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IsWindow             Determine If Window Handle Is Valid

BOOL IsWindow(hWnd)
HWND hWnd;

Returns:    None-zero if window handle is valid, otherwise zero.

Note:           Most operations that require a handle to a window will
                result in a Fatal error if the handle is invalid! To end-users
                running the retail (non-debugging) kernel the system will
                hang with no warning!  If you have any reason to doubt that
                a window handle is valid, use IsWindow.
                [Voice of experience!]