How to inherit from a generic parameter?

Question

I'm basically wanting to do this:

class UILockable<T> : T     where T : UIWidget { } 

However, this doesn't work. I've seen people recommend that you do this:

class UILockable<T>     where T : UIWidget {     private T _base; } 

This would require me to override each function UILockable would need and forward it to T. This is impossible since T may derive from UIWidget and have unique abstract/virtual methods of its own.

Is there no way to simply inherit from T?

Answer 1

You can't inherit from the generic type parameter. C# generics are very different from C++ templates. Inheriting from the type parameter requires the class to have a completely different representation based on the type parameter, which is not what happens with .NET generics. They are identical at the IL and native level (for all reference type arguments).

Answer 2

Think about it this way: When Type B inherits from type A, you're declaring that B is similar to A (where similarity means that you can use B everywhere you expect to use A). Now because such similarity is not symmetric you have that B is similar to A but A is not similar to B. Furthermore, B and C can both be similar to A (i.e. they both descend from A) without being similar to each other.

So what you want to declare is that Unlockable is similar to everything that is similar to UIWidget, but that's impossible because type similarity is not transitive (i.e. if B is similar to A and C is similar to A you can't say B is similar to C).