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Q143046: FIX: Error When Closing a Program Through the Control Box

Article: Q143046
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0
Operating System(s): 
Keyword(s): kbVBp kbVBp400bug kbGrpDSVB kbDSupport
Last Modified: 14-FEB-2002

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The information in this article applies to:

- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SYMPTOMS
========

An application error occurs when you close a Visual Basic program through the
control box if the Visual Basic program executes a command after an object
created from a class module is set to nothing. This problem occurs only in the
32-bit edition of Visual Basic 4.0.

RESOLUTION
==========

The workaround to this issue is execute all the code in the event before setting
the object to nothing. The statement to set the object to nothing should be the
last line in the event procedure.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This bug has been fixed in Visual Basic 5.0.

MORE INFORMATION
================

Steps to Reproduce Issue
------------------------

1. Start the 32-bit version of Visual Basic 4.0 or if it is already running,
  click New Project on the File menu.

2. Use the following steps to create a class module.

  a. Click Class Module on the Insert Menu. The code window for the class
     module appears.

  b. In the Properties window of the Class1 class module, set the Instancing
     Property to 2-Createable MultiUse, and the Public property to True.

  c. Add the following code to the Class1 code window:

        Option Explicit
        Public Name As String

        Private Sub Class_Terminate()
           Form1.Print Name
        End Sub
     

3. Add the following code to the Form1 code window.

        Option Explicit
        Public A As New Class1

        Private Sub Form_Load()
           Show
           Set A = New Class1
           A.Name = "A"

           'To workaround this bug, cut the next line
           Set A = Nothing

           Print "A Is " & IIf(A Is Nothing, "Nothing", "Something")
           'And uncomment this next line.
           'Set A = Nothing
        End Sub

4. Press the F5 key or click Start on the Run menu to run the program.

5. Click Close from the Control Bar menu on the form. One of the following
  application error displays and Visual Basic ends:


Window 2000:

  The instruction at "0x00214c5f" referenced memory at "0xffffffff". The memory
  could not be "written".

Windows NT 32-bit:

  An application error has occurred and an application error log is being
  generated, VB32.exe Exception: access violation (0xc0000005), Address:
  0x001d02d7.

-or-

  VB32.EXE-Application Error dialog box. The instruction at "0x001aaff8"
  referenced memory at "0x007dbd6b". The memory could not be "written".

Windows 95 or Windows 98 32-bit:

  VB32 caused an invalid page fault in module <unknown> at 0000:a0000001.

Additional query words: kbCtrl

======================================================================
Keywords          : kbVBp kbVBp400bug kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400
Version           : WINDOWS:4.0
Issue type        : kbbug
Solution Type     : kbfix

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