Question

abstract class A<T> where T:A<T>
{
    public event Action<T> Event1;
}

class B : A<B>
{
    //has a field called Action<B> Event1;
}

Is there a more elegant way to do this? I want stuff (events, etc) in the base class to be able to use the subclass’ type.

Answer 1

The pattern you are using does not actually implement the constraint you want. Suppose you want to model “an animal can only be friendly with something of its own kind”:

abstract class Animal<T> where T : Animal<T>
{
    public abstract void GetFriendly(T t);
}

class Cat : Animal<Cat>
{
    public override void GetFriendly(Cat cat) {}
}

Have we succeeded in implementing the desired constraint? No.

class EvilDog : Animal<Cat>
{
    public override void GetFriendly(Cat cat) {}
}

Now an evil dog can be friendly with any Cat, and not friendly with other evil dogs.

The type constraint you want is not possible in the C# type system. Try Haskell if you need this sort of constraint enforced by the type system.

See my article on this subject for more details:

http://blogs.msdn.com/b/ericlippert/archive/2011/02/03/curiouser-and-curiouser.aspx