Could you think of a C# method accepting an ancestor, and forbidding a descendant of a class at compile time?

The answer to this probably is: why do you need such a reptile.

Well, I don't. I didn't meant to create such a method, but generics help a lot!

public class BinaryTreeNode<Node>
  where Node: BinaryTreeNode<Node>
{
  public Node parent;
  public Node left;
  public Node right;
}

public class MyNode: BinaryTreeNode<MyNode>
{
  public int key;
}

public class MyRoot: MyNode
{
}

public class Test
{
  public void test()
  {
    MyRoot root = new MyRoot();

    // print((MyNode)root); // This works.
    print(root); // This does not work.
  }

  private static void print<T>(T node)
    where T: BinaryTreeNode<T>
  {
    Console.WriteLine("print me");
  }
}

By the way, BinaryTreeNode is an "abstract" class, as you cannot instantiate it but inherit only.