Base class constraint on generic class specifying the class itself

Question

Yesterday, I was explaining C#\'s generic constraints to my friends. When demonstrating the where T : CLASSNAME constraint, I whipped up something like this:

Answer 1

Well.

public class Implementation : UnusableClass
{
}

is perfectly valid, and as such makes

var unusable = new UnusableClass();

and

UnusableClass.method(new Implementation());

valid.

So, yes, it can be instantiated by supplying an inheriting type as the type parameter, and similarly with the call to the static method. It's for instance useful for tree-like structures where you want to generically specify the type of children the node has, while it being the same type itself.