CWinThread and CDialog - getting them work together

Posted on 2008-06-25
1 Endorsement
Last Modified: 2013-12-14

I have created a new thread - that works perfect. Now I want to display a CDialog in this thread. In a Thread Montor a new thread with the name of the dialog is shown but i can not see the dialog. I have called the dialog without a thread - that worked. That have I done wrong?

Below some code which may help.

Greedings Sonja
// Call of the thread
  CMyThread * MyThread = new CMyThread();
  MyThread ->CreateThread(CREATE_SUSPENDED);
  MyThread ->m_Parameter = Parameter;
  MyThread ->m_pDialog = pDialog;
  MyThread ->ResumeThread();
// MyThread.cpp - class CMyThread : public CWinThread
int MyThread::Run()
  //pDialog initialized to NULL in the constructor of CMyWnd class
   CMyDialog* pDialog = new CMyDialog();
   //Check if new succeeded and we got a valid pointer to a dialog object
   if(pDialog != NULL)
      BOOL ret = pDialog->Create(IDD_MYDIALOG,NULL); // <- In the debugger it stops here
      if(!ret)   //Create failed.
         AfxMessageBox("Error creating Dialog");
      AfxMessageBox("Error Creating Dialog Object");
  return 0;

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Question by:Sonja_M
LVL 19

Expert Comment

ID: 21866855
You should avoid to open a dialog in a different thread
LVL 44

Expert Comment

ID: 21867022
There are two types of thread.  A worker thread (does NOT support windows) and a UI thread (does support windows).
Which type are you using?  I suspect the first.
LVL 19

Expert Comment

ID: 21873986
>> int MyThread::Run()

Don't forget that when Run() returns, the thread ends.  So your thread will only be around for a split second.  

Generally it is a good idea not to override CWinThread::Run().  A better place to put your code is in InitInstance().  Add this function to your class and move the code to create the dialog into it.  This should resolve the problem.

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LVL 19

Expert Comment

ID: 21874527
OK this works.  The key is in setting CWinThread::m_pMainWnd to be your dialog:

BOOL MyThread::InitInstance()
	CMyDialog* pDialog = new CMyDialog();
	BOOL ret = pDialog->Create(IDD_MYDIALOG,NULL); 
	m_pMainWnd = pDialog;
	m_pMainWnd->ShowWindow( SW_SHOW );
	return CWinThread::InitInstance();

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LVL 19

Accepted Solution

mrwad99 earned 499 total points
ID: 21874798
>> The key is in setting CWinThread::m_pMainWnd to be your dialog:

Sorry but that is not true, I made a mistake.  The key is in returning TRUE (not CWinThread::InitInstance(), which returns FALSE, thereby shutting down the thread) from InitInstance().  You don't have to do anything with m_pMainWnd.

Revised code follows.
BOOL MyThread::InitInstance()
        CMyDialog* pDialog = new CMyDialog();
        BOOL ret = pDialog->Create(IDD_MYDIALOG,NULL); 
        pDialog->ShowWindow( SW_SHOW );
        return TRUE;

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Author Closing Comment

ID: 31470621
Thank you! Now it is working! Problem solved.
LVL 19

Expert Comment

ID: 21876669
Glad to help.

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