October 27, 2016
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Convert modal dialogs to modeless

  • October 16, 1998
  • By Jon S. Kyle
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Many postings to CodeGuru and similar discussion groups are questions about modeless dialogs and how to keep them from blocking the parent thread. I use a user interface thread to start a conventional modal dialog that behaves modeless-ly and it's so minimalist and easy I thought others may be interested...


  1. In your header file define a CWinThread-derived class...
      class CDialogThread : public CWinThread { DECLARE_DYNCREATE(CDialogThread) CDialogThread() {}; virtual BOOL InitInstance(); };
  2. Put this in your implementation file (where CSomeDialog is a conventional dialog class defined the usual way).
      IMPLEMENT_DYNCREATE(CDialogThread, CWinThread) BOOL CDialogThread::InitInstance() { CSomeDialog dlg; dlg.DoModal(); return FALSE; }
  3. To create an instance of your (now-modeless) modal dialog, do this...
      AfxBeginThread ( RUNTIME_CLASS(CDialogThread) );
  4. You can end the dialog the normal way by calling CDialog::EndDialog(), CDialog::OnCancel() or CDialog::OnOK(). All those special caveats about modeless dialogs (like "don't call EndDialog") no longer apply with this approach.

  5. One point to note is the return of 'FALSE' from the InitInstance() override. This tells MFC that the thread didn't start successfully, so MFC does our cleanup for us! Note that by this time the dialog has already returned and we're through with the thread. This is the same thing MFC-generated code does in the InitInstance() of dialog-based applications.

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