Why can't I implement an Interface this way? [duplicate]

Answer 1

From the C# Language Specification:

13.4.4 Interface mapping

For purposes of interface mapping, a class member A matches an interface member B when:

• A and B are methods, and the name, type, and formal parameter lists of A and B are identical.

Note that it does not say anything about inheritance or convertability! The return type must be identical.