BUG: Application Error Deleting VB ActiveX Control From ActiveX Control Test Container (236138)

SYMPTOMS

Under Windows NT, when deleting a Visual Basic 5.0 or Visual Basic 6.0 UserControl sited in the ActiveX Control Test Container, the following Application Error might occur:

The instruction at "0x0041842c" referenced memory at "0x00000000". The memory could not be "read"

CAUSE

This is caused by an errant WM_SETCURSOR message, which is sent after the DELETE key has been pressed to remove the control from the container. The code for the ActiveX Control Test Container (TSTCON32.EXE) does not check for a valid object before attempting to obtain a specific interface.

RESOLUTION

The source code for the ActiveX Control Test Container is located on the Visual Studio 6.0 MSDN CD in the "Samples\VC98\mfc\ole\tstcon" directory. The bug can be corrected by modifying the tcitem.cpp. Replace the HitTest method with the code below and recompile the tstcon project:

DWORD CTestContainer98Item::HitTest( CPoint point )
{
   HRESULT hResult;
   IViewObjectExPtr pViewObjectEx;
   DWORD dwHitResult;

   if( !m_rect.PtInRect( point ) )
   {
	  // Trivially reject the point.
	  return( HITRESULT_OUTSIDE );
   }

   if (!m_lpObject)  return( HITRESULT_OUTSIDE );  

   hResult = m_lpObject->QueryInterface( IID_IViewObjectEx,    
	     (void**)&pViewObjectEx );

   if( SUCCEEDED( hResult ) )
   {
	  hResult = pViewObjectEx->QueryHitPoint( DVASPECT_CONTENT,
                    &m_rect, point, 0, &dwHitResult );
	  if( SUCCEEDED( hResult ) )
	  {
		 return( dwHitResult );
	  }
   }

   return( HITRESULT_HIT );
}

MORE INFORMATION

The problem is caused by the following code segment, which is called after the object has been deleted and m_lpObject has been set to NULL:

hResult = m_lpObject->QueryInterface( IID_IViewObjectEx,          
	  (void**)&pViewObjectEx );

Place the following code segment just prior to this QueryInterface method call to check whether m_lpObject is NULL, avoiding the problem:

if (!m_lpObject)  return( HITRESULT_OUTSIDE );

Steps to Reproduce Behavior

Start a new Visual Basic ActiveX Control Project. Usercontrol1 is created by default.
Compile this ActiveX control as Project1.OCX.
Start Microsoft Visual C++.
On the Tools menu, select ActiveX Control Test Container.
On the Edit menu of the ActiveX Control Test Container, select the Insert New Control option. Select Project1.UserControl1 from the list box and click OK.
Select the Project1.OCX control sited in the container and press the DELETE key. The error message described in the symptoms section above should appear.