Casting to nullable generics

Casting to nullable generics

• pecasis

  Yes, I need Nullable<T> but with T taken form the generic, which is not supported as far as I have seen.

  So this cannot be solved in C# 2 I cannot convert a struct into its nullable representation without knowing the struct at design time

  
  

• david.r.johnson

  Yes, I agree, that's fine. The problem is with values, which are not null.

  So the question (from my point of view) is, how can I create an instance of Nullable type with the Type specified

  
  

• man_25

  While you cannot do it directly, you can always do what Comparer<T>.Default does and create specialized classes based on T and use Reflection to bypass the constraints (the compiler could admitedly be better here and provide a syntax for "casting" in constraints, but it's not that big of a deal).
  
  Anyway, what I mean to say is you can do it this way (You may want to add more specializing and possibly checks for DBNull and whatnot, but this does what you were trying to do):
  
  namespace Example {
using System;

internal class ValueConverter<T> {
internal static readonly ValueConverter<T> Instance = Initialize();
private static ValueConverter<T> Initialize() {
Type type = typeof(T);
Type underlyingType = Nullable.GetUnderlyingType(type);

if(underlyingType != null) {
// if T is nullable, use reflection to create an instance of NullableValueConverter
Type converterType = typeof(NullableValueConverter<>);
converterType = converterType.MakeGenericType(underlyingType);
return (ValueConverter<T>)Activator.CreateInstance(converterType);
}
else {
// otherwise use a normal converter
return new ValueConverter<T>();
}
}
internal ValueConverter() { }
internal virtual T ConvertValue(object value) {
return (T)Convert.ChangeType(value, typeof(T));
}
}
internal class NullableValueConverter<T> : ValueConverter<T > where T : struct {
public NullableValueConverter() : base() { }
internal override T ConvertValue(object value) {
if(value == null)
return null;
return (T)Convert.ChangeType(value, typeof(T));
}
}

class Program {

public static T ConvertValue<T>(object value) {
return ValueConverter<T>.Instance.ConvertValue(value);
}

static void Main(string[] args) {
Console.WriteLine(ConvertValue<DateTime >(null));
Console.WriteLine(ConvertValue<DateTime >(DateTime.Now));
Console.WriteLine(ConvertValue<DateTime >(DateTime.Now.ToString()));
Console.WriteLine(ConvertValue<DateTime>(DateTime.Now.ToString()));
}
}
}


  

• Javier Canones

  there is no constructor with nullable type.
  
  

• Adam_W

  you can use nullable<T>,but this only supports struct.
  
  

• yashpal singh

  Hello

  If you have problems with a Datareaders you should take a look at the "safeDatareader" from CSLA from Rocky Lhotka. Its a Datareader that will avoid all the NULL hassle you get if you know that you dont have to really care about NULLs. It is included in its CSLA code which you can download from its website... The datareader works exactly like a normal one with the only difference that it handels NULLs transperently. Also it supports the full datareader interface so it can be used exactly like the normal one... And while we are at the topic of NULL in DBs and .NET you should also look at the smartdate class

  Take a look at this site

  http://www.lhotka.net/

  Hope this helps...

  P.s. If you NEED to handle NULLs then just use the "plain" datareader

  
  
  

• Gautam Goenka

  Unfortunately I'm using the DataReader. But thanks for the tip with DataRow.
  

• IdahoErick

  You know, it may or may not be as readable, but if you know the type of value you're trying to get out of the DataRow, you can always use the "as" operator.
  
  However, if for whatever reason the value doesn't match the type you're casitng to you get null. This could be a problem if you're targeting multiple database platforms, but otherwise I rarely have trouble with it. When using a DataReader, however, I like to have a wrapper class that has methods returning nullable values instead.
  
  
  // nullables
  DateTime updateDate = dataRow["UpdateDate"] as DateTime ;
  int contactId = dataRow["ContactId"] as int ;
  // non-nullables
  bool flagged = dataRow["Flagged"] as bool false;
  
  

• Kay Chan

  Hi.. thank you, very nice example...

  but I think I would stay at my frequently called helper function

  public static TVal FromDB<TVal>(object value)
  {
  TVal r = default(TVal);
  if (value != DBNull.Value)
  {
  r = (TVal)Convert.ChangeType(value, typeof(TVal));
  }
  return r;
  }

  and check for the DBNull.Value explicitly myself without that function, when casting tu nullable type.

  Please go ahead and vote for http://lab.msdn.microsoft.com/ProductFeedback/viewfeedback.aspx feedbackid=1af9f75d-e566-41ff-ae00-84729ef8469e !

  
  

• Bertrand JURADO

  Dear miloush,

  change into "DoIt<DateTime>(DateTime.Now);"

  
  
  

• LondonSte

  then you should try to check if the value is null.

  because Convert.ChangeType(Object, Type) this method does not support null to pass in.

  MSDN:

  Parameters

  value

  An Object that implements the IConvertible interface.

  conversionType

  A Type.

  
  
  

• Inkyskin_UK

  Nope.

  The thing is, that if value is null, it should return null.

  
  

• j42

  Hi,

  thanks for the code, I've looked into it, but I need to handle NULLs because I'm casting to nullable types.

  The DataReader that will do well with nullable types can be written that way of course, but this is a little off-topic, since the problem I'm presenting here is (as it seems to me) that the nullable types does not have constructors.

  
  
• Casting to nullable generics