you can type   of Program cs  to open the file Program cs

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I was reading thru the book  Pro ASP NET MVC Framework   APress  and observed something the author was doing with a Dictionary object that was foreign to me    He added a new Key Value Pair without using the Add   method  He then overwrote that same Key Value pair without having to check if that key already existed  For example   Dictionary lt string  int gt  nameAgeDict   new Dictionary lt string  int gt     nameAgeDict  Joe     34          no error  will just auto-add key value nameAgeDict  Joe     41          no error  key value just get overwritten nameAgeDict Add  Joe   30        ERROR  key already exists   There are many cases where I don t need to check if my Dictionary already has a key or not and I just want to add the respective key value pair  overwriting the existing key value pair  if necessary   Prior to this discovery  I would always have to check to see if the key already existed before adding it

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double dSqrd   Math Pow d 2 0      is more accurate than   double dSqrd   d   d     Here we can lose precision

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Just learned  anonymous types can infer property names from the variable name   string hello    world   var o   new   hello    Console WriteLine o hello

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Something I missed for a long time  you can compare strings with    string  equals  String   StringComparison InvariantCultureIgnoreCase    instead of doing     string  ToLower       String  ToLower

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Atrribute Targets  Everyone has seen one  Basically  when you see this    assembly  ComVisible false     The  assembly   portion of that attribute is the target  In this case  the attribute is applied to the assembly  but there are others    return  SomeAttr  int Method3     return 0       In this sample the attribute is applied to the return value

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Robbie Rocketpants      but my instincts tell me that this   would cut a maximum of two type casts   operations down to a maximum of one     If you do the cast as you were suggesting in example 1  using is  amp  as   it results in 2 calls to the  is  operator  Because when you do  c   obj as MyClass   first it calls  is  behind the scenes  then if it fails that it simply returns null   If you do the cast as you were suggesting in example 2   c    MyClass obj   Then this actually performs the  is  operation again  then if it fails that check it throws an exception  InvalidCastException    So  if you wanted to do a lightweight dynamic cast  it s best to do the 3rd example you provided   MyClass c  if  obj is MyClass        c   obj as MyClass    if  c    null        vs  MyClass c   obj as MyClass   if  c    null        You can see which is quicker  more consise and clearer

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I find it incredible what type of trouble the compiler goes through to sugar code the use of Outer Variables   string output    helo world    Action action        gt  Console WriteLine output   output    hello    action      This actually prints hello   Why  Because the compiler creates a nested class for the delegate  with public fields for all outer variables and inserts setting-code before every single call to the delegate    Here is above code  reflectored    Action action   lt  gt c  DisplayClass1 CS  lt  gt 8  locals2  CS  lt  gt 8  locals2   new  lt  gt c  DisplayClass1    CS  lt  gt 8  locals2 output    helo world    action   new Action CS  lt  gt 8  locals2  lt Main gt b  0   CS  lt  gt 8  locals2 output    hello    action      Pretty cool I think

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Is constructor chain already cited   namespace constructorChain       using System       public class Class1           public string x          public string y           public Class1                 x    class1               y                          public Class1 string y                this                 this y   y                       public class Class2   Class1           public Class2 int y                base y ToString                                        constructorChain Class1 c1   new constructorChain Class1            constructorChain Class1 c12   new constructorChain Class1  Hello  Constructor             constructorChain Class2 c2   new constructorChain Class2 10           Console WriteLine   0   1    c1 x  c1 y           Console WriteLine   0   1    c12 x  c12 y           Console WriteLine   0   1    c2 x  c2 y            Console ReadLine

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Use  throw   instead of  throw ex   to preserve stack trace  If re-throwing an exception without adding additional information  use  throw  instead of  throw ex   An empty  throw  statement in a catch block will emit specific IL that re-throws the exception while preserving the original stack trace   throw ex  loses the stack trace to the original source of the exception

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Convert enum values to a string value  Given the enum  enum Country       UnitedKingdom       UnitedStates      UnitedArabEmirates      using it   public static void PrintEnumAsString  Country country         Console Writeline  country ToString          will print the name of the enum value as a string  e g   UnitedKingdom

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You can add and remove delegates with less typing   Usual way   handler    new EventHandler func     Less typing way   handler    func

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At first - DebuggerTypeProxy    DebuggerTypeProxy typeof HashtableDebugView    class MyHashtable   Hashtable       private const string TestString             This should not appear in the debug window         internal class HashtableDebugView               private Hashtable hashtable          public const string TestStringProxy                 This should appear in the debug window                The constructor for the type proxy class must have a             constructor that takes the target type as a parameter          public HashtableDebugView Hashtable hashtable                        this hashtable   hashtable                      At second   ICustomTypeDescriptor

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When a class implements INotifyPropertyChanged and you want to inform the binding system  WPF  Silverlight  etc   that multiple bound properties of an object  ViewModel  have changed you can raise the PropertyChanged-Event with null or String Empty   This is documented in MSDN  but code examples and articles often don  t explain this possibility  I found it very useful   public class BoundObject   INotifyPropertyChanged        private int  value      private string  text       public event PropertyChangedEventHandler PropertyChanged       public int Value           get               return  value                    set               if   value    value                     value   value                  OnPropertyChanged  Value                                       public string Text           get               return  text                    set               if   text    value                     text   value                  OnPropertyChanged  Text                                       public void Init             text    InitialValue            value   1          OnPropertyChanged string Empty              public void Reset              text    DefaultValue            value   0          OnPropertyChanged string Empty              private void OnPropertyChanged string propertyName            PropertyChangedEventArgs e   new PropertyChangedEventArgs propertyName            if  PropertyChanged    null                PropertyChanged this  e

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TryParse method for each primitive type is great when validating user input   double doubleValue if   Double TryParse myDataRow  myColumn    out doubleValue            set validation error

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Cool trick to emulate functional  wildcard  arguments  like     in Haskell  when using lambdas       b        gt  b DoStuff        only interested in b here

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The following one is not hidden  but it s quite implicit  I don t know whether samples like the following one have been published here  and I can t see are there any benefits  probably there are none   but I ll try to show a  weird  code  The following sample simulates for statement via functors in C   delegates   anonymous delegates  lambdas   and closures  Other flow statements like if  if else  while and do whle  are simulated as well  but I m not sure for switch  perhaps  I m too lazy      I ve compacted the sample source code a little to make it more clear   private static readonly Action EmptyAction        gt       private static readonly Func lt bool gt  EmptyCondition        gt    return true      private sealed class BreakStatementException   Exception     private sealed class ContinueStatementException   Exception     private static void Break     throw new BreakStatementException      private static void Continue     throw new ContinueStatementException       private static void For Action init  Func lt bool gt  condition  Action postBlock  Action statement        init   init    EmptyAction      condition   condition    EmptyCondition      postBlock   postBlock    EmptyAction      statement   statement    EmptyAction      for   init    condition    postBlock               try               statement              catch   BreakStatementException                 break            catch   ContinueStatementException                 continue                     private static void Main         int i   0     avoiding error  Use of unassigned local variable  i   if not using  for  init block     For      gt  i   0       gt  i  lt  10       gt  i                 gt                if   i    5                   Continue                Console WriteLine i                        If I m not wrong  this approach is pretty relative to the functional programming practice  Am I right

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You can have generic methods in a non-generic class

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There are some really hidden keywords and features in C  related to the TypedReference undocumented class  The following keywords are undocumented      makeref   reftype   refvalue   arglist   Examples of use      Create a typed reference int i   1  TypedReference tr1     makeref i      Get the type of a typed reference Type t     reftype tr1      Get the value of a typed referece int j     refvalue tr1  int       Create a method that accepts and arbitrary number of typed references void SomeMethod   arglist           Call the method int x   1  string y    Foo   Object o   new Object    SomeMethod   arglist x y o       And finally iterate over method parameters void SomeMethod   arglist        ArgIterator ai   new ArgIterator   arglist   while ai GetRemainingCount    gt 0          TypedReference tr   ai GetNextArg          Console WriteLine TypedReference ToObject tr

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Unions  the C   shared memory kind  in pure  safe C   Without resorting to unsafe mode and pointers  you can have class members share memory space in a class struct   Given the following class    StructLayout LayoutKind Explicit   public class A        FieldOffset 0       public byte One        FieldOffset 1       public byte Two        FieldOffset 2       public byte Three        FieldOffset 3       public byte Four        FieldOffset 0       public int Int32      You can modify the values of the byte fields by manipulating the Int32 field and vice-versa   For example  this program       static void Main string   args                A a   new A   Int32   int MaxValue             Console WriteLine a Int32           Console WriteLine   0 X   1 X   2 X   3 X    a One  a Two  a Three  a Four            a Four   0          a Three   0          Console WriteLine a Int32           Outputs this   2147483647 FF FF FF 7F 65535   just add using System Runtime InteropServices

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I especially like the nullable DateTime  So if you have some cases where a Date is given and other cases where no Date is given I think this is best to use and IMHO easier to understand as using DateTime MinValue or anything else     DateTime  myDate   null   if  myDate HasValue          doSomething   else         soSomethingElse

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Maybe not an advanced technique  but one I see all the time that drives me crazy   if  x    1       x   2    else      x   3      can be condensed to   x    x  1    2   3

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If 3rd-party extensions are allowed  then C5 and Microsoft CCR  see this blog post for a quick introduction  are a must-know   C5 complements  Net s somewhat lacking collections library  not Set      and CCR makes concurrent programming easier  I hear it s due to be merged with Parallel Extensions

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I like    if DEBUG              Code run in debugging mode   else              Code run in release mode   endif

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Accessing local variables from anonymous methods allows you to wrap just about any code with new control flow logic  without having to factor out that code into another method   Local variables declared outside the method are available inside the method such as the endOfLineChar local variable in the example here   http   aaronls wordpress com 2010 02 02 retrying-on-exception-conditionally

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I d say using certain system classes for extension methods is very handy  for example System Enum  you can do something like below      Flags  public enum ErrorTypes   int       None   0      MissingPassword   1      MissingUsername   2      PasswordIncorrect   4    public static class EnumExtensions        public static T Append lt T gt   this System Enum type  T value  where T   struct               return  T  ValueType    int  ValueType  type    int  ValueType  value               public static T Remove lt T gt   this System Enum type  T value  where T   struct               return  T  ValueType    int  ValueType type  amp    int  ValueType value               public static bool Has lt T gt   this System Enum type  T value  where T   struct               return    int  ValueType type  amp   int  ValueType value      int  ValueType value                    used like the following     ErrorTypes error   ErrorTypes None  error   error Append ErrorTypes MissingUsername   error   error Append ErrorTypes MissingPassword   error   error Remove ErrorTypes MissingUsername      then you can check using other methods if  error Has ErrorTypes MissingUsername                 This is just an example of course - the methods could use a little more work

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I couldn t figure out what use some of the functions in the Convert class had  such as Convert ToDouble int   Convert ToInt double   until I combined them with Array ConvertAll   int   someArrayYouHaveAsInt  double   copyOfArrayAsDouble   Array ConvertAll lt int  double gt                                   someArrayYouHaveAsInt                                  new Converter lt int double gt  Convert ToDouble      Which avoids the resource allocation issues that arise from defining an inline delegate closure  and slightly more readable    int   someArrayYouHaveAsInt  double   copyOfArrayAsDouble   Array ConvertAll lt int  double gt                                   someArrayYouHaveAsInt                                  new Converter lt int double gt                                     delegate int i    return  double i

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The Or assignment operator is quite nice  You can write this   x    y   instead of this   x   x   y   This is often practical if you have to a variable or property  x in the example  that starts out as false but you want to change it to the value of some other boolean variable property only when that other value is true

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With reference to the post w  perma link  Hidden Features of C     there is another way to accomplish the same - indentation   line breaks  Check this out    XmlWriterSettings xmlWriterSettings   new XmlWriterSettings    xmlWriterSettings NewLineOnAttributes   true  xmlWriterSettings Indent   true    XmlWriter xml   XmlWriter Create   C  file xml   xmlWriterSettings       Start writing the data using xml WriteStartElement    xml WriteElementString       xml WriteEndElement   etc   I am not sure whether this is an unknown feature though

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Foreach uses Duck Typing  Paraphrasing  or shamelessly stealing from Krzysztof Cwalinas blog on this  More interesting trivia than anything   For your object to support foreach  you don t have to implement IEnumerable  I e  this is not a constraint and it isn t checked by the compiler  What s checked is that   Your object provide a public method GetEnumerator that   takes no parameters return a type that has two members   a parameterless method MoveNext that returns a boolean a property Current with a getter that returns an Object     For example     class Foo       public Bar GetEnumerator     return new Bar           public struct Bar               public bool MoveNext                         return false                     public object Current                       get   return null                          the following complies just fine  Foo f   new Foo    foreach  object o in f        Console WriteLine  Krzysztof Cwalina s da man

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Four switch oddities by Eric Lippert

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I just want to mention  because of the OP metioning where T   struct  that one of the C  compiler gotchas is that   where T   Enum   will NOT compile  It throws the error  Constraint cannot be special class  System Enum

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You can switch on string   switch name      case  Dave       return true    case  Bob       return false    default      throw new ApplicationException        Very handy  and a lot cleaner than a bunch of if-else statements

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Using   for variable names that are keywords   var  object   new object    var  string       var  if   IpsoFacto

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I don t know if this is a hidden feature       Any string function

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Collection Initializer inside Object Initializer   MailMessage mail   new MailMessage      To     new MailAddress  a example com    new MailAddress  b example com         Subject    Password Recovery       You can initialize a whole tree in a single expression

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The data type can be defined for an enumeration   enum EnumName    byte  char  int16  int32  int64  uint16  uint32  uint64        A   1      B   2

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HttpContext Current Server Execute    is great for rendering HTML to strings for AJAX callbacks   You can use this with a component instead of piecing together HTML string snippets   I was able to cut page bloat down a couple of hundred KB with virtually no mess   I used it like this   Page pageHolder   new Page    UserControl viewControl    UserControl pageHolder LoadControl   MyComponent ascx    pageHolder Controls Add viewControl   StringWriter output   new StringWriter    HttpContext Current Server Execute pageHolder  output  false   return output ToString

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There s also the ThreadStaticAttribute to make a static field unique per thread  so you can have strongly typed thread-local storage   Even if extension methods aren t that secret  LINQ is based on them   it may not be so obvious as to how useful and more readable they can be for utility helper methods     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         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            result   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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I see a lot of people replicate the functionality of Nullable lt T gt  GetValueOrDefault T

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I m becoming a big fan of extension methods since they can add much wanted functionality to existing code or code you can t edit  One of my favorites I add in to everything I do now is for string IsNullOrEmpty    public static class Strings       public static bool IsNullOrEmpty this string value                return string IsNullOrEmpty value             This lets you shorten your code a bit like this  var input   Console ReadLine    if  input IsNullOrEmpty          Console WriteLine  try again

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Avoid checking for null event handlers  Adding an empty delegate to events at declaration  suppressing the need to always check the event for null before calling it is awesome  Example   public delegate void MyClickHandler object sender  string myValue   public event MyClickHandler Click   delegate        add empty delegate    Let you do this  public void DoSomething         Click this   foo        Instead of this  public void DoSomething            Unnecessary      MyClickHandler click   Click      if  click    null     Unnecessary                 click this   foo              Please also see this related discussion and this blog post by Eric Lippert on this topic  and possible downsides

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Closures  Since anonymous delegates were added to 2 0  we have been able to develop closures  They are rarely used by programmers but provide great benefits such as immediate code reuse  Consider this piece of code   bool changed   false   if  model Prop1    prop1        changed   true      model Prop1   prop1    if  model Prop2    prop2        changed   true      model Prop2   prop2           etc     Note that the if-statements above perform similar pieces of code with the exception of one line of code  i e  setting different properties  This can be shortened with the following  where the varying line of code is entered as a parameter to an Action object  appropriately named setAndTagChanged   bool changed   false  Action lt Action gt  setAndTagChanged    action    gt          changed   true       action         if  model Prop1    prop1  setAndTagChanged      gt  model Prop1   prop1   if  model Prop2    prop2  setAndTagChanged      gt  model Prop2   prop2     In the second case  the closure allows you to scope the change variable in your lambda  which is a concise way to approach this problem   An alternate way is to use another unused feature  the  or equal  binary assignment operator  The following code shows how   private bool conditionalSet bool condition  Action action        if  condition  action        return condition             bool changed   false  changed    conditionalSet model Prop1    prop1       gt  model Prop1   prop1   changed    conditionalSet model Prop2    prop2       gt  model Prop2   prop2

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This isn t a C  specific feature but it is an addon that I find very useful   It is called the Resource Refactoring Tool   It allows you to right click on a literal string and extract it into a resource file   It will search the code and find any other literal strings that match and replace it with the same resource from the Resx file   http   www codeplex com ResourceRefactoring

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The ability to use LINQ to do inline work on collections that used to take iteration and conditionals can be incredibly valuable  It s worth learning how all the LINQ extension methods can help make your code much more compact and maintainable

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The  default  keyword in generic types   T t   default T     results in a  null  if T is a reference type  and 0 if it is an int  false if it is a boolean  etcetera

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In reading the book on development of the  NET framework   A good piece of advice is not to use bool to turn stuff on or off  but rather use ENums   With ENums you give yourself some expandability without having to rewrite any code to add a new feature to a function

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Not sure why anyone would ever want to use Nullable lt bool  though   -    True  False  FileNotFound

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One great class I like is System Xml XmlConvert which can be used to read values from xml tag   Especially  if I am reading a boolean value from xml attribute or element  I use   bool myFlag    System Xml XmlConvert ToBoolean myAttribute Value     Note  since boolean type in xml accepts 1 and 0 in addition to  true  and  false  as valid values  using string comparison in this case is error-prone

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Working with enums   Convert a string to an Enum   enum MyEnum       FirstValue      SecondValue      ThirdValue    string enumValueString    FirstValue   MyEnum val    MyEnum Enum Parse typeof MyEnum   enumValueString  true     I use this to load the value of CacheItemPriority in my ASP NET applications from a settings table in a database so that I can control caching  along with other settings  dynamically without taking the application down    When comparing variables of type enum  you don t have to cast to int   MyEnum val   MyEnum SecondValue  if  val  lt  MyEnum ThirdValue           Do something

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Relection is so powerfull when used carefully  I used it in an e-mail templating system  The template manager would be passed an object and the html templates would have embedded fields that referred to Properties that could be retrieved off the passed object using reflection  Worked out very nicely

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Many people don t realize that they can compare strings using  OrdinalIgnoreCase instead of having to do someString ToUpper     This removes the additional string allocation overhead   if  myString ToUpper      theirString ToUpper              becomes  if  myString Equals  theirString  StringComparison OrdinalIgnoreCase

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TransactionScope and DependentTransaction in System Transactions is a lightweight way to use transaction processing in  NET - it s not just for Database transactions either String IsNullOrEmpty is one that I am surprised to learn a lot of developers don t know about List ForEach - iterate through your generic list using a delegate method   There are more  but that is the three obvious ones of the top of my head

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Reflection Emit and Expression trees come to mind     Don t miss Jeffrey Richter s CLR via C  and Jon Skeet s   See here for some resources   http   www codeproject com KB trace releasemodebreakpoint aspx  http   www codeproject com KB dotnet Creating Dynamic Types aspx  http   www codeproject com KB cs lambdaexpressions aspx

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A few from me - make of them what you will   The attribute    assembly  InternalsVisibleTo  SomeAssembly      Allows you to expose out the internal methods properties or data from your assembly to another assembly called  SomeAssembly   All protected private stuff remains hidden     Static constructors   otherwise called  Type Constructor     public MyClass     public static MyClass               type init goes here                      The keyword internal  So useful in so many ways

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lambdas and type inference are underrated  Lambdas can have multiple statements and they double as a compatible delegate object automatically  just make sure the signature match  as in   Console CancelKeyPress         sender  e    gt            Console WriteLine  CTRL C detected  n            e Cancel   true           Note that I don t have a new CancellationEventHandler nor do I have to specify types of sender and e  they re inferable from the event  Which is why this is less cumbersome to writing the whole delegate  blah blah  which also requires you to specify types of parameters   Lambdas don t need to return anything and type inference is extremely powerful in context like this   And BTW  you can always return Lambdas that make Lambdas in the functional programming sense  For example  here s a lambda that makes a lambda that handles a Button Click event   Func lt int  int  EventHandler gt  makeHandler        dx  dy    gt   sender  e    gt            var btn    Button  sender          btn Top    dy          btn Left    dx          btnUp Click    makeHandler 0  -1   btnDown Click    makeHandler 0  1   btnLeft Click    makeHandler -1  0   btnRight Click    makeHandler 1  0     Note the chaining   dx  dy    gt   sender  e    gt   Now that s why I m happy to have taken the functional programming class  -   Other than the pointers in C  I think it s the other fundamental thing you should learn  -

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Having just learned the meaning of invariance  covariance and contravariance  I discovered the in and out generic modifiers that will be included in  NET 4 0  They seem obscure enough that most programmers would not know about them   There s an article at Visual Studio Magazine which discusses these keywords and how they will be used

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If you are trying to create a comma delimited string from a list of items   string   itemList      Example 1    Example 2    Example 3     CommaDelimitedStringCollection commaStr   new CommaDelimitedStringCollection    commaStr AddRange itemList     outputs Example 1 Example 2 Example 3   Look here for another example

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The extern alias keyword to reference two versions of assemblies that have the same fully-qualified type names

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The Environment UserInteractive property       The UserInteractive property reports   false for a Windows process or a   service like IIS that runs without a   user interface  If this property is   false  do not display modal dialogs or   message boxes because there is no   graphical user interface for the user   to interact with

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Attributes in general  but most of all DebuggerDisplay  Saves you years

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Not hidden  but pretty neat  I find this a more succinct substitute for a simple if-then-else that just assigns a value based on a condition   string result                  i  lt  2                   question                less than 2            answer               i  lt  5                   question               less than 5             answer                  i  lt  10                  question                less than 10           answer                something else         default answer

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To call the base class constructor just put base   inline with the constructor  To call the base class method you can just put base MethodName   inside the derived class method  class ClassA      public ClassA int a            Do something        public void Method1              Do Something        class ClassB   ClassA     public ClassB int a    base a     calling the base class constructor           Do something        public void Method2           base Method1                     calling the base class method         Of course you can call the methods of the base class by just saying base MethodName

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Dictionary initializers are always useful for quick hacks and unit tests where you need to hardcode some data   var dict   new Dictionary lt int  string gt      10   Hello       20   World

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In dealing with interop between C   and C   many people don t realize that C   CLI is a great option   Say you have a C   DLL and a C  DLL which depends on the C   DLL   Often  the easiest technique is to compile some  or all  modules of the C   DLL with the  clr switch  To have the C  call the C   DLL is to write managed C   wrapper classes in the C   DLL   The C   CLI classes can call the native C   code much more seamlessly than C   because the C   compiler will automatically generate P invokes for you  has a library specifically for interop  plus language features for interop like pin ptr   And it allows managed and native code to coexist within the same binary   On the C  side  you just call into the DLL as you would any other  NET binary

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This isn t C  per se  but I haven t seen anyone who really uses System IO Path Combine   to the extent that they should  In fact  the whole Path class is really useful  but no one uses it   I m willing to bet that every production app has the following code  even though it shouldn t   string path   dir          fileName

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The generic event handler   public event EventHandler lt MyEventArgs gt  MyEvent    This way you don t have to declare your own delegates all the time

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I picked this one up when using ReSharper   Implicit Method Group Conversion     If given this  var myStrings   new List lt string gt     abc   def   xyz      Then this  myStrings ForEach s   gt  Console WriteLine s      Is equivalent to this  myStrings ForEach Console WriteLine     See  Implicit Method Group Conversion in C   for more

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With LINQ it s possible to create new functions based on parameters  That s very nice if you have a tiny function which is exectued very often  but the parameters need some time to calculate       public Func lt int gt  RandomGenerator               get                       var r   new Random                return      gt    return r Next                            void SomeFunction                 var result1   RandomGenerator             var x   RandomGenerator          var result2   x

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dynamic keyword in C  4 0  You can use dynamic keyword  if you want your method calls to be resolved only at the runtime    dynamic invoker new DynamicInvoker    dynamic result1 invoker MyMethod1    dynamic result2 invoker MyMethod2      Here I m implementing a dynamic invoker   public class DynamicInvoker   IDynamicObject               public MetaObject GetMetaObject                System Linq Expressions Expression parameter                        return new DynamicReaderDispatch  parameter                        public class DynamicDispatcher   MetaObject               public DynamicDispatcher  Expression parameter                        base parameter  Restrictions Empty              public override MetaObject Call CallAction action  MetaObject   args                          You ll get MyMethod1 and MyMethod2 here  and what ever you call              Console WriteLine  Logic to invoke Method   0     action Name               return this    Return a meta object

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Two of my personal favourites  which I see rarely used    Snippets  particularly for properties  which was made even better for Visual Studio 2008  The ObsoleteAttribute

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I love abusing the fact that static templated classes don t share their static members   Here s a threadsafe  at creation time  and cheap substitute to any Dictionary lt Type     gt  when the Type instance is known at compile-time   public static class MyCachedData lt T gt       static readonly CachedData Value      static MyCachedData           Value    Heavy computation  such as baking IL code or doing lots of reflection on a type           Cheers  Florian

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Definitely the Func lt   types when used with statement lambdas in  NET 3 5  These allow customizable functions  and can be a great aid in offering user customizable objects without subclassing them or resorting to some limited system like keeping track of a variable that lists what button or key the user wants to monitor  Also  they can be called just like regular methods and can be assigned like variables  The only downside that I can think of is that you re limited to 5 arguments  Although by that point you might want to consider a different solution    Edit  Providing some examples       public Func lt InputHelper  float gt   horizontalCameraMovement    InputHelper input    gt         return  input LeftStickPosition X    moveRate     zoom    public Func lt InputHelper  float gt   verticalCameraMovement    InputHelper input    gt         return  -input LeftStickPosition Y    moveRate     zoom        public void Update InputHelper input                position    new Vector2  horizontalCameraMovement input    verticalCameraMovement input                In this example  you can write a function that does arbitrary calculation and returns a float that will determine the amount that the camera moves by  Not the best code but it gets the point across   private int foo  public int FooProperty       get               if   onFooGotten      true              return  foo            set               if  onFooSet      true               foo   value              public Func lt bool gt   onFooGotten        gt           do whatever        return true    public Func lt bool gt   onFooSet        gt          do whatever        return true      This isn t the best example  as I haven t really explored this use yet  but it shows an example of using a lambda function for a quick event raiser without the hassle of delegates  Edit  thought of another one  Nullables  The closest thing C  has to optional parameters

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yield  would come to my mind  Some of the attributes like  DefaultValue    are also among my favorites   The  var  keyword is a bit more known  but that you can use it in  NET 2 0 applications as well  as long as you use the  NET 3 5 compiler and set it to output 2 0 code  does not seem to be known very well   Edit  kokos  thanks for pointing out the    operator  that s indeed really useful  Since it s a bit hard to google for it  as    is just ignored   here is the MSDN documentation page for that operator     Operator  C  Reference

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Exception Filters  So  hidden  you can t even use them  at least from C   without a post-compilation patch

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Here is a TIP of how you can use  Region directive to document your code

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Being able to have enum types have values other than int  the default   public enum MyEnum   long       Val1   1      Val2   2     Also  the fact that you can assign any numeric value to that enum   MyEnum e    MyEnum 123

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Marketing events as non-serializable    field NonSerializable  public event SomeDelegate SomeEvent

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Having read through all 9 pages of this I felt I had to point out a little unknown feature     This was held true for  NET 1 1  using compression decompression on gzipped files  one had to either    Download ICSharpCode ZipLib Or  reference the Java library into your project and use the Java s in-built library to take advantage of the GZip s compression decompression methods    It is underused  that I did not know about   still use ICSharpCode ZipLib still  even with  NET 2 3 5  was that it was incorporated into the standard BCL version 2 upwards  in the System IO Compression namespace    see the MSDN page  GZipStream Class

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typedefs  Someone posted that they miss typedefs but you can do it like this  using ListOfDictionary   System Collections Generic List lt System Collections Generic Dictionary lt string  string gt  gt     and declare it as  ListOfDictionary list   new ListOfDictionary

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Returning IQueryable projections  protected void LdsPostings Selecting object sender  LinqDataSourceSelectEventArgs e           var dc   new MyDataContext        var query   dc Posting AsQueryable         if  isCondition1                query   query Where q   gt  q PostedBy    Username           e Result   QueryProjection query           return                      if  isConditionN                query   query Where q   gt  q Status StatusName     submitted            query   query Where q   gt  q ReviewedBy    Username           e Result   QueryProjection query           return            and rather than coding the projection multiple times  create a single method   private IQueryable QueryProjection IQueryable lt Posting gt  query        return query Select p   gt  new               p PostingID          p Category CategoryName          p Type TypeName          p Status StatusName          p Description          p Updated          p PostedBy          p ReviewedBy

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InternalsVisibleTo attribute is one that is not that well known  but can come in increadibly handy in certain circumstances  It basically allows another assembly to be able to access  internal  elements of the defining assembly

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On-demand field initialization in one line   public StringBuilder Builder       get   return  builder      builder   new StringBuilder           I m not sure how I feel about C  supporting assignment expressions  but hey  it s there  -

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AppDomain UnhandledException Event is also candidate for being hidden      This event provides notification of uncaught exceptions  It allows the application to log information about the exception before the system default handler reports the exception to the user and terminates the application  If sufficient information about the state of the application is available  other actions may be undertaken     such as saving program data for later recovery  Caution is advised  because program data can become corrupted when exceptions are not handled    We can see  even on this site  a lot of people are wondering why their application is not starting  why it crashed  etc  The AppDomain UnhandledException event can be very useful for such cases as it provides the possibility at least to log the reason of application failure

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Environment NewLine   for system independent newlines

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This means T must have a public parameterless constructor     class MyClass lt T gt  where T   new

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I like the EditorBrowsableAttribute  It lets you control whether a method property is displayed or not in Intellisense  You can set the values to Always  Advanced  or Never   From MSDN     Remarks  EditorBrowsableAttribute is a hint to a designer indicating whether a property or method is to be displayed  You can use this type in a visual designer or text editor to determine what is visible to the user  For example  the IntelliSense engine in Visual Studio uses this attribute to determine whether to show a property or method   In Visual C   you can control when advanced properties appear in IntelliSense and the Properties Window with the Hide Advanced Members setting under Tools   Options   Text Editor   C   The corresponding EditorBrowsableState is Advanced

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PreviousPage property    The System Web UI Page representing the page that transferred control to the current page    It is very useful

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TrueForAll Method of List lt T gt     List lt int gt  s   new List lt int gt    6  1  2     bool a   s TrueForAll p   gt  p  gt  0

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Aliased generics   using ASimpleName   Dictionary lt string  Dictionary lt string  List lt string gt  gt  gt     It allows you to use ASimpleName  instead of Dictionary lt string  Dictionary lt string  List lt string gt  gt  gt    Use it when you would use the same generic big long complex thing in a lot of places

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The volatile keyword to tell the compiler that a field can be modified by multiple threads concurrently

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Nested classes can access private members of a outer class   public class Outer       private int Value   get  set         public class Inner               protected void ModifyOuterMember Outer outer  int value                        outer Value   value                      And now together with the above feature you can also inherit from nested classes as if they were top level classes as shown below   public class Cheater   Outer Inner       protected void MakeValue5 Outer outer                ModifyOuterMember outer  5             These features allow for some interesting possibilities as far as providing access to particular members via somewhat hidden classes

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I don t think someone has mentioned that appending   after a value type name will make it nullable   You can do    DateTime  date   null    DateTime is a structure

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Several people have mentioned using blocks  but I think they are much more useful than people have realised    Think of them as the poor man s AOP tool   I have a host of simple objects that capture state in the constructor and then restore it in the Dispose   method   That allows me to wrap a piece of functionality in a using block and be sure that the state is restored at the end   For example   using new CursorState this  BusyCursor             Do stuff     CursorState captures the current cursor being used by form  then sets the form to use the cursor supplied   At the end it restores the original cursor   I do loads of things like this  for example capturing the selections and current row on a grid before refreshing and so on

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Array initialization without specifying the array element type   var pets   new      Cat    Dog    Bird

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One interesting thing I ve learned is that different parts of the framework and C  language were written at different times  hence inconsistencies  For example  the framework itself violates many FxCop rules because the rules weren t all in place when the framework was written   Also  the using statement was intended for delinieating  scopes  and not specifically for disposing resources  It was written after the lock statement  Eric Gunnerson once mentioned something along the lines of that if the using statement came first  they might have not needed to write the lock statement  though who knows  maybe they would have anyways   because the using statement might have been sufficient

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Attaching    to a Type to make it nullable  ex  int    c  dir  instead of   C  dir

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I find the use of the conditional break function in Visual Studio very useful  I like the way it allows me to set the value to something that  for example can only be met in rare occasions and from there I can examine the code further

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You can change rounding scheme using   var value   -0 5  var value2   0 5  var value3   1 4   Console WriteLine  Math Round value  MidpointRounding AwayFromZero       out  -1 Console WriteLine Math Round value2  MidpointRounding AwayFromZero      out  1 Console WriteLine Math Round value3  MidpointRounding ToEven      out  1

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I don t condone it  but I was surprised that goto is still around ducks incoming projectiles

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When you need to  a synchronously communicate between objects about occurance of an event there is special purpose interface called ISynchronizeInvoke   Quoting MSDN article  link        Objects that implement this interface can receive notification that an event has occurred  and they can respond to queries about the event  In this way  clients can ensure that one request has been processed before they submit a subsequent request that depends on completion of the first    Here is a generic wrapper   protected void OnEvent lt T gt  EventHandler lt T gt  eventHandler  T args  where T   EventArgs       if  eventHandler    null  return       foreach  EventHandler lt T gt  singleEvent in eventHandler GetInvocationList                  if  singleEvent Target    null  amp  amp  singleEvent Target is ISynchronizeInvoke                        var target    ISynchronizeInvoke singleEvent Target               if  target InvokeRequired                    target BeginInvoke singleEvent  new object     this  args                     continue                                  singleEvent this  args             and here is an example usage   public event EventHandler lt ProgressEventArgs gt  ProgressChanged   private void OnProgressChanged int processed  int total        OnEvent ProgressChanged  new ProgressEventArgs processed  total

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More of a runtime feature  but I recently learned that there are two garbage collectors  The workstation gc and the server gc  Workstation is the default on client versions of windows  but server is much faster on multicore machines     lt configuration gt      lt runtime gt         lt gcServer enabled  true   gt      lt  runtime gt   lt  configuration gt    Be careful  The server gc requires more memory

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Ability to create instance of the type based on the generic parameter like this  new T

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One feature that I only learned about here on Stack Overflow was the ability to set an attribute on the return parameter     AttributeUsage  AttributeTargets ReturnValue    public class CuriosityAttribute Attribute      public class Bar        return  Curiosity      public Bar ReturnANewBar                 return new Bar              This was truly a hidden feature for me  -

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Saw a mention of List ForEach above  2 0 introduced a bevy of predicate-based collection operations - Find  FindAll  Exists  etc   Coupled with anonymous delegates you can almost achieve the simplicity of 3 5 s lambda expressions

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Using     operator with FlagAttribute and enum Sometime we would use Flag attribute with enum to perform bitwise manipulation on the enumeration     Flags   public enum Colors        None    0      Red     1      Blue    2      White   4      Black   8      Green   16      All     31   It seems pretty bad         Notice that  the value of option  All  which in enum is quite strange  Instead of that we can use     operator with flagged enum     Flags   public enum Colors        None    0      Red     1      Blue    2      White   4      Black   8      Green   16      All      0   much better now  that mean 0xffffffff in default

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Nullable GetValueOrDefault

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Lambda Expressions  Func lt int  int  int gt  add    a  b    gt   a   b     Obscure String Formats  Console WriteLine   0 D10    2      0000000002  Dictionary lt string  string gt  dict   new Dictionary lt string  string gt           David    C            Johann    Perl           Morgan    Python       Console WriteLine    0 10   1  10     Programmer    Language      Console WriteLine   -  PadRight  21   -        foreach  string key in dict Keys        Console WriteLine    0  10   1  10    key  dict key

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Easily determine type with which variable was declared  from my answer    using System  using System Collections Generic   static class Program       public static Type GetDeclaredType lt T gt  T x                return typeof T                 Demonstrate how GetDeclaredType works     static void Main string   args                IList lt string gt  iList   new List lt string gt             List lt string gt  list   null           Console WriteLine GetDeclaredType iList  Name           Console WriteLine GetDeclaredType list  Name             Results   IList 1 List 1   And its name  borrowed from  Get variable name     static void Main string   args        Console WriteLine  Name is   0     GetName new  args         Console ReadLine       static string GetName lt T gt  T item  where T   class       var properties   typeof T  GetProperties        return properties 0  Name      Result  Name is  args

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Another note on event handlers  you can simply create a raise extension method like so   public static class EventExtensions       public static void Raise lt T gt  this EventHandler lt T gt   event                                   object sender  T args  where T   EventArgs           if  event   null                 event sender  args                       Then you can use it to raise events   public class MyImportantThing       public event EventHandler lt MyImportantEventEventArgs gt  SomethingHappens              public void Bleh             SomethingHappens Raise this  new MyImportantEventEventArgs   X true               This method has the added advantage of enforcing a coding standard  using EventHandler lt  gt     There isn t a point in writing the same exact function over and over and over again  Perhaps the next version of C  will finally have an InlineAttribute that can be placed on the extension method and will cause the compiler to inline the method definition  which would make this way nearly standard  and the fastest    edit  removed temp variable inside extension method based on comments

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I couldn t see this looking above - one that I didn t realise you could do until recently is to call one constructor from another   class Example       public Example int value1            this value1   Default Value                    public Example int value1  string value2                m Value1   value1          m value2   value2             int m Value1      string m value2

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I find this technique interesting while working with linqxml   public bool GetFooSetting XElement ndef      return  bool  ndef Element  MyBoolSettingValue      true      as opposed to   public bool GetFooSetting XElement ndef      return ndef Element  MyBoolSettingValue      null   bool Parse ndef Element  MyBoolSettingValue       true

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The delegate syntax have evolved over successive versions of C   but I still find them difficult to remember  Fortunately the Action lt  gt  and Func lt  gt  delegates are easy to remember   For example    Action lt int gt  is a delegate method that takes a single int argument and returns void  Func lt int gt   is a delegate method that takes no arguments and returns an int  Func lt int  bool gt  is a delegate method that takes a single int argument and returns a bool    These features were introduced in version 3 5 of the  NET framework

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Keeps DataGridView from showing the property    System ComponentModel Browsable false   public String LastActionID get  private set     Lets you set a friendly display for components  like a DataGrid or DataGridView     System ComponentModel DisplayName  Last Action    public String LastAction get  private set     For your backing variables  if you don t want anything accessing them directly this makes it tougher    System ComponentModel EditorBrowsable System ComponentModel EditorBrowsableState Never       private DataController p dataSources

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A couple of things I like   -If you create an interface similar to   public interface SomeObject lt T gt  where T   SomeObject lt T gt   new     you force anything that inherits from this interface to  contain a parameterless constructor  It is very useful for a  couple of things I ve run across   -Using anonymous types to create a useful object on the fly   var myAwesomeObject   new  Name  Foo   Size 10     -Finally  many Java developers are familiar with syntax like   public synchronized void MySynchronizedMethod       However  in C  this is not valid syntax   The workaround is a method attribute     MethodImpl MethodImplOptions Synchronized    public void MySynchronizedMethod

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The  if DEBUG pre-processor directive  It is Useful for testing and debugging  though I usually prefer to go the unit testing route    string customerName   null   if DEBUG   customerName    Bob   endif   It will only execute code block if Visual Studio is set to compile in  Debug  mode  Otherwise the code block will be ignored by the compiler  and grayed out in Visual Studio

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Expression to initialize a Dictionary in C  3 5   new Dictionary lt string  Int64 gt       Testing   123     Test   125

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You can  use  multiple objects in one using statement   using  Font f1  new Font  Arial   10 0f   f2   new Font  Arial   10 0f            Use f1 and f2      Note that there is already an answer stating that you can do this   using  Font f1  new Font  Arial   10 0f   using  Font f2   new Font  Arial   10 0f            Which is different from mine

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I think a lot of people know about pointers in C but are not sure if it works in C   You can use pointers in C  in an unsafe context   static void Main         int i      unsafe                                 pointer pi has the address of variable i         int  pi    amp i              pointer ppi has the address of variable pi         int   ppi    amp pi             ppi addess of pi  - gt  pi addess of i  - gt  i 0          i   0             dereference the pi  i e   pi is i         Console WriteLine  i    0     pi      output  i   0            since  pi is i  equivalent to i             pi             Console WriteLine  i    0     pi      output  i   1            since  ppi is pi  one more dereference   pi is i             equivalent to i    2           ppi    2          Console WriteLine  i    0     pi     output  i   3           Console ReadLine

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Explicit interface member implementation  wherein an interface member is implemented  but hidden unless the instance is cast to the interface type

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Near all the cool ones have been mentioned   Not sure if this one s well known or not  C  property field constructor initialization   var foo   new Rectangle           Fill   new SolidColorBrush c        Width   20       Height   20       This creates the rectangle  and sets the listed properties   I ve noticed something funny - you can have a comma at the end of the properties list  without it being a syntax error   So this is also valid   var foo   new Rectangle           Fill   new SolidColorBrush c        Width   20       Height   20

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If you want to exit your program without calling any finally blocks or finalizers use FailFast   Environment FailFast

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Partial Methods  Charlie Calvert explains partial methods on his blog  Scott Cate has a nice partial method demo here   Points of extensibility in Code Generated class  LINQ to SQL  EF  Does not get compiled into the dll if it is not implemented  check it out with  NET Reflector

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System Diagnostics Debug Assert  false     will trigger a popup and allow you to attach a debugger to a running  NET process during execution  Very useful for those times when for some reason you can t directly debug an ASP NET application

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Literals can be used as variables of that type  eg   Console WriteLine 5 ToString     Console WriteLine 5M GetType          Returns  System Decimal  Console WriteLine  This is a string     Replace                Just a bit of trivia     There s quite a few things people haven t mentioned  but they have mostly to do with unsafe constructs   Here s one that can be used by  regular  code though   The checked unchecked keywords   public static int UncheckedAddition int a  int b        unchecked   return a   b       public static int CheckedAddition int a  int b        checked   return a   b       or  return checked a   b       public static void Main          Console WriteLine  Unchecked      UncheckedAddition Int32 MaxValue    1         Wraps around      Console WriteLine  Checked      CheckedAddition Int32 MaxValue    1        Throws an Overflow exception     Console ReadLine

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Thought about  dp AnonCast and decided to try it out a bit  Here s what I come up with that might be useful to some      using the concepts of dp s AnonCast static Func lt T gt  TypeCurry lt T gt  Func lt object gt  f  T type        return      gt   T f        And here s how it might be used   static void Main string   args         var getRandomObjectX   TypeCurry GetRandomObject          new   Name   default string   Badges   default int           do            var obj   getRandomObjectX             Console WriteLine  Name    0  Badges    1                obj Name              obj Badges          while  Console ReadKey   Key    ConsoleKey Escape       static Random r   new Random    static object GetRandomObject         return new           Name   Guid NewGuid   ToString   Substring 0  4           Badges   r Next 0  100

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The params keyword  i e   public void DoSomething params string   theStrings      foreach string s in theStrings             Something with the Strings            Called like  DoSomething    The        cat        sat        on        the        mat

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System Runtime Remoting Proxies RealProxy  It enables Aspect Oriented Programming in C   and you can also do a lot of other fancy stuff with it

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Labeling my endregions      region stuff1   region stuff1a          endregion stuff1a  endregion stuff1

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One thing not many people know about are some of the C -introduced preprocessor directives   You can use  error This is an error  to generate a compiler error and  warning This is a warning   I usually use these when I m developing with a top-down approach as a  todo  list   I ll  error Implement this function  or  warning Eventually implement this corner case as a reminder

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How about the FlagsAttribute on an enumeration  It allows you to perform bitwise operations    took me forever to find out how to do bitwise operations in  NET nicely

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You can create delegates from extension methods as if they were regular methods  currying the this parameter   For example    static class FunnyExtension       public static string Double this string str    return str   str        public static int Double this int num    return num   num        Func lt string gt  aaMaker    a  Double  Func lt string  string gt  doubler   FunnyExtension Double   Console WriteLine aaMaker             Prints  aa  Console WriteLine doubler  b          Prints  bb    Note that this won t work on extension methods that extend  a value type  see this question

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Math Max and Min to check boundaries  I  ve seen this in a lot of code   if  x  lt  lowerBoundary        x   lowerBoundary      I find this smaller  cleaner and more readable   x   Math Max x  lowerBoundary     Or you can also use a ternary operator   x     x  lt  lowerBoundary    lowerBoundary   x

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Honestly the experts by the very definition should know this stuff   But to answer your question  Built-In Types Table  C  Reference   The compiler flagging for numbers are widely known for these   Decimal   M Float   F Double   D     for example double d   30D    However these are more obscure   Long   L Unsigned Long   UL Unsigned Int   U

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I like the keyword continue   If you hit a condition in a loop and don t want to do anything but advance the loop just stick in  continue     E g    foreach object o in ACollection      if NotInterested       continue

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true and false operators are really weird   More comprehensive example can be found here   Edit  There is related SO question What   s the false operator in C  good for

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You can use generics to check  compile time  if a method argument implements two interfaces   interface IPropA        string PropA   get  set        interface IPropB        string PropB   get  set       class TestClass        void DoSomething lt T gt  T t  where T   IPropA  IPropB                MessageBox Show t PropA           MessageBox Show t PropB             Same with an argument that is inherited from a base class and an interface

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Here s a useful one for regular expressions and file paths    c   program files  oldway    c  program file newway    The   tells the compiler to ignore any escape characters in a string

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lomaxx I also learned the other day  the same time I learned your tip  is that you can now have disparate access levels on the same property   public string Name   get  private set     That way only the class itself can set the Name property   public MyClass string name    Name   name

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Nesting Using Statements  Usually we do it like this   StringBuilder sb   new StringBuilder    using  StringWriter sw   new StringWriter          using  IndentedTextWriter itw   new IndentedTextWriter sw                            But we can do it this way   StringBuilder sb   new StringBuilder    using  StringWriter sw   new StringWriter    using  IndentedTextWriter itw   new IndentedTextWriter sw

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ContextBoundObject  Not so much a C  thing as a  NET thing  It s another way of achieving DI although it can be hardwork  And you have to inherit from it which can be off putting   http   msdn microsoft com en-us library system contextboundobject aspx  I ve used it to add logging when I decorate a class method with a custom logging attribute

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When debugging  you can type  exception in the Watch QuickWatch Immediate window and get all the info on the exception of the current frame   This is very useful if you ve got 1st chance exceptions turned on

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I just wanted to copy that code without the comments  So  the trick is to simply press the Alt button  and then highlight the rectangle you like  e  g  below    protected void GridView1 RowCommand object sender  GridViewCommandEventArgs e                  if  e CommandName     sel                             lblCat Text   e CommandArgument ToString                        In the above code if I want to select    e CommandName     sel   lblCat Text   e Comman   Then I press ALt key and select the rectangle and no need to uncomment the lines   Check this out

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I tend to find that most C  developers don t know about  nullable  types  Basically  primitives that can have a null value   double  num1   null   double num2   num1    -100    Set a nullable double  num1  to null  then set a regular double  num2  to num1 or -100 if num1 was null   http   msdn microsoft com en-us library 1t3y8s4s VS 80  aspx  one more thing about Nullable type   DateTime  tmp   new DateTime    tmp   null  return tmp ToString      it is return String Empty  Check this link for more details

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If you want to prevent the garbage collector from running the finalizer of an object  just use GC SuppressFinalize object    In a similar vein  GC KeepAlive object   will prevent the garbage collector from collecting that object by referencing it  Not very commonly used  at least in my experience  but nice to know just in  case

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Apologies for posting so late  I am new to Stack Overflow so missed the earlier opportunity   I find that EventHandler lt T gt  is a great feature of the framework that is underutilised    Most C  developers I come across still define a custom event handler delegate when they are defining custom events  which is simply not necessary anymore   Instead of   public delegate void MyCustomEventHandler object sender  MyCustomEventArgs e    public class MyCustomEventClass        public event MyCustomEventHandler MyCustomEvent      you can go   public class MyCustomEventClass        public event EventHandler lt MyCustomEventArgs gt  MyCustomEvent      which is a lot more concise  plus you don t get into the dilemma of whether to put the delegate in the  cs file for the class that contains the event  or the EventArgs derived class

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Mixins  Basically  if you want to add a feature to several classes  but cannot use one base class for all of them  get each class to implement an interface  with no members   Then  write an extension method for the interface  i e   public static DeepCopy this IPrototype p            Of course  some clarity is sacrificed  But it works

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I didn t know the  as  keyword for quite a while   MyClass myObject    MyClass  obj    vs  MyClass myObject   obj as MyClass    The second will return null if obj isn t a MyClass  rather than throw a class cast exception

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The Yield keyword is often overlooked when it has a lot of power  I blogged about it awhile ago and discussed benefits  differed processing  and happens under the hood of yield to help give a stronger understanding   Using Yield in C

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arglist as well   DllImport  msvcrt40 dll    public static extern int printf string format    arglist    static void Main string   args       printf  Hello  s  n     arglist  Bart

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I think if you have to use nullable   types  it s better to use Nullable lt  T    rather than the question mark   notation  It makes it eye-achingly   obvious that magic is occurring  Not   sure why anyone would ever want to use   Nullable lt  bool  though    In a VB NET Web service where the parameter might not be passed through  because the partners request wasn t consistent or reliable   but had to pass validation against the proposed type  Boolean for  if is search request    Chalk it up to  another demand by management         and yes  I know some people think it s not the right way to do these things  but IsSearchRequest As Nullable Of Boolean  saved me losing my mind that night

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I quite enjoy implicit generic parameters on functions  For example  if you have   public void DoStuff lt T gt  T value     Instead of calling it like this   DoStuff lt int gt  5     You can   DoStuff 5     And it ll work out the generic type from the parameter s type     This doesn t work if you re calling the method through reflection  I remember having some weird problems on Mono

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ConditionalAttribute  Allows you to tell the compiler to omit the call to the method marked with the attribute under certain conditions   define    The fact that the method call is omitted also means that its parameters are not evaluated  This is very handy and it s what allows you to call expensive validation functions in Debug Assert   and not worry about them slowing down your release build

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Before lambda comes into play  it s anonymous delegate  That could be used for blanket code similar to Ruby s blockgiven  I haven t tested how lambda works though because I want to stick with  NET 2 0 so far   For example when you want to make sure you remember to close your HTML tags   MyHtmlWriter writer new MyHtmlWriter    writer writeTag  html      delegate             writer writeTag  head           delegate                         writer writeTag  title                             I am sure if lambda is an option  that could yield much cleaner code

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Don t know if this is a secret per se but I loved the added Enumerable  adds to IEnumerable  class in System Linq   http   msdn microsoft com en-us library system linq enumerable members aspx  While the yield keyword is already listed  Iterator blocks are freaking amazing  I used them to build Lists that would be tested to see if they were co-prime  It basically allows you to go though a function that returns values one by one and stop any time   Oh  I almost forgot the best class in the world when you can t optimize it any more  The BackgroundWorker      http   msdn microsoft com en-us library system componentmodel backgroundworker aspx

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Only for reference - enum binary operations using the extension method   using System  using System Collections Generic  using System Linq  using System Text  using System Linq Expressions   namespace BinaryOpGenericTest        Flags      enum MyFlags               A   1          B   2          C   4             static class EnumExtensions               private static Dictionary lt Type  Delegate gt  m operations   new Dictionary lt Type  Delegate gt              public static bool IsFlagSet lt T gt  this T firstOperand  T secondOperand                                                     where T   struct                        Type enumType   typeof T                 if   enumType IsEnum                                throw new InvalidOperationException  Enum type parameter required                                Delegate funcImplementorBase   null              m operations TryGetValue enumType  out funcImplementorBase                Func lt T  T  bool gt  funcImplementor   funcImplementorBase as Func lt T  T  bool gt                if  funcImplementor    null                                funcImplementor   buildFuncImplementor secondOperand                                return funcImplementor firstOperand  secondOperand                       private static Func lt T  T  bool gt  buildFuncImplementor lt T gt  T val                                                              where T   struct                       var first   Expression Parameter val GetType     first                var second   Expression Parameter val GetType     second                 Expression convertSecondExpresion   Expression Convert second  typeof int                var andOperator   Expression Lambda lt Func lt T  T  bool gt  gt  Expression Equal                                                                                                         Expression And                                                                                                              Expression Convert first  typeof int                                                                                                                 convertSecondExpresion                                                                                                          convertSecondExpresion                                                                                                new     first  second                 Func lt T  T  bool gt  andOperatorFunc   andOperator Compile                m operations typeof T     andOperatorFunc              return andOperatorFunc                        class Program               static void Main string   args                        MyFlags flag   MyFlags A   MyFlags B               Console WriteLine flag IsFlagSet MyFlags A                            Console WriteLine EnumExtensions IsFlagSet flag  MyFlags C                Console ReadLine

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One that I just learned recently is that you can still call methods on a nullable value      It turns out what when you have a nullable value   decimal  MyValue   null    where you might think you would have to write   MyValue    null   null   MyValue  ToString     you can instead write   MyValue ToString     I ve been aware that I could call MyValue HasValue and MyValue Value   but it didn t fully click that I could call ToString

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Not sure Microsoft would like this question  especially with so many responses   I m sure I once heard a Microsoft head say      a hidden feature is a wasted feature       or something to that effect

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Not hidden  but I think that a lot of developers are not using the HasValue and Value properties on the nullable types           int  x   null          int y          if  x HasValue              y   x Value

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new modifier  Usage of the  new  modifier in C  is not exactly hidden but it s not often seen  The new modifier comes in handy when you need to  hide  base class members and not always override them  This means when you cast the derived class as the base class then the  hidden  method becomes visible and is called instead of the same method in the derived class   It is easier to see in code   public class BaseFoo       virtual public void DoSomething                 Console WriteLine  Foo             public class DerivedFoo   BaseFoo       public new void DoSomething                 Console WriteLine  Bar             public class DerivedBar   BaseFoo       public override void DoSomething                 Console WriteLine  FooBar             class Program       static void Main string   args                BaseFoo derivedBarAsBaseFoo   new DerivedBar            BaseFoo derivedFooAsBaseFoo   new DerivedFoo             DerivedFoo derivedFoo   new DerivedFoo             derivedFooAsBaseFoo DoSomething      Prints  Foo  when you might expect  Bar          derivedBarAsBaseFoo DoSomething      Prints  FooBar           derivedFoo DoSomething      Prints  Bar             Ed  Do I get extra points for puns  Sorry  couldn t be helped

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Instead of using int TryParse   or Convert ToInt32    I like having a static integer parsing function that returns null when it can t parse   Then I can use    and the ternary operator together to more clearly ensure my declaration and initialization are all done on one line in a easy-to-understand way      public static class Parser       public static int  ParseInt string s            int result          bool parsed   int TryParse s  out result           if  parsed  return result          else return null                       This is also good to avoid duplicating the left side of an assignment  but even better to avoid duplicating long calls on the right side of an assignment  such as a database calls in the following example   Instead of ugly if-then trees  which I run into often    int x   0  YourDatabaseResultSet data   new YourDatabaseResultSet    if  cond1      if  int TryParse x input  x            data   YourDatabaseAccessMethod  my proc name   2  x             else          x   -1             do something to report  Can t Parse              else       x   y      data   YourDatabaseAccessMethod  my proc name           new SqlParameter   param1   2          new SqlParameter   param2   x        You can do   int x   cond1    Parser ParseInt x input     -1    y  if  x  gt   0   data   YourDatabaseAccessMethod  my proc name        new SqlParameter   param1   2       new SqlParameter   param2   x      Much cleaner and easier to understand

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C  3 0 s LINQ query comprehensions are full-blown monadic comprehensions a la Haskell  in fact they were designed by one of Haskell s designers   They will work for any generic type that follows the  LINQ pattern  and allows you to write in a pure monadic functional style  which means that all of your variables are immutable  as if the only variables you used were IDisposables and IEnumerables in using and foreach statements   This is helpful for keeping variable declarations close to where they re used and making sure that all side-effects are explicitly declared  if there are any at all    interface IFoo lt T gt      T Bar  get         class MyFoo lt T gt    IFoo lt T gt       public MyFoo T t   Bar   t       public T Bar  get  private set          static class Foo      public static IFoo lt T gt  ToFoo lt T gt  this T t   return new MyFoo lt T gt  t         public static void Do lt T gt  this T t  Action lt T gt  a    a t         public static IFoo lt U gt  Select lt T U gt  this IFoo lt T gt  foo  Func lt T U gt  f          return f foo Bar  ToFoo                             using  var file   File OpenRead  objc h       var x   from f in file ToFoo              let s   new Scanner f             let p   new Parser  scanner   s             select p Parse        x Do p   gt            drop into imperative code to handle file           in Foo monad if necessary

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Regarding foreach  It does not use  duck typing   as duck typing IMO refers to a runtime check  It uses structural type checking  as opposed to nominal  at compile time to check for the required method in the type

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Arbitrary nested scopes          1  For finer scoping behaviour    anywhere inside members      using only braces      with no control statement     void MyWritingMethod          int sameAge   35             scope some work         string name    Joe           Log Write name   sameAge ToString                      scope some other work         string name    Susan           Log Write name   sameAge ToString                   I ll never mix up Joe and Susan again     Inside large  confusing or archaic members  not that they should ever exist  however   it helps me prevent against using wrong variable names  Scope stuff to finer levels      2  For code beautification or visual semantics  For example  this XML writing code follows the indentation level of the actual generated XML  i e  Visual Studio will indent the scoping braces accordingly   XmlWriter xw   new XmlWriter          lt root gt  xw WriteStartElement  root             lt game gt      xw WriteStartElement  game                     lt score gt   lt  score gt          for  int i   0  i  lt  scores Length    i     multiple scores             xw WriteElementString  score   scores i  ToString                   lt  game gt      xw WriteEndElement         lt  root gt  xw WriteEndElement      3  Mimic a  with  statement   Also another use to keep temp work out of the main scope  Provided by Patrik   sometimes used to mimic the VB  with-statement  in C    var somePerson   this GetPerson        whatever         var p   somePerson       p FirstName    John        p LastName    Doe                   p City    Gotham         For the discerning programmer

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String interning  This is one that I haven t seen come up in this discussion yet  It s a little obscure  but in certain conditions it can be useful   The CLR keeps a table of references to literal strings  and programmatically interned strings   If you use the same string in several places in your code it will be stored once in the table  This can ease the amount of memory required for allocating strings   You can test if a string is interned by using String IsInterned string  and you can intern a string using String Intern string    Note  The CLR can hold a reference to an interned string after application or even AppDomain end  See the MSDN documentation for details

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Not a C  specific thing  but I am a ternary operations junkie   Instead of   if  boolean Condition          Do Function   else         Do something else     you can use a succinct  booleanCondtion   true operation   false operation    e g   Instead of  int value   param  if  doubleValue        value    2    else       value    3      you can type  int value   param    tripleValue   3   2     It does help write succinct code  but nesting the damn things can be nasty  and they can be used for evil  but I love the little suckers nonetheless

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Framework Feature  I don t know but I was quite suprised about VisualStyleRenderer and the whole System Windows Forms VisualStyles-Namespace  Pretty cool

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You can combine the protected and internal accessor to make it public within the same assembly  but protected in a diffrent assembly  This can be used on fields  properties  method and even constants

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Extension methods can be called on null  this will not cause a NullReferenceException to be thrown   Example application  you can define an alternative for ToString   called ToStringOrEmpty   which will return the empty string when called on null

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Falling through switch-cases can be achieved by having no code in a case  see case 0   or using the special goto case  see case 1  or goto default  see case 2  forms   switch                 case 0     shares the exact same code as case 1     case 1             do something         goto case 2      case 2             do something else         goto default      default             do something entirely different         break

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I apologize if this one has been mentioned  but I use this a lot     An add-in for Visual Studio was developed by Alex Papadimoulis   It s used for pasting regular text as string  string builder  comment or region   http   weblogs asp net alex papadimoulis archive 2004 05 25 Smart-Paster-1 1-Add-In---StringBuilder-and-Better-C 2300 -Handling aspx  In this plugin  I also don t know if this has been mentioned  I noticed that strings are pasted with the string literal prefix       I knew about these  but I didn t know about using a double quote within a literal to escape the quote     For example  string s    A line of text    Environment NewLine    Another with a   quote         can be expressed as  string s     A line of text  Another with a   quote

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Width in string Format    Console WriteLine  Product   0 -7  Price   1 5    product1  price1   Console WriteLine  Product   0 -7  Price   1 5    product2  price2     produces    from  Prabir s Blog   Hidden C  feature

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This relates to static constructors   This is a method for performing static destruction  i e  cleaning up resources when the program quits    First off the class   class StaticDestructor            lt summary gt          The delegate that is invoked when the destructor is called           lt  summary gt      public delegate void Handler        private Handler doDestroy            lt summary gt          Creates a new static destructor with the specified delegate to handle the destruction           lt  summary gt           lt param name  method  gt The delegate that will handle destruction  lt  param gt      public StaticDestructor Handler method                doDestroy   method              StaticDestructor                 doDestroy              Then as a member of the class you wish to have a  static destructor  do   private static readonly StaticDestructor destructor   new StaticDestructor       delegate                   Cleanup here            This will now be called when final garbage collection occurs   This is useful if you absolutely need to free up certain resources   A quick and dirty program exhibiting this behavior   using System   namespace TestStaticDestructor       class StaticDestructor               public delegate void Handler            private Handler doDestroy           public StaticDestructor Handler method                        doDestroy   method                      StaticDestructor                         doDestroy                         class SomeClass               static SomeClass                         Console WriteLine  Statically constructed                        static readonly StaticDestructor destructor   new StaticDestructor              delegate                                 Console WriteLine  Statically destructed                                         class Program               static void Main string   args                        SomeClass someClass   new SomeClass                someClass   null              System Threading Thread Sleep 1000                       When the program exits  the  static destructor  is called

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In no particular order   Lists lt  gt  Mutex   The new property definitions shortcut in Framework 3 5

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Everything else  plus   1  implicit generics  why only on methods and not on classes    void GenericMethod lt T gt   T input              Infer type  so GenericMethod lt int gt  23     You don t need the  lt  gt   GenericMethod 23          Is enough    2  simple lambdas with one parameter   x   gt  x ToString     simplify so many calls   3  anonymous types and initialisers     Duck-typed  works with any  Add method  var colours   new Dictionary lt string  string gt           red     ff0000            green     00ff00            blue     0000ff        int   arrayOfInt     1  2  3  4  5        Another one   4  Auto properties can have different scopes   public int MyId   get  private set        Thanks  pzycoman for reminding me   5  Namespace aliases  not that you re likely to need this particular distinction    using web   System Web UI WebControls  using win   System Windows Forms   web  Control aWebControl   new web  Control    win  Control aFormControl   new win  Control

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OK  it may seem obvious  but I want to mention the Object Equals method  the static one  with two arguments    I m pretty sure many people don t even know about it  or forget it s there  yet it can really help in some cases  For instance  when you want to compare two objects for equality  not knowing if they re null  How many times did you write something like that    if   x    y       x    null  amp  amp  y    null   amp  amp  x Equals y                  When you can just write    if  Object Equals x  y                  Object Equals is actually implemented exactly like in the first code sample

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Here are some interesting hidden C  features  in the form of undocumented C  keywords     makeref    reftype    refvalue    arglist   These are undocumented C  keywords  even Visual Studio recognizes them   that were added to for a more efficient boxing unboxing prior to generics  They work in coordination with the System TypedReference struct   There s also   arglist  which is used for variable length parameter lists   One thing folks don t know much about is System WeakReference -- a very useful class that keeps track of an object but still allows the garbage collector to collect it   The most useful  hidden  feature would be the yield return keyword  It s not really hidden  but a lot of folks don t know about it  LINQ is built atop this  it allows for delay-executed queries by generating a state machine under the hood  Raymond Chen recently posted about the internal  gritty details

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Method groups aren t well known   Given   Func lt Func lt int int gt  int int gt  myFunc1    i  j    gt  i j   Func lt int  int gt  myFunc2   i   gt  i   2    You can do this   var x   myFunc1 myFunc2  1     instead of this   var x   myFunc1 z   gt  myFunc2 z   1

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- coalescing operator   using  statement   directive  - great keyword that can be used for more than just calling Dispose   readonly - should be used more   netmodules - too bad there s no support in Visual Studio

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Brad Barker  I think if you have to use nullable types  it s better to use Nullable lt  T  rather than the question mark notation   It makes it eye-achingly obvious that magic is occurring  Not sure why anyone would ever want to use Nullable lt  bool  though   -   Krzysztof Cwalina  one of the authors of Framwork Design Guidlines  has a good post here  http   blogs msdn com kcwalina archive 2008 07 16 Nullable aspx  And Mike Hadlow has a nice post on Nullability Voodoo

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Static constructors   Instances   public class Example       static Example                    Code to execute during type initialization            public Example                    Code to execute during object initialization           Static classes   public static class Example       static Example                    Code to execute during type initialization           MSDN says      A static constructor is used to initialize any static data  or to perform a particular action that needs performed once only  It is called automatically before the first instance is created or any static members are referenced    For example   public class MyWebService       public static DateTime StartTime       static MyWebService                 MyWebService StartTime   DateTime Now             public TimeSpan Uptime               get   return DateTime Now - MyWebService StartTime              But  you could also just as easily have done   public class MyWebService       public static DateTime StartTime   DateTime Now       public TimeSpan Uptime               get   return DateTime Now - MyWebService StartTime              So you ll be hard-pressed to find any instance when you actually need to use a static constructor   MSDN offers useful notes on static constructors         A static constructor does not take access modifiers or have parameters    A static constructor is called automatically to initialize the class    before the first instance is created   or any static members are referenced    A static constructor cannot be called directly    The user has no control on when the static constructor is executed in the    program    A typical use of static constructors is when the class is   using a log file and the   constructor is used to write   entries to this file    Static constructors are also useful when creating wrapper classes for   unmanaged code  when the constructor   can call the LoadLibrary method    If a static constructor throws an exception  the runtime will not   invoke it a second time  and the type    will remain uninitialized for the   lifetime of the application domain in    which your program is running       The most important note is that if an error occurs in the static constructor  a TypeIntializationException is thrown and you cannot drill down to the offending line of code  Instead  you have to examine the TypeInitializationException s InnerException member  which is the specific cause

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Easier-on-the-eyes   condensed ORM-mapping using LINQ  Consider this table    MessageId  INT   MessageText  NVARCHAR MAX   MessageDate  DATETIME       And this structure   struct Message       Int32 Id      String Text      DateTime Date        Instead of doing something along the lines of   List lt Message gt  messages   new List lt Message gt      foreach  row in DataTable Rows        var message   new Message               Id   Convert ToInt32 row  MessageId             Text   Convert ToString row  MessageText             Date   Convert ToDateTime row  MessageDate                messages Add message       You can use LINQ and do the same thing with fewer lines of code  and in my opinion  more style  Like so   var messages   DataTable AsEnumerable   Select r   gt  new Message       Id   Convert ToInt32 r  MessageId         Text   Convert ToString r  MessageText         Date   Convert ToDateTime r  MessageDate       ToList      This approach can be nested  just like loops can

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RealProxy lets you create your own proxies for existing types   This is super-advanced and I haven t seen anyone else use it -- which may mean that it s also really not that useful for most folks -- but it s one of those things that s good to know      Basically  the  NET RealProxy class lets you create what is called a transparent proxy to another type   Transparent in this case means that it looks completely like the proxied target object to its client -- but it s really not   it s an instance of your class  which is derived from RealProxy     This lets you apply powerful and comprehensive interception and  intermediation  services between the client and any methods or properties invoked on the real target object   Couple this power with the factory pattern  IoC etc   and you can hand back transparent proxies instead of real objects  allowing you to intercept all calls to the real objects and perform actions before and after each method invocation   In fact  I believe this is the very functionality  NET uses for remoting across app domain  process  and machine boundaries    NET intercepts all access  sends serialized info to the remote object  receives the response  and returns it to your code   Maybe an example will make it clear how this can be useful  I created a reference service stack for my last job as enterprise architect which specified the standard internal composition  the  stack   of any new WCF services across the division   The model mandated that the data access layer for  say  the Foo service implement IDAL lt Foo gt    create a Foo  read a Foo  update a Foo  delete a Foo   Service developers used supplied common code  from me  that would locate and load the required DAL for a service    IDAL lt T gt  GetDAL lt T gt        retrieve data access layer for entity T   Data access strategies in that company had often been  well  performance-challenged   As an architect  I couldn t watch over every service developer to make sure that he she wrote a performant data access layer   But what I could do within the GetDAL factory pattern was create a transparent proxy to the requested DAL  once the common service model code located the DLL and loaded it   and use high-performance timing APIs to profile all calls to any method of the DAL   Ranking laggards then is just a matter of sorting DAL call timings by descending total time   The advantage to this over development profiling  e g  in the IDE  is that it can be done in the production environment as well  to ensure SLAs      Here is an example of test code I wrote for the  entity profiler   which was common code to create a profiling proxy for any type with a single line    Test  Category  ProfileEntity    public void MyTest            this is the object that we want profiled         we would normally pass this around and call        methods on this instance      DALToBeProfiled dal   new DALToBeProfiled            To profile  instead we obtain our proxy        and pass it around instead      DALToBeProfiled dalProxy    DALToBeProfiled EntityProfiler Instance dal           or        DALToBeProfiled dalProxy2   EntityProfiler lt DALToBeProfiled gt  Instance dal           Now use proxy wherever we would have used the original           All methods  timings are automatically recorded        with a high-resolution timer     DoStuffToThisObject dalProxy           Output profiling results     ProfileManager Instance ToConsole        Again  this lets you intercept all methods and properties called by the client on the target object   In your RealProxy-derived class  you have to override Invoke    System ComponentModel EditorBrowsable System ComponentModel EditorBrowsableState Never    SecurityPermission SecurityAction LinkDemand       Flags   SecurityPermissionFlag Infrastructure      per FxCop public override IMessage Invoke IMessage msg        IMethodCallMessage msgMethodCall   msg as IMethodCallMessage      Debug Assert msgMethodCall    null      should not be null - research Invoke if this trips  KWB 2009 05 28         The MethodCallMessageWrapper        provides read write access to the method         call arguments       MethodCallMessageWrapper mc           new MethodCallMessageWrapper msgMethodCall           This is the reflected method base of the called method       MethodInfo mi    MethodInfo mc MethodBase       IMessage retval   null          Pass the call to the method and get our return value     string profileName   ProfileClassName         mi Name       using  ProfileManager Start profileName                 IMessage myReturnMessage              RemotingServices ExecuteMessage  target  msgMethodCall            retval   myReturnMessage             return retval      Isn t it fascinating what  NET can do  The only restriction is that the target type must be derived from MarshalByRefObject   I hope this is helpful to someone

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Some concurrency utilities in the BCL might qualify as hidden features   Things like System Threading Monitor are used internally by the lock keyword  clearly in C  the lock keyword is preferrable  but sometimes it pays to know how things are done at a lower level  I had to lock in C   CLI  so I encased a block of code with calls to Monitor Enter   and Monitor Exit

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Conditional string Format   Applies different formatting to a number depending on whether the number is positive  negative  or zero   string s   string Format   0 positive negative zero    i     e g   string format    000 -   0     string pos   1 ToString format          001 string neg    -1  ToString format       -1 string zer   0 ToString format           0

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Two things I like are Automatic properties so you can collapse your code down even further   private string  name  public string Name       get               return  name            set                name   value            becomes  public string Name   get  set     Also object initializers   Employee emp   new Employee    emp Name    John Smith   emp StartDate   DateTime Now      becomes  Employee emp   new Employee  Name  John Smith   StartDate DateTime Now

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The  TODO  property and the tasks list    TODO   something     Adding that to your code  the spacing is important  throws an item in your task list  and double clicking the item will jump you to the appropriate location in your code

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Don t forget about goto

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My favorite attribute  InternalsVisibleTo  At assembly level you can declare that another assembly can see your internals  For testing purposes this is absolutely wonderful    Stick this in your AssemblyInfo cs or equivalent and your test assembly get full access to all the internal stuff that requires testing    assembly  InternalsVisibleTo  MyLibrary Test  PublicKey 0024   5c042cb      As you can see  the test assembly must have a strong name to gain the trust of the assembly under test   Available in  Net Framework 2 0   Compact Framework 2 0  and XNA Framework 1 0

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I must admit that i m not sure wether this performs better or worse than the normal ASP NET repeater onItemDatabound cast code  but anyway here s my 5 cent   MyObject obj   e Item DataItem as MyObject  if obj    null        Do work

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The built-in  2 0  MethodInvoker delegate is useful when you want to Invoke BeginInvoke inline code   This avoids needing to create an actual delegate and separate method       void FileMessageEvent object sender  MessageEventArgs e                 if  this InvokeRequired    true                        this BeginInvoke  MethodInvoker delegate                         lblMessage Text e Message                        Application DoEvents                                                           Resolves the error    Cannot convert anonymous method to type  System Delegate  because it is not a delegate type

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Pointers in C     They can be used to do in-place string manipulation  This is an unsafe feature so the unsafe keyword is used to mark the region of unsafe code  Also note how the fixed keyword is used to indicate that the memory pointed to is pinned and cannot be moved by the GC  This is essential a pointers point to memory addresses and the GC can move the memory to different address otherwise resulting in an invalid pointer       string str    some string       Console WriteLine str       unsafe               fixed char  s   str                        char  c   s              while  c      0                                  c   Char ToUpper  c                                                           Console WriteLine str     I wouldn t ever do it but just for the sake of this question to demonstrate this feature

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I didn t see this   for         The same as  while  true

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Not sure if this one got mentioned yet but the ThreadStatic attribute is a realy useful one  This makes a static field static just for the current thread    ThreadStatic  private static int  ThreadStaticInteger    You should not include an initializer because it only get executed once for the entire application  you re better off making the field nullable and checking if the value is null before you use it   And one more thing for ASP NET applications threads are reused so if you modify the value it could end up being used for another page request   Still I have found this useful on several occasions  For example in creating a custom transaction class that   using  DbTransaction tran   new DbTransaction          DoQuery             DoQuery                 The DbTransaction constructor sets a ThreadStatic field to its self and resets it to null in the dispose method  DoQuery checks the static field and if    null uses the current transaction if not it defaults to something else  We avoid having to pass the transaction to each method plus it makes it easy to wrap other methods that were not originaly meant to be used with transaction inside a transaction      Just one use

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Type-inference for factory methods  I don t know if this has been posted already  I scanned the first post  couldn t find it    This is best shown with an example  assuming you have this class  to simulate a tuple   in in an attempt to demonstrate all the language features that make this possible I will go through it step by step   public class Tuple lt V1  V2 gt    Tuple       public readonly V1 v1      public readonly V2 v2       public Tuple V1 v1  V2 v2              this v1   v1        this v2   v2            Everyone knows how to create an instance of it  such as   Tuple lt int  string gt  tup   new Tuple lt int  string gt  1   Hello  World       Not exactly rocket science  now we can of course change the type declaration of the variable to var  like this   var tup   new Tuple lt int  string gt  1   Hello  World       Still well known  to digress a bit here s a static method with type parameters  which everyone should be familiar with   public static void Create lt T1  T2 gt             stuff     Calling it is  again common knowledge  done like this   Create lt float  double gt       What most people don t know is that if the arguments to the generic method contains all the types it requires they can be inferred  for example   public static void Create lt T1  T2 gt  T1 a  T2 b           stuff     These two calls are identical   Create lt float  string gt  1 0f   test    Create 1 0f   test      Since T1 and T2 is inferred from the arguments you passed  Combining this knowledge with the var keyword  we can by adding a second static class with a static method  such as   public abstract class Tuple       public static Tuple lt V1  V2 gt  Create lt V1  V2 gt  V1 v1  V2 v2                return new Tuple lt V1  V2 gt  v1  v2             Achieve this effect   var tup   Tuple Create 1   Hello  World       This means that the types of the  variable  tup   the type-parameters of  Create  and the return value of  Create  are all inferred from the types you pass as arguments to Create  The full code looks something like this   public abstract class Tuple       public static Tuple lt V1  V2 gt  Create lt V1  V2 gt  V1 v1  V2 v2                return new Tuple lt V1  V2 gt  v1  v2            public class Tuple lt V1  V2 gt    Tuple       public readonly V1 v1      public readonly V2 v2       public Tuple V1 v1  V2 v2                this v1   v1          this v2   v2              Example usage  var tup   Tuple Create 1   test      Which gives you fully type inferred factory methods everywhere

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Empty blocks with braces are allowed   You can write code like this        service DoTonsOfWork args       It s helpful when you want to try something without a using or try    finally that you ve already written     using var scope   new TransactionScope        service DoTonsOfWork args

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Need to return an empty IEnumerable   public IEnumerable lt T gt  GetEnumerator      yield break

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A couple other attributes from the System Diagnostics namespace are quite helpful   DebuggerBrowsable will let you hide variables from the debugger window  we use it for all private backing variables of exposed properties    Along with that  DebuggerStepThrough makes the debugger step over that code  very useful for dumb properties  probably should be converted to auto-properties if you can take a dependency to the C  3 0 compiler    As an example   DebuggerBrowsable DebuggerBrowsableState Never   private string nickName  public string NickName           DebuggerStepThrough      get   return nickName         DebuggerStepThrough      set   this nickName   value

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I can t comment yet  but note that by default Visual Studio 2008 automatically steps over properties  so the DebuggerStepThrough attribute is no longer needed in that case   Also  I haven t noticed anyone showing how to declare a parameter-less lambda  useful for implementing Action lt  gt        gt  DoSomething x     You should also read up on closures - I m not clever enough to explain them properly  But basically it means that the compiler does clever stuff so that the x in that line of code will still work even if it goes  out of scope  after creating the lambda   I also discovered recently that you can pretend to ignore a lambda parameter   e       gt  DoSomething e    It s not really ignoring it  it s just that   is a valid identifier  So you couldn t ignore both of the parameters like that  but I think it is a kind of neat way to indicate that we don t care about that parameter  typically the EventArgs which is  Empty

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You can store colors in Enum   public enum MyEnumColors   uint       Normal            0xFF9F9F9F      Active            0xFF8EA98A      Error             0xFFFF0000

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I am bit late in this conversation and I would like to contribute the following  It may be a new thing for some developers   public class User       public long UserId   get  set        public String Name   get  set        public String Password   get  set        public String Email   get  set        The usual way to declare and initialize it is with a constructor or like following   User user   new User    user UserId   1  user Name    myname   etc   But I learned following way to initialize it  I know Visual Basic developers will love it because it s like with operator available only in VB NET and not in C  that is as follows   User user   new User         UserId   1      Name    myname       Email    myemail domain com       Password    mypassword

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I m late to this party  so my first choices are already taken   But I didn t see anyone mention this gem yet   Parallel Extensions to the  NET Framework  It has things like replace with Parallel For or foreach with Parallel ForEach    Parallel Sample   In your opinion  how many CLR object can be created in one second   See fallowing example    using System  using System Collections Generic  using System Diagnostics  using System Threading  using System Threading Tasks   namespace ObjectInitSpeedTest      class Program               Note  don t forget to build it in Release mode         static void Main                       normalSpeedTest                          parallelSpeedTest                Console ForegroundColor   ConsoleColor White             Console WriteLine  Press a key                   Console ReadKey                     private static void parallelSpeedTest                       Console ForegroundColor   ConsoleColor Yellow             Console WriteLine  parallelSpeedTest                long totalObjectsCreated   0             long totalElapsedTime   0              var tasks   new List lt Task gt                var processorCount   Environment ProcessorCount              Console WriteLine  Running on  0  cores   processorCount               for  var t   0  t  lt  processorCount  t                                tasks Add Task Factory StartNew                      gt                                      const int reps   1000000000                     var sp   Stopwatch StartNew                       for  var j   0  j  lt  reps    j                                              new object                                            sp Stop                        Interlocked Add ref totalObjectsCreated  reps                      Interlocked Add ref totalElapsedTime  sp ElapsedMilliseconds                                                                   let s complete all the tasks            Task WaitAll tasks ToArray                 Console WriteLine  Created  0 N  objects in 1 sec n    totalObjectsCreated    totalElapsedTime   processorCount     1000                    private static void normalSpeedTest                       Console ForegroundColor   ConsoleColor Green             Console WriteLine  normalSpeedTest                const int reps   1000000000             var sp   Stopwatch StartNew               sp Start               for  var j   0  j  lt  reps    j                              new object                            sp Stop                Console WriteLine  Created  0 N  objects in 1 sec n    reps   sp ElapsedMilliseconds    1000

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I like to use the using directive to rename some classes for easy reading like this      defines a descriptive name for a complexed data type using MyDomainClassList   System Collections Generic List lt    MyProjectNameSpace MyDomainClass gt           MyDomainClassList myList   new MyDomainClassList       instead of  List lt MyDomainClass gt  myList   new List lt MyDomainClass gt          This is also very handy for code maintenance  If you need to change the class name  there is only one place you need to change  Another example   using FloatValue    float     you only need to change it once to decimal  double          FloatValue val1

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C  allows you to add property setter methods to concrete types that implement readonly interface properties even though the interface declaration itself has no property setter  For example   public interface IReadOnlyFoo      object SomeReadOnlyProperty   get        The concrete class looks like this   internal class Foo   IReadOnlyFoo      public object SomeReadOnlyProperty   get  internal set        What s interesting about this is that the Foo class is immutable if you cast it to the IReadOnlyFoo interface      Create a Foo instance Foo foo   new Foo        This statement is legal foo SomeReadOnlyProperty   12345      Make Foo read only IReadOnlyFoo readOnlyFoo   foo      This statement won t compile readOnlyFoo SomeReadOnlyProperty   54321

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Mixins are a nice feature  Basically  mixins let you have concrete code for an interface instead of a class  Then  just implement the interface in a bunch of classes  and you automatically get mixin functionality  For example  to mix in deep copying into several classes  define an interface  internal interface IPrototype lt T gt        Add functionality for this interface  internal static class Prototype     public static T DeepCopy lt T gt  this IPrototype lt T gt  target          T copy      using  var stream   new MemoryStream                var formatter   new BinaryFormatter          formatter Serialize stream   T target         stream Seek 0  SeekOrigin Begin         copy    T  formatter Deserialize stream         stream Close              return copy          Then implement interface in any type to get a mixin

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This isn t a C  specific type  but I just found the ISurrogateSelector and ISerializationSurrogate interfaces --  http   msdn microsoft com en-us library system runtime serialization isurrogateselector aspx  http   msdn microsoft com en-us library system runtime serialization isurrogateselector aspx  Using these in conjunction with the BinaryFormatter allows for non-serializable objects to be serialized via the implementation of a surrogate class   The surrogate pattern is well-understood in computer science  particularly when dealing with the problem of serialization   I think that this implementation is just tucked away as a parameter of the constructor to BinaryFormatter  and that s too bad   Still - VERY hidden

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Well    Don t use it  but a lot of people don t know C  supports the evil goto    static void Example         int i   0  top      Console WriteLine i ToString         if  i    0                i            goto top

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I love using the   character for SQL queries  It keeps the sql nice and formatted and without having to surround each line with a string delimiter   string sql     SELECT firstname  lastname  email                FROM users                WHERE username    username AND password    password

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You can use any Unicode character in C  names  for example   public class MyClass       public string H  rf  ner                 return  Yes  it works              You can even use Unicode escapes  This one is equivalent to the above   public class MyClass       public string H u00e5rf  ner                 return  Yes  it  still  works

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Properties to display when viewing components Properties in design view   private double  Zoom   1    Category  View     Description  The Current Zoom Level    public double Zoom   get   return  Zoom   set    Zoom   value       Makes things a lot easier for other users of your component libraries

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It s not actually a C  hidden feature  but I recently discovered the WeakReference class and was blown away by it  although this may be biased by the fact that it helped me found a solution to a particular problem of mine

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C    CLR    Thread MemoryBarrier  Most people wouldn t have used it and there is some inaccurate information on MSDN  But if you know intricacies then you can do nifty lock-free synchronization  volatile  Thread VolatileRead  Thread VolatileWrite  There are very very few people who gets the use of these and even fewer who understands all the risks they avoid and introduce     ThreadStatic variables  There was only one situation in past few years I ve found that ThreadStatic variables were absolutely god send and indispensable  When you want to do something for entire call chain  for example  they are very useful  fixed keyword  It s a hidden weapon when you want to make access to elements of large array almost as fast as C    by default C  enforces bound checks that slows down things   default typeName  keyword can be used outside of generic class as well  It s useful to create empty copy of struct  One of the handy feature I use is DataRow columnName  ToString   always returns non-null value  If value in database was NULL  you get empty string  Use Debugger object to break automatically when you want developer s attention even if s he hasn t enabled automatic break on exception      if DEBUG       if  Debugger IsAttached            Debugger Break       endif    You can alias complicated ugly looking generic types so you don t have to copy paste them again and again  Also you can make changes to that type in one place  For example          using ComplicatedDictionary   Dictionary lt int  Dictionary lt string  object        ComplicatedDictionary myDictionary   new ComplicatedDictionary

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The   tells the compiler to ignore any   escape characters in a string    Just wanted to clarify this one    it doesn t tell it to ignore the escape characters  it actually tells the compiler to interpret the string as a literal   If you have   string s     cat              dog              fish    it will actually print out as  note that it even includes the whitespace used for indentation    cat              dog              fish

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I just used this one  Set a field null and return it without an intermediate variable   try       return  field    finally        field   null

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Lately I learned about the String Join method  It is really useful when building strings like columns to select by a query

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Separate static fields depending on the generic type of the surrounding class       public class StaticConstrucEx2Outer lt T gt         Will hold a different value depending on the specicified generic type  public T SomeProperty   get  set      static StaticConstrucEx2Outer       Console WriteLine  StaticConstrucEx2Outer     typeof T  Name        public class StaticConstrucEx2Inner lt U  V gt       static StaticConstrucEx2Inner         Console WriteLine  Outer  lt  0  gt    Inner  lt  1  gt  lt  2  gt        typeof T  Name      typeof U  Name      typeof V  Name          public static void FooBar           public class SCInner      static SCInner        Console WriteLine  SCInner init  lt  0  gt    typeof T  Name          public static void FooBar             StaticConstrucEx2Outer lt int gt  StaticConstrucEx2Inner lt string  DateTime gt  FooBar    StaticConstrucEx2Outer lt int gt  SCInner FooBar     StaticConstrucEx2Outer lt string gt  StaticConstrucEx2Inner lt string  DateTime gt  FooBar    StaticConstrucEx2Outer lt string gt  SCInner FooBar     StaticConstrucEx2Outer lt string gt  StaticConstrucEx2Inner lt string  Int16 gt  FooBar    StaticConstrucEx2Outer lt string gt  SCInner FooBar     StaticConstrucEx2Outer lt string gt  StaticConstrucEx2Inner lt string  UInt32 gt  FooBar     StaticConstrucEx2Outer lt long gt  StaticConstrucEx2Inner lt string  UInt32 gt  FooBar      Will produce the following output  Outer  lt Int32 gt    Inner  lt String gt  lt DateTime gt  SCInner init  lt Int32 gt   Outer  lt String gt    Inner  lt String gt  lt DateTime gt  SCInner init  lt String gt   Outer  lt String gt    Inner  lt String gt  lt Int16 gt   Outer  lt String gt    Inner  lt String gt  lt UInt32 gt   Outer  lt Int64 gt    Inner  lt String gt  lt UInt32 gt

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Other underused operators are checked and unchecked   short x   32767       32767 is the max value for short short y   32767  int z1    checked  short  x   y        will throw an OverflowException int z2    unchecked  short  x   y       will return -2 int z3     short  x   y                 will return -2

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This may be pretty basic to database application developers  but it took me a while to realize that null is not the same as DBNull value     You have to use DBNull value when you want to see if a value from a database record is null

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Another way of geting IEnumerable through yield without explicity creating an IEnumerable object  public IEnumerable lt Request gt  SomeMethod IEnumerable lt Request gt  requests        foreach  Request request in requests         yield return DoSomthing request

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Generics and the Curiously-Recurring Template Pattern really help with some static method property declarations   Suppose you are building a class hierarchy   class Base      class Foo  Base      class Bar  Base       Now  you want to declare static methods on your types that should take parameters  or return values  of the same type or static properties of the same type  For example  you want   class Base       public static Base Get                    Return a suitable Base           class Foo  Base       public static Foo Get                    Return a suitable Foo           class Bar  Base       public static Bar Get                    Return a suitable Bar            If these static methods basically all do the same thing  then you have lots of duplicated code on your hands  One solution would be to drop type safety on the return values and to always return type Base  However  if you want type safety  then the solution is to declare the Base as   class Base lt T gt  where T  Base lt T gt        public static T Get lt T gt                     Return a suitable T            and you Foo and Bar as   class Foo  Base lt Foo gt       class Bar  Base lt Bar gt        This way  they will automatically get their copies of the static methods   This also works wonders to encapsulate the Singleton pattern in a base class  I know the code below is not thread-safe  it just to demonstrate a point    public class Singleton lt T gt  where T  Singleton lt T gt   new       public static T Instance   get  private set       static Singleton lt T gt            Instance   new T            I realize that this forces you to have a public parameterless constructor on your singleton subclass but there is no way to avoid that at compile time without a where T  protected new   construct  however one can use reflection to invoke the protected private parameterless constructor of the sub-class at runtime to achieve that

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Returning anonymous types from a method and accessing members without reflection      Useful  probably not  private void foo         var user   AnonCast GetUserTuple    new   Name   default string   Badges   default int          Console WriteLine  Name   0  Badges   1    user Name  user Badges      object GetUserTuple         return new   Name    dp   Badges   5              Using the magic of Type Inference    static T AnonCast lt T gt  object obj  T t       return  T  obj

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From CLR via C        When normalizing strings  it is highly   recommended that you use   ToUpperInvariant instead of   ToLowerInvariant because Microsoft has   optimized the code for performing   uppercase comparisons    I remember one time my coworker always changed strings to uppercase before comparing  I ve always wondered why he does that because I feel it s more  natural  to convert to lowercase first  After reading the book now I know why

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I call this AutoDebug because you can drop right into debug where and when you need based on a bool value which could also be stored as a project user setting as well   Example     Place at top of your code public UseAutoDebug   true      Place anywhere in your code including catch areas in try catch blocks Debug Assert  this UseAutoDebug     Simply place the above in try catch blocks or other areas of your code and set UseAutoDebug to true or false and drop into debug anytime you wish for testing   You can leave this code in place and toggle this feature on and off when testing  You can also save it as a Project Setting  and manually change it after deployment to get additional bug information from users when if needed as well   You can see a functional and working example of using this technique in this Visual Studio C  Project Template here  where it is used heavily   http   code msdn microsoft com SEHE

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Not really hidden  but useful   When you ve got an enum with flags  you can use shift-left to make things clearer   e g    Flags  public enum ErrorTypes       None                0      MissingPassword     1  lt  lt  0      MissingUsername     1  lt  lt  1      PasswordIncorrect   1  lt  lt  2

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Events are really delegates under the hood and any delegate object can have multiple functions attached to it  and detatched from it using the    and -  operators  respectively   Events can also be controlled with the add remove  similar to get set except they re invoked when    and -  are used   public event EventHandler SelectiveEvent object sender  EventArgs args       add         if  value Target    null  throw new Exception  No static handlers             SelectiveEvent    value             remove         SelectiveEvent -  value             EventHandler  SelectiveEvent

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Instead of doing something cheesy like this   Console WriteLine   0  item s  found    count     I use the following inline trick   Console WriteLine   0  item 1  found    count  count  1         s      This will display  item  when there s one item or  items  when there are more  or less  than 1  Not much effort for a little bit of professionalism

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field  NonSerialized  public event EventHandler Event    This way  the event listener is not serialized   Just  NonSerialized  does not work  because NonSerializedAttribute can only be applied to fields

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I just found out about this one today -- and I ve been working with C  for 5 years   It s the namespace alias qualifier   extern alias YourAliasHere    You can use it to load multiple versions of the same type  This can be useful in maintenance or upgrade scenarios where you have an updated version of your type that won t work in some old code  but you need to upgrade it to the new version  Slap on a namespace alias qualifier  and the compiler will let you have both types in your code

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What about IObservable   Pretty much everybody knows IEnumerable but their mathematical dual seems to be unknown IObservable  Maybe because its new in  NET 4   What it does is instead of pulling the information  like an enumerable  it pushes information to the subscriber s  of the observerable   Together with the Rx extensions it will change how we deal with events  Just to illustrate how powerful it is check a very short example here

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Programmers moving from C C   may miss this one   In C      modulus operator  works on floats

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From Rick Strahl   You can chain the    operator so that you can do a bunch of null comparisons   string result   value1    value2    value3    String Empty

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Here s one I discovered recently which has been useful   Microsoft VisualBasic Logging FileLogTraceListener   MSDN Link  This is a TraceListener implementation which has a lot of features  such as automatic log file roll over  which I previously would use a custom logging framework for   The nice thing is that it is a core part of  NET and is integrated with the Trace framework  so its easy to pick up and use immediately   This is  hidden  because its in the Microsoft VisualBasic assembly    but you can use it from C  as well

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I m pretty sure everyone is familiar with operator overloading  but maybe some aren t   class myClass       private string myClassValue            public myClass string myString                myClassValue   myString             public override string ToString                 return myClassValue             public static myClass operator  lt  lt  myClass mc  int shiftLen                string newString               for  int i   shiftLen  i  lt  mc myClassValue Length  i                newString    mc myClassValue i  ToString            mc myClassValue   newString ToString            return mc             public static myClass operator  gt  gt  myClass mc  int shiftLen                char   newString   new char shiftLen   mc myClassValue Length            for  int i   shiftLen  i  lt  mc myClassValue Length  i                newString i     mc myClassValue i - shiftLen            mc myClassValue   new string newString           return mc             public static myClass operator   myClass mc  string args                if  args Trim   Length  gt  1              mc myClassValue    args          return mc             public static myClass operator - myClass mc  string args                if  args Trim   Length  gt  1                        Regex rgx   new Regex args               mc myClassValue   rgx Replace mc myClassValue                         return mc            I think it s pretty cool to be able to shift a string left and right using  lt  lt  and    or to remove a set of strings that follow a regular expression pattern using -   myClass tmpClass   new myClass    HelloWorld123    tmpClass -    World   tmpClass  lt  lt   2  Console WriteLine tmpClass

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The Action and Func delegate helpers in conjunction with lambda methods  I use these for simple patterns that need a delegate to improve readability  For example  a simple caching pattern would be to check if the requested object exists in the cache  If it does exist  return the cached object  If it doesn t exist  generate a new instance  cache the new instance and return the new instance  Rather that write this code 1000 times for each object I may store retrieve from the cache I can write a simple pattern method like so     private static T CachePattern lt T gt  string key  Func lt T gt  create  where T   class       if  cache key     null                cache Add key  create                return cache key  as T          then I can greatly simplify my cache get set code by using the following in my custom cache manager  public static IUser CurrentUser       get               return CachePattern lt IUser gt   CurrentUserKey        gt  repository NewUpUser               Now simple  everyday  code patterns can be written once and reused much more easily IMHO  I don t have to go write a delegate type and figure out how I want to implement a callback  etc  If I can write it in 10 seconds I m much less apt  to resort to cutting pasting simple code patterns whether they be lazy initialization and some other examples shown above

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lainMH   Nullable booleans are useful when retrieving values from a database that are nullable and when putting values back in  Sometimes you want to know the field has not been set

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When defining custom attributes you can use them with  MyAttAttribute  or with  MyAtt   When classes exist for both writings  then a compilation error occures   The   special character can be used to distinguish between them    AttributeUsage AttributeTargets All   public class X  Attribute      AttributeUsage AttributeTargets All   public class XAttribute  Attribute      X          Error  ambiguity class Class1      XAttribute       Refers to XAttribute class Class2       X          Refers to X class Class3       XAttribute       Refers to XAttribute class Class4

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Also useful  but not commonly used   Constrained Execution Regions   A quote from BCL Team blog       Constrained execution regions  CER s    exist to help a developer write her   code to maintain consistency   The CLR   doesn t guarantee that the developer s   code is correct  but the CLR does   hoist all of the runtime-induced   failure points  ie  async exceptions    to either before the code runs  or   after it has completed   Combined with   constraints on what the developer can   put in a CER  these are a useful way   of making strong guarantees about   whether your code will execute   CER s   are eagerly prepared  meaning that   when we see one  we will eagerly JIT   any code found in its   statically-discoverable call graph     If the CLR s host cares about stack   overflow  we ll probe for some amount   of stack space as well  though perhaps   not enough stack space for any   arbitrary method     We also delay   thread aborts until the CER has   finished running    It can be useful when making edits to more than one field of a data structure in an atomic fashion  So it helps to have transactions on objects   Also CriticalFinalizerObject seems to be hidden at least who are not writing unsafe code   A CriticalFinalizerObject guarantees that garbage collection will execute the finalizer  Upon allocation  the finalizer and its call graph are prepared in advance

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I am so so late to this question  but I wanted to add a few that I don t think have been covered  These aren t C -specific  but I think they re worthy of mention for any C  developer   AmbientValueAttribute This is similar to DefaultValueAttribute  but instead of providing the value that a property defaults to  it provides the value that a property uses to decide whether to request its value from somewhere else  For example  for many controls in WinForms  their ForeColor and BackColor properties have an AmbientValue of Color Empty so that they know to get their colors from their parent control   IsolatedStorageSettings This is a Silverlight one  The framework handily includes this sealed class for providing settings persistence at both the per-application and per-site level   Flag interaction with extension methods  Using extension methods  flag enumeration use can be a lot more readable       public static bool Contains            this MyEnumType enumValue            MyEnumType flagValue                return   enumValue  amp  flagValue     flagValue              public static bool ContainsAny            this MyEnumType enumValue            MyEnumType flagValue                return   enumValue  amp  flagValue   gt  0           This makes checks for flag values nice and easy to read and write  Of course  it would be nicer if we could use generics and enforce T to be an enum  but that isn t allowed  Perhaps dynamic will make this easier

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Ed  I m a bit reticent about posting this as it s little more than nitpicking  However  I would point out that in your code sample   MyClass c    if  obj is MyClass      c   obj as MyClass   If you re going to use  is   why follow it up with a safe cast using  as   If you ve ascertained that obj is indeed MyClass  a bog-standard cast   c    MyClass obj      is never going to fail   Similarly  you could just say   MyClass c   obj as MyClass  if c    null               I don t know enough about  NET s innards to be sure  but my instincts tell me that this would cut a maximum of two type casts operations down to a maximum of one  It s hardly likely to break the processing bank either way  personally  I think the latter form looks cleaner too

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If you re trying to use curly brackets inside a String Format expression     int foo   3  string bar    blind mice   String Format    I am in brackets     0   1    foo  bar     Outputs   I am in brackets   3 blind mice

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A lot of this is explained already  in the standard  It s a good read for any beginner as well as expert  it s a lot to read  but it s the official standard  and it s filled with juicy details   Once you fully understand C   it s time to take this further to understand the fundamentals of the Common Language Infrastructure  The architecture and underpinnings of C    I ve met a variety of programmers that don t know the difference between an object and a ValueType except the adherent limitations thereof    Familiarize yourself with these two documents and you ll never become that person

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Generic constraints      Constructor constraint  T has a default empty constructor class Node lt K T gt  where T   new           Reference Value Type constraints   T is a struct public class MyClass lt T gt  where T   struct            T is a reference type public class MyClass lt T gt  where T   class          public class MyClass lt T gt  where T   SomeBaseClass  ISomeInterface

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I didn t discover - for almost a year - that Strongly Typed DataRows contain an Is ColumnName Null   method   For example   Units UnitsDataTable dataTable   new Units UnitsDataTable     foreach  Units UnitsRow row in dataTable Rows        if  row IsPrimaryKeyNull                        if  row IsForeignKeyNull

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My favourite is the  global     keyword to escape namespace hell with some of our 3rd party code providers     Example   global  System Collections Generic List lt global  System String gt  myList       new global  System Collections Generic List lt global  System String gt

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In addition to duncansmart s reply  also extension methods can be used on Framework 2 0  Just add an ExtensionAttribute class under System Runtime CompilerServices namespace and you can use extension methods  only with C  3 0 of course    namespace System Runtime CompilerServices       public class ExtensionAttribute   Attribute

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I think one of the most under-appreciated and lesser-known features of C    NET 3 5  are Expression Trees  especially when combined with Generics and Lambdas  This is an approach to API creation that newer libraries like NInject and Moq are using   For example  let s say that I want to register a method with an API and that API needs to get the method name  Given this class   public class MyClass        public void SomeMethod        Do Something          Before  it was very common to see developers do this with strings and types  or something else largely string-based    RegisterMethod typeof MyClass    SomeMethod      Well  that sucks because of the lack of strong-typing  What if I rename  SomeMethod   Now  in 3 5 however  I can do this in a strongly-typed fashion   RegisterMethod lt MyClass gt  cl   gt  cl SomeMethod       In which the RegisterMethod class uses Expression lt Action lt T gt  gt  like this   void RegisterMethod lt T gt  Expression lt Action lt T gt  gt  action  where T   class       var expression    action Body as MethodCallExpression        if  expression    null                   TODO  Register method         Console WriteLine expression Method Name             This is one big reason that I m in love with Lambdas and Expression Trees right now

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Preprocessor Directives can be nifty if you want different behavior between Debug and Release modes   http   msdn microsoft com en-us library ed8yd1ha aspx

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Dictionary TryGetValue K key  out V value   Works as a check and a get in one  Rather than   if dictionary ContainsKey key          value   dictionary key               you can just do   if dictionary TryGetValue key  out value              and the value has been set

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I have often come across the need to have a generic parameter-object persisted into the viewstate in a base class   public abstract class BaseListControl lt ListType KeyType ParameterType gt                     UserControl                   where ListType   BaseListType                   amp  amp  ParameterType   BaseParameterType  new        private const string viewStateFilterKey    FilterKey        protected ParameterType Filters               get                       if  ViewState viewStateFilterKey     null                  ViewState viewStateFilterKey   new ParameterType                 return ViewState viewStateFilterKey  as ParameterType                    set                       ViewState viewStateFilterKey    value                       Usage   private void SomeEventHappened object sender  EventArgs e        Filters SomeValue   SomeControl SelectedValue     private void TimeToFetchSomeData         GridView DataSource   Repository GetList Filters       This little trick with the  where ParameterType   BaseParameterType  new  is what makes it really work   With this property in my baseclass  I can automate handling of paging  setting filter values to filter a gridview  make sorting really easy  etc   I am really just saying that generics can be an enormously powerful beast in the wrong hands

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David in Dakota   Console WriteLine   -  PadRight  21   -         I used to do this  until I discovered that the String class has a constructor that allows you to do the same thing in a cleaner way   new String  -  22

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How about Expression Trees  They are the heart of LINQ and allow for defered execution   Taken from David Hayden s blog   In C  3 0  you can define a delegate as follows using a lambda expression   Func lt int int gt  f   x   gt  x   1    This delegate is compiled into executable code in your application and can be called as such   var three   f 2      2   1   The code works as you would expect  Nothing fancy here   Expression Trees  When you define the delegate as an Expression Tree by using System Query Expression   Expression lt Func lt int int gt  gt  expression   x   gt  x   1    The delegate is no longer compiled into executable code  but compiled as data that can be converted and compiled into the original delegate   To actually use the delegate represented as an Expression Tree in your application  you would have to compile and invoke it in your application   var originalDelegate   expression Compile     var three   originalDelegate Invoke 2

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This trick for calling private methods using Delegate CreateDelegate is extremely neat    var subject   new Subject    var doSomething    Func lt String  String gt       Delegate CreateDelegate typeof Func lt String  String gt    subject   DoSomething    Console WriteLine doSomething  Hello Freggles       Here s a context where it s useful

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There are operators for performing implicit and explicit user-defined type conversion between the declared class and one or more arbitrary classes  The implicit operator effectively allows the simulation of overloading the assignement operator  which is possible in languages such as C   but not C    It doesn t seem to be a feature one comes across very often  but it is in fact used in the LINQ to XML  System Xml Linq  library  where you can implicitly convert strings to XName objects  Example   XName tagName    x Name     I discovered this feature in this article about how to simulate multiple inheritance in C

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FIXED   Power of Pointers in C  - This topic is too big  but I will just outline simple things    In C we had facility of loading structure like     struct cType     char type 4      int  size     char name 50      char email 100      cType myType  fread file   amp mType  sizeof mType      We can use fixed keyword in  unsafe  method to read byte array aligned structure    Layout LayoutKind Sequential  Pack 1   public unsafe class CType      public fixed byte type 4       public int size      public fixed byte name 50       public fixed byte email 100       Method 1  Reading from regular stream in to byte buffer and mapping byte array to individual bytes of struct   CType mType   new CType    byte   buffer   new byte Marshal SizeOf CType    stream Read buffer 0 buffer Length      you can map your buffer back to your struct    fixed CType  sp    amp mType        byte  bsp    byte   sp      fixed byte  bp    amp buffer                 for int i 0 i lt buffer Length i                              bsp      bp                bsp   bp                         Method 2  you can map Win32 User32 dll s ReadFile to directly read bytes     CType mType   new CType    fixed CType  p    amp mType        User32 ReadFile fileHandle   byte   p  Marshal SizeOf mType  0

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fixed statement  This statement prevents the garbage collector from relocating a movable variable  Fixed can also be used to create fixed size buffers      The fixed statement sets a pointer to a managed variable and  pins  that variable during the execution of statement    stackalloc  The stackalloc allocates a block of memory on the stack

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I didn t start to really appreciate the  using  blocks until recently   They make things so much more tidy

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Full access to the call stack   public static void Main       StackTrace stackTrace   new StackTrace                 get call stack   StackFrame   stackFrames   stackTrace GetFrames        get method calls  frames        write call stack method names   foreach  StackFrame stackFrame in stackFrames          Console WriteLine stackFrame GetMethod   Name        write method name         So  if you ll take the first one - you know what function you are in  If you re creating a helper tracing function - take one before the last one - you ll know your caller

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Here is a new method of the string class in C  4 0   String IsNullOrWhiteSpace String value    It s about time

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Ability to use LINQ Expressions to perform strongly-typed reflection   static void Main string   args      var domain    matrix     Check      gt  domain     Console ReadLine       static void Check lt T gt  Expression lt Func lt T gt  gt  expr      var body     MemberExpression expr Body     Console WriteLine  Name is   0    body Member Name     Console WriteLine  Value is   0      FieldInfo body Member      GetValue   ConstantExpression body Expression  Value          output     Name is   domain     Value is   matrix    More details are available at How to Find Out Variable or Parameter Name in C

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This will not compile   namespace ns       class Class1               Nullable lt int gt  a            The type or namespace name  Nullable  could not be found  are you missing a using directive or an assembly reference    lt -- missing  using System    But  namespace ns       class Class1               int  a            will compile    NET 2 0

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You can put several attributes in one pair of square brackets        OperationContract  ServiceKnownType typeof Prism    ServiceKnownType typeof Cuboid        Shape GetShape

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I didn t find anyone who is using string Join to join strings using a separator  Everyone keeps writing the same ugly for-loop  var sb   new StringBuilder    var count   list Count    for int i   0  i  lt  count  i        if  sb Length  gt  0  sb Append seperator     sb Append list i       return sb ToString      instead of  return string Join separator  list ToArray

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You type  prop  and then press  TAB  twice  it generates useful code for your properties and can speed your typing   I know this works in VS 2005  I use it  but I don  t know in previous versions

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Zero parameter Lambdas      gt Console ReadLine

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One of the most useful features Visual Studio has is  Make object id   It generates an id and  attaches  to the object so wherever you look at the object you will also see the id  regardless of the thread    While debugging right click on the variable tooltip and there you have it  It also works on watched autos locals variables

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Advanced Debugging Display The already mentioned attributes DebuggerDisplay and DebuggerBrowsable control the visibility of elements and the textual value displayed  Simply overriding ToString   will cause the debugger to use the output of that method  If you want more complex output you can use create a Debugger Visualizer  several examples are available here  Son Of Strike Microsoft provide a debugger extension known as SOS  This is an extremely powerful  though often confusing  extension which is an excellent way to diagnose  leaks   more accurately unwanted references to objects no longer required  Symbol Server for framework source Following these instructions will allow you to step through the source of some parts of the framework  Changes in 2010 Several enhancements and new features exist in Visual Studio 2010   Debugging Parallel Tasks Parallel Stacks allow viewing multiple threads call stacks at the same time  Historical Debugging lets you see events and non-local variables back in time  so long as you enable the collection in advance   Potentially a significant change to how you debug things

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The usage of the default keyword in generic code to return the default value for a type   public class GenericList lt T gt        private class Node                              public Node Next          public T Data             private Node head                  public T GetNext                 T temp   default T            Node current   head          if  current    null                        temp   current Data              current   current Next                    return temp            Another example here

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I found that only few developers know about this feature   If you need a method that works with a value-type variable via some interface  implemented by this value type   it s easy to avoid boxing during the method call   Example code   using System  using System Collections   interface IFoo       void Foo      struct MyStructure   IFoo       public void Foo             public static class Program       static void MethodDoesNotBoxArguments lt T gt  T t  where T   IFoo           t Foo              static void Main string   args            MyStructure s   new MyStructure            MethodThatDoesNotBoxArguments s             IL code doesn t contain any box instructions    method private hidebysig static void  MethodDoesNotBoxArguments lt  IFoo  T gt    T t  cil managed        Code size       14  0xe     maxstack  8   IL 0000   ldarga s   t   IL 0002   constrained    T   IL 0008   callvirt   instance void IFoo  Foo     IL 000d   ret      end of method Program  MethodDoesNotBoxArguments   method private hidebysig static void  Main string   args  cil managed      entrypoint      Code size       15  0xf     maxstack  1    locals init   0  valuetype MyStructure s    IL 0000   ldloca s   s   IL 0002   initobj    MyStructure   IL 0008   ldloc 0   IL 0009   call       void Program  MethodDoesNotBoxArguments lt valuetype MyStructure gt    0    IL 000e   ret      end of method Program  Main   See Richter  J  CLR via C   2nd edition  chapter 14  Interfaces  section about Generics and Interface Constraints   See also my answer to another question

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Just learned the joys of  UnmanagedFunctionPointerAttribute CallingConvention CDecl   from trying to interface with an unmanaged C   function library that defined callbacks without   stdcall

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The InternalsVisibleToAttribute specifies that types that are ordinarily visible only within the current assembly are visible to another assembly  Article on msdn

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This one is not  hidden  so much as it is misnamed   A lot of attention is paid to the algorithms  map    reduce   and  filter   What most people don t realize is that  NET 3 5 added all three of these algorithms  but it gave them very SQL-ish names  based on the fact that they re part of LINQ       map     Select Transforms data   from one form into another       reduce     Aggregate Aggregates   values into a single result       filter     Where Filters data   based on a criteria   The ability to use LINQ to do inline work on collections that used to take iteration and conditionals can be incredibly valuable  It s worth learning how all the LINQ extension methods can help make your code much more compact and maintainable

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I didn t knew about Generic methods which could help avoid using Method Overloadding  Below are overloaded methods to print int and double numbers       private static void printNumbers int    intNumbers                 foreach int element in intNumbers                        Console WriteLine element                         private static void printNumbers double   doubleNumbers                foreach  double element in doubleNumbers                        Console WriteLine element                     Generic method which help to have one method for both of the above      private static void printNumbers lt E gt  E    Numbers                foreach  E element in Numbers                        Console WriteLine element

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You can limit the life and thus scope of variables by using     brackets         string test2    3       Console Write test2      Console Write test2     compile error   test2 only lives within the brackets

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I ve read through all seven pages  and I m missing these   String Join  I ve seen a lot of for-loops to convert a list of items to a string with separators  It s always a pain to make sure you doin t start with a separator and don t end with a separator  A built-in method makes this easier   String Join      new String      a    b    c       TODO in comment  Not really a C  feature  more of a Visual Studio feature  When you start your comment with TODO  it s added to your Visual Studio Task List  View -  Task List  Comments      TODO  Implement this  throw new NotImplementedException      Extension methods meets Generics  You can combine extension methods with Generics  when you think of the tip earlier in this topic  you can add extensions to specific interfaces  public static void Process lt T gt  this T item  where T ITest ITest2      Enumerable Range  Just want a list of integers   Enumerable Range 0  15    I ll try to think of some more

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Open generics are another handy feature especially when using Inversion of Control   container RegisterType typeof IRepository lt  gt    typeof NHibernateRepository lt  gt

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Use of   before a string that contains escape char  Basically when a physical path is used to assign in a string variable everybody uses     where escape character is present in a string   e g  string strPath  D  websites web1 images     But escape characters can be ignored using   before the string value    e g   string strPath   D  websites web1 images

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A couple I can think of    field  NonSerialized    public EventHandler event SomeEvent    This prevents the event from being serialised   The  field   indicates that the attribute should be applied to the event s backing field   Another little known feature is overriding the add remove event handlers   public event EventHandler SomeEvent       add                                 remove

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A few hidden features I ve come across    stackalloc which lets you allocate arrays on the stack Anonymous methods with no explicit parameter list  which are implicitly convertible to any delegate type with non-out ref parameters  very handy for events  as noted in an earlier comment  A lot of people aren t aware of what events really are  an add remove pair of methods  like get set for properties   field-like events in C  really declare both a variable and an event The    and    operators can be overloaded to return types other than bool  Strange but true  The query expression translation in C  3 is really  simple  in some ways - which means you can get it to do some very odd things  Nullable types have special boxing behaviour  a null value gets boxed to a null reference  and you can unbox from null to the nullable type too

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My favorite trick is using the null coalesce operator and parentheses to automagically instantiate collections for me   private IList lt Foo gt   foo   public IList lt Foo gt  ListOfFoo        get   return  foo      foo   new List lt Foo gt

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FlagsAttribute  a small but nice feature when using enum to make a bitmasks    Flags  public enum ConfigOptions       None      0      A         1  lt  lt  0      B         1  lt  lt  1      Both      A   B    Console WriteLine  ConfigOptions A ToString      Console WriteLine  ConfigOptions Both ToString         Will print     A    A  B

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string Empty   I know it s not fantastical  ludicrously odd   but I use it all the time instead of      And it s pretty well hidden until someone tells you it s there

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The  region  string  and  endregion pair is very neat for grouping code  outlining     region Using statements using System  using System IO  using       using        endregion   The code block can be compressed to a single describing line of text  Works inside functions aswell

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The C     null coalescing operator -   Not really hidden  but rarely used  Probably because a lot of developers run a mile when they see the conditional   operator  so they run two when they see this one  Used   string mystring   foo     foo was null    rather than  string mystring  if  foo  null      mystring    foo was null   else     mystring   foo

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JavaScript-like anonymous inline-functions  Return a String   var s   new Func lt String gt       gt        return  Hello World            Return a more complex Object   var d   new Func lt Dictionary lt Int32  String gt  gt       gt        return new Dictionary lt Int32  String gt                  0   Foo               1   Bar               2                         A real-world use-case   var tr   new TableRow     tr Cells AddRange       new                 new TableCell   Text                 new TableCell   Text                 new TableCell   Text                  new TableCell                       Text   new Func lt String gt       gt                                return   Result of a chunk of logic  without having to define                          the logic outside of the TableCell constructor                                        new TableCell   Text                 new TableCell   Text                   Note  You cannot re-use variable names inside the inline-function s scope     Alternative syntax     The one-liner Func lt Int32  Int32  String gt  Add    a  b    gt  Convert ToString a   b       Multiple lines Func lt Int32  Int32  String gt  Add    a  b    gt        var i   a   b       return i ToString           Without parameters Func lt String gt  Foo        gt          Without parameters  multiple lines Func lt String gt  Foo        gt        return          Shorten a string and add horizontal ellipsis     Func lt String  String gt  Shorten   s   gt  s Length  gt  100   s Substring 0  100      amp hellip     s

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Most of the P Invoke stuff is a bit strange   Example of attributes    DllImport   gdi32 dll      return   MarshalAs UnmanagedType I4    StructLayout LayoutKind Sequential

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Not sure if this one has been mentioned or not  11 pages     But the OptionalField attribute for classes is amazing when you are versioning classes objects that are going to be serialized   http   msdn microsoft com en-us library ms229752 VS 80  aspx

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ViewState getters can be one-liners   Using a default value   public string Caption       get   return  string   ViewState  Caption       Foo          set   ViewState  Caption     value       public int Index       get   return  int   ViewState  Index      0         set   ViewState  Index     value        Using null as the default   public string Caption       get   return  string  ViewState  Caption          set   ViewState  Caption     value       public int  Index       get   return  int   ViewState  Index          set   ViewState  Index     value        This works for anything backed by a dictionary

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IEnumerable s SelectMany  which flattens a list of lists into a single list  Let s say I have a list of Orders  and each Order has a list of LineItems on that order   I want to know the total number of LineItems sold      int totalItems   Orders Select o   gt  o LineItems  SelectMany i   gt  i  Sum

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On the basis that this thread should be entitled  things you didn t know about C  until recently despite thinking you already knew everything   my personal feature is asynchronous delegates   Until I read Jeff Richter s C  CLR book  excellent book  everyone doing  NET should read it  I didn t know that you could call any delegate using BeginInvoke   EndInvoke   I tend to do a lot of ThreadPool QueueUserWorkItem calls  which I guess is much like what the delegate BeginInvoke is doing internally   but the addition of a standardised join rendezvous pattern may be really useful sometimes

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To test if an IEnumerable lt T gt  is empty with LINQ  use      IEnumerable lt T gt  Any       At first  I was using  IEnumerable lt T gt  Count      0       Which unnecessarily causes all items in the IEnumerable lt T gt  to be enumerated   As an improvement to this  I went on to use  IEnumerable lt T gt  FirstOrDefault      null       Which is better     But IEnumerable lt T gt  Any   is the most succinct and performs the best

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ThreadStaticAttribute is a favorite of mine   Also  NonSerializableAttribute is useful    Can you tell I do a lot of server stuff using remoting

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What about using this    if DEBUG             Console Write  Debugging     else             Console Write  Final     endif   When you have your solution compiled with DEBUG defined it will output  Debugging    If your compile is set to Release it will write  Final

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Object ReferenceEquals Method  Determines whether the specified Object instances are the same instance   Parameters    objA  System Object - The first Object to compare   objB  System Object - The second Object to compare     Example    object o   null   object p   null   object q   new Object      Console WriteLine Object ReferenceEquals o  p     p   q   Console WriteLine Object ReferenceEquals p  q     Console WriteLine Object ReferenceEquals o  p      Difference to      and   Equals    Basically  Equals   tests of object A has the same content as object B   The method System Object ReferenceEquals   always compares references  Although a class can provide its own behavior for the equality operator  below   that re-defined operator isn t invoked if the operator is called via a reference to System Object    For strings there isn t really a difference  because both    and Equals have been overriden to compare the content of the string   See also this answer to another question   How do I check for nulls in an          operator overload without infinite recursion

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Some    weirdness      Delegate target      target0   target as CallTargetWithContext0        target1   target as CallTargetWithContext1        target2   target as CallTargetWithContext2        target3   target as CallTargetWithContext3        target4   target as CallTargetWithContext4        target5   target as CallTargetWithContext5         Delegate  targetN   target as CallTargetWithContextN      Interesting to note the last cast that is needed for some reason  Bug or by design

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I like looking up stuff in a list like -  bool basketContainsFruit string fruit      return new      apple    orange    banana    pear    Contains fruit       Rather than -  bool basketContainsFruit string fruit      return fruit     apple     fruit     orange     fruit     banana         fruit     pear       Doesn t come up that much in practice  but the idea of matching items against the subject of the search can be really quite useful   succinct

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I love the fact that I can use LINQ to objects on plain old  NET 2 0  i e  without requiring  NET 3 5 to be installed everywhere   All you need is an implementation of all the query operator Extension methods - see LINQBridge

[c#] Hidden Features of C#?

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