There can be only one...

Introduction

When reading Joseph Newcomer's article about this matter, I thought there must be a way to do this simpler. So based on his excellent work the past few years I've spent all my time devoted on this theorem. In the mean time I never saw any daylight and I've lost all my friends (Just kidding' ;)...

But after this long research and keeping it secret for too long I finally bring out the goods just to make sure such an important discovery like this doesn't fade away in history... uhum...

So here we go:

1. We need a registered message that our application will respond to. Make this one global in your application.

   static const UINT WM_CHECK_ITS_ME = RegisterWindowMessage("MY_UNIQUE_GUID");

2. Now we create a mutex somewhere at the start of your program:

   HANDLE hMutexOneInstance = CreateMutex( NULL, FALSE, _T("MY_UNIQUE_GUID"));

3. Check the last error to see what happened:

   bool AlreadyRunning = ( GetLastError() == ERROR_ALREADY_EXISTS || 
                              GetLastError() == ERROR_ACCESS_DENIED);

4. Then we send out a message to all top level windows to make sure it pops up and becomes the active window. The trick here is to use HWND_BROADCAST, which is a little bit undocumented.

   if ( AlreadyRunning )
   {    
    DWORD result;
    LRESULT ok = SendMessageTimeout(HWND_BROADCAST,
                                          WM_CHECK_ITS_ME,
                                          0, 0, 
                                          SMTO_BLOCK |
                                          SMTO_ABORTIFHUNG,
                                          200,
                                          &result);
    return FALSE; // Here we quit this application
    // or exit(0);
    // or PostQuitMessage() ... etc...
   }

5. The already running application needs to respond the registered window message, so we add a message handler to the application.
   
   If you are using MFC edit your messagemap:

   ON_REGISTERED_MESSAGE( WM_CHECK_ITS_ME, OnCheckItsMe )

   or WTL:

   MESSAGE_HANDLER(WM_CHECK_ITS_ME, OnCheckItsMe)

   or plain Windows API:

   LRESULT CALLBACK MyWindowProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp)
   {
     if (msg == WM_CHECK_ITS_ME)
     {
       ...
       return 0;
     }
     ....
     return ...;
   }

6. Now in the handler for this message we just make sure this application becomes the foreground and active one.

   LRESULT CMainFrame::OnCheckItsMe(...)
   {
   // If we are using Chris Maunder's CSystemTray it's handy
   
    m_SystemTray.MaximiseFromTray(this->m_hWnd); 
    
   // or else you can call all sorts of restore/maximise functions:
   
    - ShowWindow(SW_RESTORE);
    - SetForegroundWindow();
    - SetWindowPlacement(m_MyWindowPlacement);
   
    return WM_CHECK_ITS_ME;
   }

7. That should work!