InvalidateRect and GetUpdateRect

What are all the cases in which the following lines yield b==0 ?
   
    InvalidateRect( hWnd, NULL, TRUE ) ;
    BOOL b = GetUpdateRect ( hWnd, NULL, TRUE ) ;
gil_moAsked:
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nietodConnect With a Mentor Commented:
1. hwnd is invalid
2. hwnd is for a window that is hidden
3., hwnd is for a window is covered by other windows.
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nietodCommented:
4.  (Possibly, but unlikely to happen) if LockWindowUpdate() was called on  the window.  (If that was done, I'm not sure what would happen in this case, but it seems likely that you'd get 0.)
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