Covariance and Contravariance on the same type argument

Question

The C# spec states that an argument type cannot be both covariant and contravariant at the same time.

This is apparent when creating a covariant or contravariant int

Answer 1

Generic type parameters cannot be both covariant and contravariant.

Why? This has to do with the restrictions which in and out modifiers impose. If we wanted to make our generic type parameter both covariant and contravariant, we would basically say:

• None of the methods of our interface returns T
• None of the methods of our interface accepts T

Which would essentially make our generic interface non-generic.

I explained it in detail under another question: