DOCUMENT:Q149488  11-JAN-2001  [vbwin]
TITLE   :FIX: Closing an MDI Form Results in a General Protection Fault
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:
OPER/SYS:
KEYWORDS:kbCtrl kbVBp400bug kbVBp500fix kbGrpDSVB

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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
========

A Visual Basic program contains an MDI child form with a ComboBox set to the
1-Simple ComboBox style. The ComboBox contains at least one item. When you
select an item in the ComboBox and then attempt to close the MDI Child form with
the Close button, the program shuts down with the following error message:

   VB32 caused a general protection fault in module USER.EXE

RESOLUTION
==========

To work around this bug, you can do one of the following:

 - Compile your program using Visual Basic version 5.0.

 - Use a ComboBox style other than the 1-Simple ComboBox.

 - Disable the Close button on the Toolbar using the Windows API functions as
   shown in the MORE INFORMATION section. Instead, use the Unload method to
   close the MDI Child form.

 - Close the form using an Unload statement in a command button or menu item
   click event. Use the following code in the Query_Unload event of the MDI
   child with the combo box to block an attempt to unload the form from the
   control menu of the close button:

         Private Sub Query_Unload(Cancel As Integer, UnloadMode As Integer)
            If UnloadMode = vbFormControlMenu Then
               Cancel = True
            End If
         End Sub

STATUS
======

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

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

This section shows how to create a sample project that demonstrates the bug and
shows how to disable the Close button on the toolbar using Windows API
functions.

Steps to Reproduce Problem
--------------------------

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

2. Set the MDIChild property of Form1 to True.

3. Add a ComboBox and two Command buttons to Form1. Set the ComboBox Style
   property to 1-Simple ComboBox.

4. Copy the following code to the Code window of Form1:

         Option Explicit

         Private Declare Function GetSystemMenu Lib "user32" _
                  (ByVal hWnd As Long, _
                  ByVal bRevert As Long) As Long

         Private Declare Function ModifyMenu Lib "user32" _
                  Alias "ModifyMenuA" _
                  (ByVal hMenu As Long, _
                  ByVal nPosition As Long, _
                  ByVal wFlags As Long, _
                  ByVal wIDNewItem As Long, _
                  ByVal lpString As Any) As Long

         Private Declare Function GetMenuItemID Lib "user32" _
               (ByVal hMenu As Long, _
               ByVal nPos As Long) As Long

         Const MF_BYPOSITION = &H400&
         Const MF_GRAYED = &H1&
         Const MF_BYCOMMAND = &H0&

         Sub Command1_click()
         ' Disables the Close button by calling a user-defined function.
            Dim hwndParent As Long
            hwndParent = Me.hWnd
            DisableItem hwndParent, "&Close", 6
         End Sub

         Sub DisableItem(hWnd As Long, _
                        sMenuCaption As String, _
                        iMenuPos As Integer)
         'User-defined function to disable the Close button on the
         'MDI Child Form toolbar.
            Dim hMenu As Long
            Dim hItem As Long

            hMenu = GetSystemMenu(hWnd, 0)
            hItem = GetMenuItemID(hMenu, iMenuPos)
            ModifyMenu hMenu, _
                        hItem, _
                        MF_BYCOMMAND Or MF_GRAYED, _
                        -9, _
                        sMenuCaption
         End Sub

         Private Sub Command2_Click()
             Unload Me
         End Sub

         Private Sub Form_Load()
             Me.Show
             Combo1.AddItem "first item"
             Command1.Caption = "Disable Close Button"
             Command2.Caption = "Unload MDI Child Form"
         End Sub

5. From the Insert menu, click MDIForm to add an MDI form to the project.
   MDIForm1 is created by default.

6. On the Run menu, click start or press the F5 key to start the program. Select
   the first item in the ComboBox. Click the Close button in the Form1 form
   toolbar. An application error occurs with an error message.

7. Run the program again, click Disable Close Button. Check the Close menu item
   in the control menu to confirm that it is grayed out. Select the first item
   in the ComboBox, and then close the form using the Unload MDI Child Form
   button, the MDI Child form closes normally.

Additional query words: kbide kbVBp kbdsd kbDSupport kbVBp400bug kbVBp500fix kbControl

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Keywords          : kbCtrl kbVBp400bug kbVBp500fix kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400
Issue type        : kbbug
Solution Type     : kbfix

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