How is Generic Covariance & Contra-variance Implemented in C# 4.0?

Question

I didn\'t attend PDC 2008, but I heard some news that C# 4.0 is announced to support Generic covariance and contra-variance. That is, List can be

Answer 1

Variance will only be supported in a safe way - in fact, using the abilities that the CLR already has. So the examples I give in the book of trying to use a List as a List (or whatever it was) still won't work - but a few other scenarios will.

Firstly, it will only be supported for interfaces and delegates.

Secondly, it requires the author of the interface/delegate to decorate the type parameters as in (for contravariance) or out (for covariance). The most obvious example is IEnumerable which only ever lets you take values "out" of it - it doesn't let you add new ones. That will become IEnumerable. That doesn't hurt type safety at all, but lets you return an IEnumerable from a method declared to return IEnumerable