Every window belongs to a window class that determines which window procedure a particular window uses to process its messages. All windows of the same class use the same window procedure. For example, the operating system defines a window procedure for the frame window class (WC_FRAME), and all frame windows use that window procedure.

An application typically defines at least one new window class and an associated window procedure. Then, the application can create many windows of that class, all of which use the same window procedure. This means that the same piece of code can be called from several sources simultaneously; therefore, you must be careful when modifying shared resources from a window procedure.

Dialog procedures have the same structure and function as window procedures. The primary difference between a dialog procedure and a window procedure is the absence of a client window in the dialog procedure; that is, the controls in a dialog procedure are the immediate child windows of the frame, whereas the controls in a normal window are the grandchildren of the frame. This makes significant differences in the code between the two; for example, WinSendDlgItemMsg does not work from a client window if you pass the client window handle as the first parameter.