BUG: Error When Closing a Program Through the Control Box

• Standard, Professional, and Enterprise Editions of Microsoft Visual Basic for Windows, 32-bit only, version 4.0

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.

WORKAROUND

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 an issue in the Microsoft products listed at the beginning of this article. Microsoft is researching this problem and will post new information here in the Microsoft Knowledge Base as it becomes available.

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.

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

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

       - 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:

   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 32-bit:

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