What is variance for generic type parameters?
This is a new feature in C# 4.0. Now, when creating a generic interface, you can specify whether there is an implicit conversion between interface instances that have different type arguments. For example, you can use an interface instance that has methods with more derived return types than originally specified (covariance) or that has methods with less derived parameter types (contravariance). The same rules are applied to generic delegates. While you can create variant interfaces and delegates yourself, this is not the main purpose for this feature. What is more important is that a set of interfaces and delegates in .NET Framework 4 have been updated to become variant.
