Dlg app finds another copy of itself running and does SetForegroundWindow

Dlg app finds another copy of itself running and does SetForegroundWindow

I'm preparing an Mfc dialog application. I want to be able to find and invoke a previously invoked instance of this application ... i.e., I don't want two instances running at the same time.  The second instance should detect this and bring the previously started instance to the foreground before exiting.

I'm using a mutex to determine if another instance is already running.  This works fine.  However, I'm having trouble finding the other instance in order to foreground it. I've seen several examples of using SetProp and GetProp, but these don't seem to work with a CWinApp dialog based app.  In the code below, I suspect I'm not using the proper hWnd in the calls to SetProp and GetProp.

I was going to use another technique involving FindWindow of a registered class, but I understand this involves changing the window class name from "#32770" to another name.  I was trying to do this in PreCreateWindow, but oddly, my overridden PreCreateWindow isn't called. My app dynamically modifies the title bar, so I don't think I can do a FindWindow of the window name itself.

My question(s): Is there a generally accepted technique for an Mfc dialog based app to find and foreground a previously invoked instance of itself?  If this involves PreCreateWindow, how is it set up so that PreCreateWindow is called by the Mfc framework? If this involves SetProp, how is this done with an Mfc dialog based app so the corresponding GetProp works ok?

TIA

CMyDialogApp::InitInstance()
{
  ::CreateMutex(NULL, TRUE, m_pszExeName);
  if (GetLastError() == ERROR_ALREADY_EXISTS) {
    CWnd* pPrevWnd = CWnd::GetDesktopWindow()->GetWindow(GW_CHILD);
    while (pPrevWnd) {
      HANDLE h = ::GetProp(pPrevWnd->GetSafeHwnd(), m_pszExeName);
      if (h != 0) {
        if (pPrevWnd->IsIconic()) {
          pPrevWnd->ShowWindow(SW_RESTORE);
        }
        pPrevWnd->SetForegroundWindow();
        pPrevWnd->GetLastActivePopup()->SetForegroundWindow();
        return FALSE;
      }
      pPrevWnd = pPrevWnd->GetWindow(GW_HWNDNEXT);
    }
    TRACE("Could not find previous instance main window!\n");
    return FALSE;
  }
  ...
  BOOL flag = ::SetProp(m_pMainWnd->GetSafeHwnd(), m_pszExeName, (HANDLE)1);
  ...
}
 
ee-userAsked:
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mblatConnect With a Mentor Commented:
Honestly at first glance I didn't find anything wrong here,
so look at this link, it has freeware that does exactly what you are looking for.  I've used it in number of my apps, so I know it works.

http://www.naughter.com/sinstance.html

Hope it helps
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kosolobovCommented:
I propose you to place property directly on desktop window with data handle carrying first instance HWND.
Something like this:

CMyDialogApp::InitInstance()
{
 ::CreateMutex(NULL, TRUE, m_pszExeName);
 if (GetLastError() == ERROR_ALREADY_EXISTS) {
     HWND h = ::GetProp(CWnd::GetDesktopWindow()->GetSafeHwnd(),    
m_pszExeName);
if (h) {
     CWnd* pPrevWnd = CWnd::FromHandle(h);
     if (h != 0) {
       if (pPrevWnd->IsIconic()) {
         pPrevWnd->ShowWindow(SW_RESTORE);
       }
       pPrevWnd->SetForegroundWindow();
       pPrevWnd->GetLastActivePopup()->SetForegroundWindow();
       return FALSE;
     }
     pPrevWnd = pPrevWnd->GetWindow(GW_HWNDNEXT);
   }
   TRACE("Could not find previous instance main window!\n");
   return FALSE;
 }
 ...
 BOOL flag = ::SetProp(CWnd::GetDesktopWindow()->GetSafeHwnd(), m_pszExeName, (HANDLE)m_pMainWnd->GetSafeHwnd());
 ...
}

ps. this source is not tested - i place it here for just general idea.
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ee-userAuthor Commented:
mblat: This looks promising ... I'm evaluating it.

kosolobov: Your reply looks very close.  I had to do some casting to get HWND h to compile with GetProp and FromHandle. However, the SetProp/GetProp doesn't seem to be working.  The flag indicates success from SetProp, but h is always 0 from the GetProp.  I tried putting the SetProp in several places, with the same result.  My impression is that it shouldn't make any difference where the ::SetProp is put.

The documentation for GetDesktopWindow indicates it returns a temporary pointer that might be invalid later. I wonder if this is involved.

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kosolobovCommented:
MFC version of GetDesktopWindow will return temp pointer on CWnd object, while HWND handle carring there will be good.
About place to make SetProp: if you use m_pMainWnd to access m_hWnd - then be sure to use it after initialization.
For MFC Dialog projects it must be in App::InitInstance()

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ee-userAuthor Commented:
I'm accepting mblat's reply. This works great and is what I'm using in my project.  Thanks!

I really like the direction of kosolobov's reply, but I couldn't get it to work after some tries. I may look at it later. Because it more directly addresses my question (and I appreciate the time and effort), I'm going to make a 50 point question, "For kosolobov".
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