Return MyNestedClass<K> when MyNestedClass is not MyNestedClass<K>?

Question

Breaking down the MS RBTree (an internal .Net abstract class), I have discovered that one method returns TreePage<K>:

private TreePage<K> AllocPage(int size)
{
    ...
}

Within the method, variables are declared as TreePage...but the class is not defined that way:

private sealed class TreePage
{
    ...
}

Yet, when I mimic the code and definition using .Net 2010 (Express), I cannot do this:

Error: The non-generic type 'RBTree.TreePage' cannot be used with type arguments

So, is there an extension method that I can't find? Is there something MS is doing that we just don't get to see?

Answer 1

When you declare a class nested in a generic class

class Foo<T>
{
    class Bar
    {
    }
}

this gets compiled to a class

Foo<T>

and a class

Foo+Bar<T>

Bar is generic, because it is nested in the generic class Foo. But the type parameter declaration is not repeated in C# (where you refer to the class as Foo<T>.Bar).

I noticed that Reflector shows the generic type parameter for classes nested in generic types, even if they don't have declared any type parameters directly. That's a bug. You need to fix the code when copy it straight out of Reflector.

Answer 2

It's actually less complicated than you might imagine. Klass and Klass<T> are two completely different types. To wit:

  class A
  {
  }

  class A<T>
  {}

  class Program
  {
    static public void Main(string[] args)
    {
      A a = new A();
      A<int> generic_a = new A<int>();
    }
  }

There's another TreePage<T> floating around there somewhere.