How to disable the window close button in an MFC Multiple Document Interface (MDI) application

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In some circumstances you may want to prevent the user from clicking the close button in the window's title bar to close a frame window in an MFC application. The close button can be removed by removing the WS_SYSMENU style from the frame window. However, the minimize, maximize, and restore buttons are also removed and cannot be added. This is in accordance to the design of Windows.

To work around this limitation, you can simulate the functionality of a window without a close button by disabling the close button. In the WM_CREATE message handler for the MDI child frame window (CMDIChildWnd-derived class) disable the close button with the following code:

CMenu *pSysMenu = GetSystemMenu(FALSE);
ASSERT(pSysMenu != NULL);
VERIFY(pSysMenu->RemoveMenu(SC_CLOSE, MF_BYCOMMAND));
				

When the child frame window is not maximized, the above code prevents the user from closing the child frame window by clicking the close button. When the child frame window is maximized, the above code makes the close button appear disabled. However, a user can still close the child frame window by clicking this window's close button. You can prevent this by trapping the SC_CLOSE command when it is sent to the child frame window and preventing further processing of this command. To do so, use the WM_SYSCOMMAND message handler for the child frame's class, as in the following example:

// CChildFrame is a CMDIChildWnd-derived class.
void CChildFrame::OnSysCommand(UINT nID, LPARAM lParam)
{
    if(nID == SC_CLOSE)
        return;
    CMDIChildWnd::OnSysCommand(nID, lParam);
}
				

(c) Microsoft Corporation 1999, All Rights Reserved. Contributions by Isaac L. Varon, Microsoft Corporation.