Generics and the special constraints

Someone recently asked me to update an old post of mine Reflection and Generics . You gotta be careful what you write in blogs as they seem to live forever.

Well, it is true, this needs a little updating. We no longer use the NewConstraintAttribute (or the other special constraint attributes) to denote the special constraint in the metadata. (In fact they are being removed from Whidbey)

Here are some examples in C# syntax:

class C<T> where T : new() { } // T must have a public default constructor

class C<T> where T : class { } // T must be a class

class C<T> where T : struct { } // T must be a struct

Here are some examples in VB syntax:

Class C(Of T As New)

End Class

Class C1(Of T As Class)

End Class

Class C2(Of T As Structure)

End Class

We now encode it the IL syntax for special constraints by putting either '.ctor', 'class' or 'valuetype' before the type parameter name (C<T> above is minimally 'C`1'<.ctor T> in IL).

So, some code to show how this works in the IL:

class Class1

{

    static void Main()

    {

        GenericParameterAttributes ga = typeof(C<>).GetGenericArguments()[0].GenericParameterAttributes;

        if ((ga & GenericParameterAttributes.SpecialConstraintMask) == GenericParameterAttributes.DefaultConstructorConstraint)

        {

            Console.WriteLine("Has New Constriant");

        }

        else

        {

            Console.WriteLine("Does not have New Constriant");

        }

    }

}

Comments

• Anonymous
  October 04, 2004
  Hi,
  I have a question regarding generics. Suppose I have made a class like this:
  
  class View<Model> : where Model : ModelBase
  
  Now I have several classes which inherits ModelBase class. For example, CarModel, AirplaneModel.
  
  Now I am making a collection class like this:
  
  
  List<View<ModelBase>> list;
  
  list.Add( new View<CarModel> );
  
  Unfortunately I cannot do that. Although
  CarModel is a child of ModelBase and the collection I am making is of View<ModelBase>, but it does not work.
  
  
• Anonymous
  October 04, 2004
  Thanks Brad for an answer.
  
  I've posted a RFC on PF for this http://lab.msdn.microsoft.com/ProductFeedback/viewFeedback.aspx?FeedbackId=FDBK16162 - but now unable to close it :-(
  
  Omar Al Zabir: Generics does not support downcasting and does NOT follow class inheritance hierarhy of their type params because in addition to input params - generics methods has output params.
  class View<T> {
  T Current { get; }
  bool Update(ref T);
  }
  See my comments on http://lab.msdn.microsoft.com/productfeedback/viewfeedback.aspx?feedbackid=FDBK13559
  
  But I think that this will be cool to create some kind of inheritance-aware generics if you will mark your generics class with some kind of special flag that one of typeparams can be used for input-only.
  Something like
  class GenericClass<[InputOnly] InT, OutT> where InT: OutT {
  OutT DoMagic(InT x) { return x;}
  }
  or
  class IComparer<[FollowInheritance] T> {
  bool Compare(T a, T b) { return false; }
  }
• Anonymous
  October 04, 2004
  Omar,
  Generics and Polymorphism don't mix.
  If that construction worked, what would list.Item return?
  Remember, a constructed type can't return generic types.
  
  You don't need generics for your model:
  
  Class View
  {
  public View(ModelBase model) {...}
  }
  
  List<View> list = new List<View>();
  list.Add(new View(new CarModel()));
  list.Add(new View(new AirPlaneModel()));
  
• Anonymous
  October 09, 2004
  The comment has been removed
• Anonymous
  October 09, 2004
  The comment has been removed
• Anonymous
  March 08, 2006
  If you opened up your CD from Krzysztof Cwalina and Brad Abram's new book Framework Design Guidelines....
• Anonymous
  June 12, 2007
  PingBack from http://www.kintespace.com/rasxlog/?p=652