Why do we need new keywords for Covariance and Contravariance in C#?

Question

Can someone explain why there is the need to add an out or in parameter to indicate that a generic type is Co or Contra variant in C# 4.0?

I've been trying to understand why this is important and why the compiler can't just figure it out..

Thanks,

Josh

Answer 1

Eric Lippert, who works on the langauge, has a series of posts on msdn that should help clarify the issues involved:
http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx

When reading the articles shown at that link, start at the bottom and work up.

Eventually you'll get to #7 (Why do we need a syntax at all?).