Q150184: FIX: Unloading a Form After Assigning Text Property Causes GPF

Article: Q150184
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0
Operating System(s): 
Keyword(s): kbnokeyword kbVBp kbVBp400bug kbVBp600fix kbGrpDSVB kbDSupport
Last Modified: 11-JAN-2001

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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, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SYMPTOMS
========

If the Text property of a Text box is set equal to the Text property of a Text
box on a separate modal form, and if that statement is followed by unloading the
modal form, a General Protection Fault results.

RESOLUTION
==========

Rather than use the Unload statement to unload the modal dialog box, use the
PostMessage API function. The declaration for PostMessage is:

     #If Win32 Then

     Private Declare Function PostMessage Lib "user32" Alias _
     "PostMessageA"(ByVal hWnd As Long, ByVal wMsg As Long, _
     ByVal wParam As Long, ByVal lParam As Long) As Long

     #Else

     Private Declare Function PostMessage Lib "User" (ByVal hWnd As Integer,
     ByVal wMsg As Integer, ByVal wParam As Integer, lParam As Any) As
     Integer

     #End If

To post a close message for a form, set the first parameter to the hWnd of the
target form, and the other three parameters as specified below:

     Const NILL = 0&

     Const WM_SYSCOMMAND = &H112

     Const SC_CLOSE = &HF060

so the following statement closes down Form2:

     PostMessage Form2.hWnd, WM_SYSCOMMAND, SC_CLOSE, NILL

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 6.0.

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

Steps To Reproduce Problem
--------------------------

1. Start a new project in Visual Basic. Form1 is created by default. Add a Text
  box to Form1. In the Click event for Form1, place the following code:

     Private Sub Form_Click()
       Form2.Show 1
     End Sub

2. From the Insert menu, select Form to add another form to the project. On
  Form2, place a Text box. Add the following code to Form2:

     Private Sub Text1_DblClick()
       Form1.Text1 = Text1
       Unload Me
     End Sub

3. Run the project by pressing F5. Click on Form1 to show Form2 modally.
  Double-click on the text box in Form2, and a General Protection Fault occurs.

In one test on Windows NT 3.51, with the 32-Bit Edition of Visual Basic, the
error message was:

  The instruction at "0x00428646" referenced memory at "0x00d10cd8". The memory
  could not be "read".

In another test with, the 16-Bit Edition of Visual Basic, the error message was:

  VB caused a General Protection Fault in module VB.EXE at 0016:2BFB.

To work around the problem, place the declaration given in the Workaround Section
above in the General Declarations section of Form2, and change the code in the
DblClick event of the Text box on Form2 to:

     Private Sub Text1_DblClick()

     Const NILL = 0&

     Const WM_SYSCOMMAND = &H112

     Const SC_CLOSE = &HF060

     Form1.Text1.Text = Text1.Text

     PostMessage Form2.hWnd, WM_SYSCOMMAND, SC_CLOSE, NILL

     End Sub

Additional query words:

======================================================================
Keywords          : kbnokeyword kbVBp kbVBp400bug kbVBp600fix kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400 kbVB16bitSearch
Version           : WINDOWS:4.0
Issue type        : kbbug
Solution Type     : kbfix

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