TIP: Dialog-Based, Single-Instance Applications

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Introduction

There are lots of articles on CodeGuru about limiting your program to a single instance. In this article, I want to show how easily it can be done by simply modifying the dialog template and adding few lines of code in InitInstance(). Although I use MFC for this sample, you can use the same technique for ATL/WTL and generic Win32 applications.

Implementation

• Create an MFC dialog-based project.
• Open the Resource Script file (.rc), find your main dialog template, and add the following lines:

  CLASS "SINGLE_INSTANCE_APP"
  
  IDD_SINGLEINSTANCE_DIALOG DIALOGEX 0, 0, 320, 200
     STYLE DS_SETFONT | DS_MODALFRAME | DS_FIXEDSYS |
           WS_POPUP | WS_VISIBLE |
           WS_CAPTION | WS_SYSMENU
     EXSTYLE WS_EX_APPWINDOW
     CLASS "SINGLE_INSTANCE_APP"
     CAPTION "Single Instance Application"
     ...
  

  This will instruct Windows to use your own windows class “SINGLE_INSTANCE_APP” instead of a standard dialog class.

• Now, you have to register the “SINGLE_INSTANCE_APP” windows class. The best place for this is InitInstance().

   WNDCLASS wc = {0};
   wc.style = CS_BYTEALIGNWINDOW|CS_SAVEBITS|CS_DBLCLKS;
   wc.lpfnWndProc = DefDlgProc;
   wc.cbWndExtra = DLGWINDOWEXTRA;
   wc.hInstance = m_hInstance;
   wc.hIcon = LoadIcon(IDR_MAINFRAME);
   wc.hCursor = ::LoadCursor(NULL, IDC_ARROW);
   wc.hbrBackground =
      CreateSolidBrush(GetSysColor(COLOR_BTNFACE));
   wc.lpszClassName = _T("SINGLE_INSTANCE_APP");
   ATOM cls = RegisterClass(&wc);

• After this, you can find your main dialog by its class name.

CWnd* pWnd =
   CWnd::FindWindow(_T("SINGLE_INSTANCE_APP"), NULL);
if (pWnd)
{
   pWnd->ShowWindow(SW_SHOW);
   pWnd->SetForegroundWindow();
   return FALSE;
}