generics - C#: No implict conversion from Class<Child> to Class<Base>

Question

Following snippet wouldn't compile. With following error:

Cannot implicitly convert type 'Container<ChildClass>' to 'Container<BaseClass>'

class BaseClass {}
class ChildClass : BaseClass {}
class Container<T> where T : BaseClass {}
class Program {
    static void Main() {
        // why doesn't this work?
        Container<BaseClass> obj = new Container<ChildClass>(); 
    }
}

Is this by design? If it is, what is the reason?

Answer 1

(made wiki, in case of dups)

C# (3.0) doesn't support covariance of lists etc. C# 4.0 will support limited [co|contra]variance, but still not lists.

The problem is that with:

Container<BaseClass> obj = new Container<ChildClass>(); 

I could do:

obj.Add(new SomeOtherSubclass()); // SomeOtherSubclass : BaseClass

which would compile, but not work.

This behaviour is supported for arrays, but largely for historic reasons.