Instance Subclassing - Win32 Programmer's Reference

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Instance Subclassing

An application subclasses an instance of a window by using the SetWindowLong function. The application passes the GWL_WNDPROC flag, the handle of the window to subclass, and the address of the subclass procedure to SetWindowLong. The subclass procedure can reside in either the application's module or a dynamic-link library (DLL). An application must list the name of the subclass procedure in the EXPORTS statement of the application's or DLL's module-definition (.DEF) file.

SetWindowLong returns the address of the window's original window procedure. The application must save the address, using it in subsequent calls to the CallWindowProc function, to pass intercepted messages to the original window procedure. The application must also have the original window procedure address to remove the subclass from the window. To remove the subclass, the application calls SetWindowLong again, passing the address of the original window procedure with the GWL_WNDPROC flag and the handle of the window.

An application can subclass any window in the system; however, when subclassing a window it does not own, the application must ensure that the subclass procedure does not destroy the original behavior of the window. Because the application does not control the window, it must not depend on information about the window that the owner might change in the future.

An application should not use the extra window bytes or the class bytes for a window without knowing exactly what the bytes mean and how the original window procedure uses them. Even so, the application should not use them unless it owns the window. If the application uses the extra window bytes of a window that another application owns and the owner changes some aspect of the extra bytes, the subclass procedure may fail. For this reason, an application should not subclass a window that belongs to a system global control class. Windows owns the system global classes, and aspects of the controls might change from one version of Windows to the next. If the application must subclass a window that belongs to a system global class, the developer may need to update the application when a new version of Windows is released.

Because instance subclassing occurs after a window is created, the application subclassing the window cannot add any extra bytes to the window. Applications that subclass a window should use the window's property list to store any data needed for an instance of the subclassed window. For more information, see Window Properties.

When an application subclasses a subclassed window, it must remove the subclasses in the reverse order they were performed. If the removal order is not reversed, an unrecoverable system error may occur.

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