c# generics on a method with the constraint that the type must be of "this" type

I have a C# class hierarchy with a common base type and two derived types. I want to declare an abstract method on the base class something like this :

public abstract IEnumerable<T> GetSiblings<T>() where T : MyBaseClass

... and I want this method to be implemented in the derived classes such that T is the type of that derived type, for each of the derived types, ie, in derived class A:

public override IEnumerable<A> GetSiblings<A>开发者_开发技巧() { ... }

... and in derived class B ...

public override IEnumerable<B> GetSiblings<B>() { ... }

Put another way, each derived class must implement the method so that it returns an IEnumerable of items of the same type. Is there any way to implement this in C# ?

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Well, you can hardly call a method generic if it only accepts a parameter of a single type, and your method signatures will have different return types which isn't allowed. Why don't you define an interface for all of these classes and simply return an IEnumerable<IMyClass>?

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You can't do this because the return types are different. Simple as that. The reason is if you create an instance of A and stuff it into your base class(cast it) then the return type will be wrong.

You might be able to to use new instead but that might break your hierarchy.

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This is not supported by the type system. It's a common enough problem, represented often as

class Animal<T> where T : Animal<T> { }
class Cat : Animal<Cat> { } // what you desire
class Dog : Animal<Cat> { } // what is possible yet not desired

But not a problem that has as yet been acted upon by the appropriate parties (be it the framework providers or C# team, not sure who).

Until it passes the critical "worth it" test as determined by costs (and opportunity costs) versus benefits, you'll have to work around it.

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I found the solution. Apparently in C# 4.0, generic parameter types can be covariant, so what I've posted above will work. C# 3.5 or lower, and it doesn't work. Took a lot of Googling.