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Limiting Applications to a Single Instance

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Here is a very simple example of how to implement a "One Instance" Application. If the application is yet running and the user tries to launch another instance, the running app is activated and line command parameter parsed to the runnin app.

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);
A 'PreTranslateMessage()' function is surcharged in the CWinApp-derived class ('COneInstanceAppApp', in the present example):
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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