KB141752 - Limiting 32-Bit MFC SDI/MDI Applications to a Single Instance

PSS ID Number: 141752

Article Last Modified on 6/15/2004

The information in this article applies to:

Microsoft Foundation Classes (MFC) 1.0
Microsoft Visual C++ .NET (2002)

This article was previously published under Q141752

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:

Onet32.exe

For additional information about how to download Microsoft Support files, click the following article number to view the article in the Microsoft Knowledge Base:

119591 How to Obtain Microsoft Support Files from Online Services

Microsoft scanned this file for viruses. Microsoft used the most current virus-detection software that was available on the date that the file was posted. The file is stored on security-enhanced servers that help to prevent any unauthorized changes to the file.

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:

238100 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:

140596 INFO: MFC 4.0 No Longer Pre-Registers Window Classes

For additional information%1, click the article number%2 below to view the article%2 in the Microsoft Knowledge Base:

%3 %4

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

Keywords: kbdownload kbArchitecture kbcode kbFAQ kbfile kbSample KbUIDesign KB141752
Technology: _IKkbbogus kbAudDeveloper kbMFC kbVCNET2002 kbVCNETSearch kbVCsearch