FIX: Access Violation Firing an Error Event in OLE Control

The information in this article applies to:

• Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2

SYMPTOMS

An access violation occurs when the error event is fired in an OLE Control. This occurs on the first firing of an error event in Release builds and only after several firings in Debug builds.

CAUSE

An invalid "this" pointer is being loaded off the stack on a call to ExternalRelease() at the end of COleControl::FireEvent().

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Microsoft Visual C++, 32-bit Edition, version 4.0.

MORE INFORMATION

To reproduce this problem, add the stock Error event to an OLE Control using ClassWizard's OLE Events tab. The name given the member function to fire the event will be FireError. In the OnLButtonDown handler of the OLE Control, call FireError() and pass the following parameters:

     FireError(CTL_E_OUTOFMEMORY, " ");

Now, run the test container, insert your control, and click the control's client area. In Release builds, this causes an access violation on the first occurrence of the event. In Debug, it takes several occurrences before the access violation is seen.

Sample Code

Below is the OnLButtonDown handler for the Circ3 tutorial sample, modified to call the error event.

/* Compile options needed:
*/

void CCirc3Ctrl::OnLButtonDown(UINT nFlags, CPoint point)

   /*  Comment the tutorial.
        CDC* pdc;

   // Flash the color of the control if within the
           ellipse.
   if (InCircle(point))
   {
      pdc = GetDC();
      FlashColor(pdc);
      ReleaseDC(pdc);

      // Fire the ClickIn event
      FireClickIn(point.x, point.y);
   }
   else
      // Fire the ClickOut event
      FireClickOut();
        */

        // Here's the call.

   FireError(CTL_E_OUTOFMEMORY, " ");
   COleControl::OnLButtonDown(nFlags, point);