Limiting 32-Bit MFC SDI/MDI Applications to a Single Instance

ID: Q141752

The information in this article applies to:

The Microsoft Foundation Classes(MFC)
- Microsoft Visual C++, 32-bit Editions, versions 4.0, 5.0, 6.0

SUMMARY

The ONET32 sample application demonstrates how to limit a Microsoft Foundation Class Library (MFC) application to a single instance. This is the 32-bit version of this sample.

MORE INFORMATION

The following files are available for download from the Microsoft Download Center. Click the file names below to download the files:

Onet32.exe

For more information about how to download files from the Microsoft Download Center, please visit the Download Center at the following Web address

http://www.microsoft.com/downloads/search.asp

and then click How to use the Microsoft Download Center.

Onet32.exe is a self-extracting file and should be executed as follows:

ONET32 -d

The -d option maintains the subdirectory structure.

There are several ways to limit a Windows-based application to a single instance. This sample program uses a technique in which a custom window class is registered for the application's main frame window. In subsequent instances, CWinApp::InitInstance searches for this window class and then activates the prior instance.

This technique was chosen because it is fairly easy to implement.

The following code fragments show the essential requirements of this technique:

Override CWinApp::InitInstance to check for, and activate, any previous instance of the application. Also, register your own window class as follows:


      // Add a static BOOL that indicates whether the class was
      // registered so that you can unregister it in ExitInstance
      static BOOL bClassRegistered = FALSE;

      BOOL COneT32App::InitInstance()
      {
          // If a previous instance of the application is already running,
          // then activate it and return FALSE from InitInstance to
          // end the execution of this instance.

          if(!FirstInstance())
            return FALSE;

          // Register your unique class name that you wish to use
          WNDCLASS wndcls;

          memset(&wndcls, 0, sizeof(WNDCLASS));   // start with NULL
                                                  // defaults

          wndcls.style = CS_DBLCLKS | CS_HREDRAW | CS_VREDRAW;
          wndcls.lpfnWndProc = ::DefWindowProc;
          wndcls.hInstance = AfxGetInstanceHandle();
          wndcls.hIcon = LoadIcon(IDR_MAINFRAME); // or load a different
                                                  // icon
          wndcls.hCursor = LoadCursor( IDC_ARROW );
          wndcls.hbrBackground = (HBRUSH) (COLOR_WINDOW + 1);
          wndcls.lpszMenuName = NULL;

          // Specify your own class name for using FindWindow later
          wndcls.lpszClassName = _T("MyNewClass");

          // Register the new class and exit if it fails
          if(!AfxRegisterClass(&wndcls))
          {
            TRACE("Class Registration Failed\n");
            return FALSE;
          }
          bClassRegistered = TRUE;

          // Rest of InitInstance goes here
          ...
          ...
          ...
      }

Add a member function to your CWinApp-derived class that actually does the work of checking for, and activating of, a previous instance:


      BOOL COneT32App::FirstInstance()
      {
        CWnd *pWndPrev, *pWndChild;

        // Determine if another window with your class name exists...
        if (pWndPrev = CWnd::FindWindow(_T("MyNewClass"),NULL))
        {
          // If so, does it have any popups?
          pWndChild = pWndPrev->GetLastActivePopup();

          // If iconic, restore the main window
          if (pWndPrev->IsIconic())
             pWndPrev->ShowWindow(SW_RESTORE);

          // Bring the main window or its popup to
          // the foreground
          pWndChild->SetForegroundWindow();

          // and you are done activating the previous one.
          return FALSE;
        }
        // First instance. Proceed as normal.
        else
          return TRUE;
      }

Override CWinApp::ExitInstance to unregister the class if it was registered as follows:


      int COneT32App::ExitInstance()
      {
        if(bClassRegistered)
          ::UnregisterClass(_T("MyNewClass"),AfxGetInstanceHandle());
        return CWinApp::ExitInstance();
      }

Override CFrameWnd::PreCreateWindow to use your window class instead of the one registered automatically by MFC as follows:


      BOOL CMainFrame::PreCreateWindow(CREATESTRUCT& cs)
      {
          // Use the specific class name you established earlier.
          cs.lpszClass = _T("MyNewClass");

          // Change the following line to call.
          // CFrameWnd::PreCreateWindow(cs) if this is an SDI application.
          return CMDIFrameWnd::PreCreateWindow(cs);
      }

NOTE: This information does not apply to WinCE-based applications. For additional information about limiting 32-bit applications to a single instance on a Windows CE environment, please click the article number below to view the article in the Microsoft Knowledge Base:

Q238100 HOWTO: Limiting 32-bit Applications to a Single Instance in WinCE

REFERENCES

MFC Technical Note #1 discusses window class registration.

For additional information about how to register MFC window classes, please click the article number below to view the article in the Microsoft Knowledge Base:

Q140596 INFO: MFC 4.0 No Longer Pre-Registers Window Classes

For additional information, which is more recent, click the article number below to view the article in the Microsoft Knowledge Base:

Q243953 HOWTO: Limit 32-bit Applications to One Instance Using C++

Additional query words: ONETIME one time Chapter 12 Advanced Windows HINSTANCE hPrevInstance single instance

Keywords : kbcode kbfile kbsample kbMFC KbUIDesign kbVC kbVC400 kbVC500 kbVC600 kbfaq
Version :
Platform : NT WINDOWS
Issue type :