Creating a generic method in C

Question

I am trying to combine a bunch of similar methods into a generic method   I have several methods that return the value of a querystring  or null if that querystring does not exist or is not in the correct format   This would be easy enough if all the types were natively nullable  but I have to use the nullable generic type for integers and dates   Here s what I have now   However  it will pass back a 0 if a numeric value is invalid  and that unfortunately is a valid value in my scenarios   Can somebody help me out   Thanks   public static T GetQueryString lt T gt  string key  where T   IConvertible       T result   default T        if  String IsNullOrEmpty HttpContext Current Request QueryString key      false                string value   HttpContext Current Request QueryString key            try                       result    T Convert ChangeType value  typeof T                        catch                         Could not convert   Pass back default value                result   default T                        return result

User · Answer

I like to start with a class like this class settings { public int X {get;set;} public string Y { get; set; } // repeat as necessary

 public settings()
 {
    this.X = defaultForX;
    this.Y = defaultForY;
    // repeat ...
 }
 public void Parse(Uri uri)
 {
    // parse values from query string.
    // if you need to distinguish from default vs. specified, add an appropriate property

 }

This has worked well on 100's of projects. You can use one of the many other parsing solutions to parse values.

User · Answer

Convert ChangeType   doesn t correctly handle nullable types or enumerations in  NET 2 0 BCL  I think it s fixed for BCL 4 0 though   Rather than make the outer implementation more complex  make the converter do more work for you  Here s an implementation I use   public static class Converter     public static T ConvertTo lt T  object value          return ConvertTo value  default T           public static T ConvertTo lt T  object value  T defaultValue          if  value    DBNull Value              return defaultValue            return  T  ChangeType value  typeof T           public static object ChangeType object value  Type conversionType          if  conversionType    null              throw new ArgumentNullException  conversionType                  if it s not a nullable type  just pass through the parameters to Convert ChangeType     if  conversionType IsGenericType    conversionType GetGenericTypeDefinition   Equals typeof Nullable lt                     null input returns null output regardless of base type       if  value    null                  return null                    it s a nullable type  and not null  which means it can be converted to its underlying type           so overwrite the passed-in conversion type with this underlying type       conversionType   Nullable GetUnderlyingType conversionType             else if  conversionType IsEnum                 strings require Parse method       if  value is string                  return Enum Parse conversionType   string  value                              primitive types can be instantiated using ToObject       else if  value is int    value is uint    value is short    value is ushort                value is byte    value is sbyte    value is long    value is ulong                  return Enum ToObject conversionType  value                 else                 throw new ArgumentException String Format  Value cannot be converted to  0  - current type is                                    not supported for enum conversions    conversionType FullName                       return Convert ChangeType value  conversionType          Then your implementation of GetQueryString lt T  can be   public static T GetQueryString lt T  string key        T result   default T       string value   HttpContext Current Request QueryString key        if   String IsNullOrEmpty value                 try                       result   Converter ConvertTo lt T  value                       catch                         Could not convert   Pass back default value                result   default T                        return result

User · Answer

You can use sort of Maybe monad  though I d prefer Jay s answer   public class Maybe lt T gt        private readonly T  value       public Maybe T value                 value   value          IsNothing   false             public Maybe                 IsNothing   true             public bool IsNothing   get  private set         public T Value               get                       if  IsNothing                                throw new InvalidOperationException  Value doesn t exist                              return  value                       public override bool Equals object other                if  IsNothing                        return  other    null                     if  other    null                        return false                    return  value Equals other              public override int GetHashCode                 if  IsNothing                        return 0                    return  value GetHashCode               public override string ToString                 if  IsNothing                        return                       return  value ToString               public static implicit operator Maybe lt T gt  T value                return new Maybe lt T gt  value              public static explicit operator T Maybe lt T gt  value                return value Value            Your method would look like       public static Maybe lt T gt  GetQueryString lt T gt  string key  where T   IConvertible               if  String IsNullOrEmpty HttpContext Current Request QueryString key      false                        string value   HttpContext Current Request QueryString key                try                               return  T Convert ChangeType value  typeof T                              catch                                 Could not convert   Pass back default value                    return new Maybe lt T gt                                      return new Maybe lt T gt

User · Answer

What about this  Change the return type from T to Nullable lt T gt   public static Nullable lt T gt  GetQueryString lt T gt  string key  where T   struct  IConvertible                       T result   default T                if  String IsNullOrEmpty HttpContext Current Request QueryString key      false                                string value   HttpContext Current Request QueryString key                    try                                       result    T Convert ChangeType value  typeof T                                        catch                                         Could not convert   Pass back default value                        result   default T                                                return result

User · Answer

I know  I know  but     public static bool TryGetQueryString lt T gt  string key  out T queryString

User · Answer

What if you specified the default value to return  instead of using default T    public static T GetQueryString lt T gt  string key  T defaultValue          It makes calling it easier too   var intValue   GetQueryString  intParm   Int32 MinValue   var strValue   GetQueryString  strParm        var dtmValue   GetQueryString  dtmPatm   DateTime Now      eg use today s date if not specified   The downside being you need magic values to denote invalid missing querystring values

[c#] Creating a generic method in C#

Examples related to c#

Examples related to generics