DestroyWindow invalidates the window handle to the dialog box, so any subsequent calls to functions that use the handle return error values. To prevent errors, the dialog box procedure should notify the owner that the dialog box has been destroyed. Many applications maintain a global variable containing the handle to the dialog box. When the dialog box procedure destroys the dialog box, it also sets the global variable to NULL, indicating that the dialog box is no longer valid.

The dialog box procedure must not call the EndDialog function to destroy a modeless dialog box.
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