Using FindWindow in an MFC application

5 Aug 2003 12:46 mfc

There are at least two reasons for using FindWindow.

To activate the previous instance of your application.
To close your application while the installer is upgrading it.

Changing window class names in MFC

Unfortunately, FindWindow is only really guaranteed to return your application’s main window if you use a unique class name. If you’re using MFC, you’ll find this a little difficult: MFC automatically generates class names of the form “Afx:400000:8:10011:0:7e0c0b”.

To use your own class name, you’ll need to override PreCreateWindow:

BOOL CMainFrame::PreCreateWindow(CREATESTRUCT& cs)
{
    if(!CMDIFrameWnd::PreCreateWindow(cs))
        return FALSE;

    // When called from LoadFrame, via GetIconWndClass, we're supposed
    // to return the default window class name (something like
    // "AfxMDIFrame42").
    //
    // This information is then modified and used to register the
    // "Afx:x:x:x:" class, using the relevant icon/cursor/etc.

    // The new class name is then passed to CreateEx.  We're called
    // again to modify styles, etc.

    // Now, we _could_ fill in the WNDCLASS structure and register the
    // class fully the first time.

    // Or, we could be lazy and let MFC fill it in for us, and then
    // just register it with a different name.  If we do this, we need
    // to be careful not to change the default class name during
    // GetIconWndClass.

    // The first time through, cs.hInstance is NULL.
    if (cs.hInstance)
    {
        static const TCHAR className[] = _T("UniqueWindowClassName");

        // Get the existing WNDCLASS.
        WNDCLASS wc;
        VERIFY(GetClassInfo(cs.hInstance, cs.lpszClass, &wc));

        // Change the name.
        wc.lpszClassName = className;

        // Register the renamed WNDCLASS.
        VERIFY(AfxRegisterClass(&wc));

        // Use the new one.
        cs.lpszClass = className;
    }

    return TRUE;
}

Activating the previous instance of your application

Put this code near the top of your InitInstance function:

    const TCHAR semaphoreName[] = _T("UniqueSemaphoreName");
    HANDLE h = CreateSemaphore(NULL, 0, 1, semaphoreName);
    if (GetLastError() == ERROR_ALREADY_EXISTS)
    {
        // Then we're already running.  Find and activate the previous instance.
        static const TCHAR className[] = _T("UniqueWindowClassName");

        HWND hwndPrevious = ::FindWindow(className, NULL);
        if (hwndPrevious)
            SetForegroundWindow(hwndPrevious);

        return FALSE;        // Bail; don't run message loop.
    }

Closing the previous instance of your application

Similar to the above:

    const TCHAR semaphoreName[] = _T("UniqueSemaphoreName");
    HANDLE h = CreateSemaphore(NULL, 0, 1, semaphoreName);
    if (GetLastError() == ERROR_ALREADY_EXISTS)
    {
        // Then we're already running.  Find and activate the previous instance.
        static const TCHAR className[] = _T("UniqueWindowClassName");

        HWND hwndPrevious = ::FindWindow(className, NULL);
        if (hwndPrevious)
            ::SendMessage(hwndPrevious, WM_CLOSE, 0, 0);

        return FALSE;        // Bail; don't run message loop.
    }