Limiting Applications to a Single Instance

The following code is added to the CWinApp-derived class in the 'InitInstance()' function, after the main window has been initialized but not displayed (m_uUserMessage is a UINT member variable of the same class)

// Define a user message indicating that another instance of
// the app is trying to run:
m_uUserMessage=RegisterWindowMessage("ANOTHER_INSTANCE");

// Test for the presence of another instance from an
// existing main window whose handle has previously
// be written to the Registry
CString strHand=GetProfileString("Control","MainWndHwnd","0");
HWND hWnd;
sscanf(strHand,"%ld",&hWnd);

// Test existing window
if( IsWindow(hWnd) )	// There is an instance running
{
 // Copy to the registry the command line data, as
 // for example a file name
 WriteProfileString("Control","CmdLineFile",cmdInfo.m_strFileName);

 // Sent user message to the running app
 PostMessage(hWnd,m_uUserMessage,0,0);

 // Push running app to the foreground
 SetForegroundWindow(hWnd);

 // Exit current instance
 return FALSE;
}

// No instance running, so save to Registry
// the handle of the current app main window

hWnd=m_pMainWnd->GetSafeHwnd();	
strHand.Format("%ld",hWnd);
WriteProfileString("Control","MainWndHwnd",strHand);

BOOL COneInstanceAppApp::PreTranslateMessage(MSG* pMsg) 
{
 if( pMsg->message == m_uUserMessage )
 {
  // Retrive relevant command line info
  CString file=GetProfileString("Control","CmdLineFile","");

  if( !file.IsEmpty() )
  {
   // Do what you want with this info!
  }
  return TRUE; // User message has been handled
 }
	
 return CWinApp::PreTranslateMessage(pMsg);
}

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