How is covariance cooler than polymorphism…and not redundant?

Question

.NET 4 introduces covariance. I guess it is useful. After all, MS went through all the trouble of adding it to the C# language. But, why is Covariance more useful than go

Answer 1

Consider an API which asks for an IContainer:

public void DrawShape(IContainer container>) { /* ... */ }

You have a Container. How can you pass your container to the DrawShape API? Without covariance, the type Container is not convertible to IContainer, requiring you to rewrap the type or come up with some other workaround.

This is not an uncommon problem in APIs that use a lot of generic parameters.