Although I’m not cool enough to actually go to PDC, I’ve been watching some of the things that have been announced. One of the things I’m most excited about is co- and contra-variance in generics, which is something that the CLR has lacked since generics were first introduced in 2.0. (Note: some of the examples on here are pulled from the excellent description of new features release by Microsoft.)

In versions of C# prior to 4.0, generics were invariant. For example, consider this simple type definition:

public class Foo<T>

Since the generic type parameter T was not constrained, the compiler understands that T should be treated as type object. That means that since a string is an object, a Foo<string> is functionally equivalent to a Foo<object>. However, because of generic invariance, ian instance of Foo<string> cannot be assigned to a variable of…

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