Generic Co- and Contravariance

Let’s take a closer look at co- and contravariance on generic interface types. At the start of this chapter, you saw how the next piece of code didn’t compile in .NET 3.5, with the assumption that MakeJuice takes in an IEnumerable<Fruit>:

IEnumerable<Apple> apples = ...;var juice = MakeJuice(from apple in apples                      where apple.Color == Color.Red                      select apple);

The reason is that IEnumerable<T> was invariant in its type parameter T. Invariance simply means that no typing relationship between the types IEnumerable<Base> and IEnumerable<Derived> is established. They’re treated as unrelated to one another.

In .NET 4, IEnumerable<T> has been marked ...

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