BUG: MessageBox with NULL Owner and MB_TASKMODAL

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 4.0, and 4.1

SYMPTOMS

Using ::MessageBox in an application built with the Software Development Kit (SDK) with a NULL owner window and MB_TASKMODAL disables all top-level windows belonging to a task, as if it were a modal dialog box.

MFC, however, allows the main window to be re-enabled when clicked. For example, if you use the following function call to bring up the message box, when you run the application, if you click one of the application's windows, the main window moves to the foreground and is enabled.

   ::MessageBox(NULL,
                "Test",
                "Test",
                MB_OK | MB_TASKMODAL);

CAUSE

The problem is due to "enable on activate" code in CFrameWnd that activates a window when it is clicked, even if it was disabled. This works around the Windows bug where clicking on the owner of a modal dialog box doesn't activate the dialog box instead.

Because MFC doesn't recognize the MessageBox as a popup belonging to the application, it will eventually call CWnd::EnableWindow(TRUE). This can be seen in the relevant portions of the call stack below:

... CWnd::EnableWindow(TRUE) _AfxHandleActivate(...) WM_ACTIVATE message is sent. ... CWnd::SetForegroundWindow()

_AfxHandleSetCursor(...)           //popups checked here.

The most recently called function is at the top of the list.

RESOLUTION

To work around this problem, use CWnd::MessageBox or AfxMessageBox instead of ::MessageBox. If you must use ::MessageBox, specify a window as the owner rather than using NULL as the first parameter. For example, to specify the application's main window as the owner, use this code:

::MessageBox(AfxGetMainWnd()->m_hWnd,

             "Test",
             "Test",
             MB_OK | MB_TASKMODAL);

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.