對話框和工作線程同步工作的標準模型

Note that you must also override the virtual methods OnOK and OnCancel to have empty
bodies, so that a mis-typed <enter>or <esc> will not terminate the dialog.  Then you have
to add an OnClose handler.  The correct code would now be

OnInitDialog:
running = TRUE;
AfxBeginThread(StartThread, this);

in your thread
CDialog* dlg = (CDialog*)pParam;
...do thread stuff
dlg->PostMessage(UWM_THREAD_IS_FINISHED);

Note that typically your thread function would be declared as a static method of the
CMyAppDlg class.  See my essay on worker threads to see how easy it is to move back from C
space to C++ space.  It's on my MVP Tips site.

OnClose:
if(running)
{
...request thread termination here, same as your stop button would do
return;
}
CDialog::OnOK();

LRESULT CMyAppDlg::OnThreadIsFinished(WPARAM, LPARAM)
{
running = FALSE;
PostMessage(WM_CLOSE);
return 0;
}

This is the standard "asynchronous shutdown" pattern.