FIX: SetWindowText(NULL) Doesn't Clear .OCX Edit Control

The information in this article applies to:

• The Microsoft Foundation Classes included with: Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1

SYMPTOMS

It is possible to use AppWizard to create an OLE Custom Control (.ocx file) that subclasses a standard Windows control. If you choose to subclass an edit control, the control will not exhibit the same behavior as a standard edit control.

Specifically, in the case where the control is not initialized with data, the user types some text into the control, and then SetWindowText(NULL) is called to clear the text, the text in the control will not be cleared. If this same sequence of events were to occur in a standard edit control, the contents of the edit control would be cleared.

CAUSE

COleControl handles the WM_SETTEXT message in its OnSetText handler. The first few lines of this function are:

LRESULT COleControl::OnSetText(WPARAM wParam, LPARAM lParam) {

   ASSERT(lParam == 0 || AfxIsValidString((LPCTSTR)lParam));

   // Is the property changing?
   if ((lParam == 0 && m_strText.IsEmpty()) ||
      (lParam != 0 && m_strText == (LPCTSTR)lParam))
      return 0;
           .
         .
         .
}

RESOLUTION

To work around this problem, override OnSetText() in the COleControl derived class, and copy the code from COleControl::OnSetText() into the overridden funtion, with the following change:

LRESULT CMyOleControl::OnSetText(WPARAM wParam, LPARAM lParam) {

   ASSERT(lParam == 0 || AfxIsValidString((LPCTSTR)lParam));

   // Is the property changing?
   if ((lParam == 0 && InternalGetText().IsEmpty()) ||
      (lParam != 0 && m_strText == (LPCTSTR)lParam))
      return 0;
                    .
         .
         .
}

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Visual C++ 32-bit Edition, version 4.2.