Launching a dialog from a secondary thread

Posted on 2003-10-21
Last Modified: 2013-11-20
I have a dialog based application that runs a function in a second thread.  Under certain conditions this function needs to launch a dialog.  I have created the dialog with a dialog template and it works fine from the primary thread, but not within the secondary thread.  It gives me an assert error when it runs the AfxGetThread() function on it in the MFC DoModal.  Is there any way to get it to work this way or do I need to dynamically create the window instead of using a dialog template?  (Or do I have a completely different problem causing the Assertion?)  Thanks,

David Johns
Question by:david_johns

Expert Comment

ID: 9595229
you should ensure your Dialog-constructor and DoModal call are in the same thread. try to construct the dialog by new CxxDialog in the threadfunction also (and don_t forget to delete the Dialogobject!)


Expert Comment

ID: 9595503
Yes...the dialog needs to be created inside the thread. The reason for this is that anything derived from CObject can not be passed between threads. If you instantiate and DoModal inside of your will be okay. I had this same problem with my CSockets. Just hollar if you need more assistance.  :)


Accepted Solution

Priyesh earned 125 total points
ID: 9596506
This will work

//thread launching modal dialog.
UINT MyThread(LPVOID lParam)
   CAboutDlg dlg ;
   dlg.DoModal() ;

BOOL CTestDialog::OnInitDialog()
   CDialog::OnInitDialog() ;

   AfxBeginThread(MyThread, NULL) ;

   return TRUE ;
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Author Comment

ID: 9596623
Thanks for the responses.  I was creating the dialog object inside the thread, but I was launching the thread with a CreateThread() instead of AfxBeginThread().  It works great after switching that.  Thanks for the help,
LVL 23

Expert Comment

by:Roshan Davis
ID: 9596756
Better create in main thread, becoz, we are creating thread for some wait operation

If a thread is in *WAIT STATE* all the dialog messages(WM_PAINT, click etc etc) will block.

So Post a message to Main thread from the worker thread,
Map that message in main thread and create the dialog from there....

UINT YourThread(LPVOID lParam)
      CYourDialog *pDlg = (CYourDialog *)lParam;

      pDlg->PostMessage(WM_USER+10, 0, 0);


And in your main Thread (in dialog class) put a message map like

      // class wizar added maps

      ON_MESSAGE(WM_USER+10, OnLaunchDialog)          <----- ADD LIKE THIS

void CYourDialog::OnLaunchDialog(WPARAM, LPARAM)

Good Luck

Expert Comment

ID: 9597763
Hi roshmon,
I hardly understand what you'r saying.
When a thread calls DoModal it hangs in a Messageloop, and code behind DoModal isn't executed until endmodal, so what could suspend the thread except a messagehandler of that dialog, and if, not to imagine this to be the mainthread.
Well modal dialogs in threads means also (as the thread could be created multiple times) that there cound be several modal dialogs in one app, what may look a little bit strange.
Also a simple Messagebox call from a thread should be harmless, I think, becaus they are usefull to hold the thread and decide for further processing.
I mention that, because I use similar in own apps and was conviced to handle well, but what you say makes me a bit pensive.
LVL 23

Expert Comment

by:Roshan Davis
ID: 9597792
Thats Not for DoModal, Its for Create. I thought u are creating dialog *dynamically* using Create Function. Leave it. its for Modeless dialogs. leave it.

Expert Comment

ID: 9597952
understood  ;-)

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