Each window is a member of a particular window class. The window class determines the default window procedure that an individual window uses to process its messages. All windows belonging to the same class use the same default window procedure. For example, the system defines a window procedure for the combo box class (COMBOBOX); all combo boxes then use that window procedure.
An application typically registers at least one new window class and its associated window procedure. After registering a class, the application can create many windows of that class, all of which use the same window procedure. Because this means several sources could simultaneously call the same piece of code, you must be careful when modifying shared resources from a window procedure. For more information, see Window Classes.
Window procedures for dialog boxes (called dialog box procedures) have a similar structure and function as regular window procedures. All points referring to window procedures in this section also apply to dialog box procedures. For more information, see Dialog Boxes.