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Q236138: BUG: Error Deleting VB ActiveX Control From Test Container

Article: Q236138
Product(s): Microsoft C Compiler
Version(s): WINDOWS:5.0,6.0
Operating System(s): 
Keyword(s): kbActiveX kbContainer kbGrpDSVB _IK
Last Modified: 18-JUN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
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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 );
  }

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

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
---------------------------

1. Start a new Visual Basic ActiveX Control Project. Usercontrol1 is created by
  default.

2. Compile this ActiveX control as Project1.OCX.

3. Start Microsoft Visual C++.

4. On the Tools menu, select ActiveX Control Test Container.

5. 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.

6. 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.

Additional query words:

======================================================================
Keywords          : kbActiveX kbContainer kbGrpDSVB _IK 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600
Version           : WINDOWS:5.0,6.0
Issue type        : kbbug
Solution Type     : kbpending

=============================================================================

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