FIX: Closing an MDI Form Results in a General Protection Fault (149488)


The information in this article applies to:

  • Microsoft Visual Basic Standard Edition, 32-bit, for Windows 4.0
  • Microsoft Visual Basic Professional Edition, 32-bit, for Windows 4.0
  • Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows 4.0
This article was previously published under Q149488

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.
Modification Type:MinorLast Reviewed:7/13/2004
Keywords:kbBug kbCtrl kbfix kbVBp500fix KB149488
©2004 Microsoft Corporation. All rights reserved.