Hi, i understand how to subclass a window by using SetWindowLong() with GWL_WNPROC, but i am not quite sure if we need to register the window class with an extra pointer byte to cbWndExtra when we want to subclass. If we dont have to, can someone give me an example of when do we exactly need to use the cbWndExtra? And how do we set the byte offset in SetWindowLong() to use that extra byte? Secondly, what is GWL_USERDATA exactly? i read it on msdn but i still dont understand. Thanks. Also