Q193367: FIX: Access Violation in MSVBVM50 When Passing a Variant Array

Article: Q193367
Product(s): Microsoft Visual Basic for Windows
Version(s): 5.0
Operating System(s): 
Keyword(s): kbGrpDSVB
Last Modified: 01-MAR-2002

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, version 5.0 
- Microsoft Visual Basic Professional Edition for Windows, version 5.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 5.0 
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SYMPTOMS
========

When calling a function of an object in an out-of-process server and passing a
Variant array by reference, you get an access violation as below:

  The instruction at '0x0f0fdbda' referenced memory at '0x0000b1a4'. The
  memory could not be 'read'.

NOTE: The actual memory address may vary. This error occurs after you apply the
Service Pack 3 for Visual Basic 5.0

RESOLUTION
==========

Refer to "Ways to Resolve the Problem" in the MORE INFORMATION section below.

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 Microsoft Visual
Basic, version 6.0.

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

Steps to Reproduce Behavior
---------------------------

The following steps require MDAC 2.0 to be installed on the machine:

- Download and install MDAC 2.0 from:

  http://www.microsoft.com/data/mdac2.htm

  You only need the minimum setup files.

- Alternatively, you can replace the existing oleaut32.dll with the version
  shipped with MDAC 2.0 or Visual Basic 6.0 (2.30.4261).

Create the Server
-----------------

1. Create an ActiveX exe project and rename it ArraySvr.

2. Add the following code to the default class module (Class1):

        Public Function UseArray(ByRef vArray As Variant) As Boolean
           UseArray = True
        End Function

        Public Function GetArray() As Variant
           Dim vReturn As Variant
           Dim vBstr As Variant
           ReDim vReturn(1, 0) As Variant
           vBstr = "Zero,Zero"
           vReturn(0, 0) = vBstr
           vBstr = "One,Zero"
           vReturn(1, 0) = vBstr
           GetArray = vReturn
        End Function

3. Compile the server.

Create and Test Client
----------------------

1. Create a Standard EXE project in Visual Basic. Form1 is created by default.

2. Add two CommandButtons and one Label to Form1.

3. Select References from the Project menu and add a reference to ArraySvr.

4. Add the following code to Form1:

        Private vModule As Variant

        Private Sub Command1_Click()
           Dim o As ArraySvr.Class1
           Set o = CreateObject("arraysvr.class1")
           vModule = o.GetArray
           Set o = Nothing
           Command2.Enabled = True
        End Sub

        Private Sub Command2_Click()
           Dim o As ArraySvr.Class1
           Dim vLocal(1, 0) As Variant
           Dim i As Long
           Dim bRet As Boolean
           For i = LBound(vModule, 1) To UBound(vModule, 1)
              vLocal(i, 0) = vModule(i, 0)
              Label1.Caption = vLocal(i, 0)
              Set o = CreateObject("arraysvr.class1")
              bRet = o.UseArray(vLocal)
           Next i
           Set o = Nothing
        End Sub

        Private Sub Form_Load()
           Command2.Enabled = False
        End Sub

5. Press the F5 key to run the client.

6. When you press Command1 and then Command2, you should see Label1 display
  "One,Zero."

7. Compile the project into Native code.

8. Run the compiled exe. When you press Command1 and then Command2, you get a
  message as shown below:

  The instruction at '0x0f0fdbda' referenced memory at '0x0000b1a4'.
  The memory could not be 'read'.

Ways to Solve Problem
---------------------

There are two ways to resolve this problem. You can either compile in P- code or
use a temporary Variant variable for the argument as shown in the modified code
below:

     Private Sub Command2_Click()
        Dim o As ArraySvr.Class1
        Dim vLocal(1, 0) As Variant
        Dim i As Long
        Dim bRet As Boolean
        Dim vTmp as Variant    'temporary variable for passing argument

        For i = LBound(vModule, 1) To UBound(vModule, 1)
           vLocal(i, 0) = vModule(i, 0)
           Label1.Caption = vLocal(i, 0)
           vTmp = vLocal
           Set o = CreateObject("arraysvr.class1")
           'instead of passing vLocal directly, we pass vTmp
           bRet = o.UseArray(vTmp)
        Next i
        Set o = Nothing
     End Sub

Additional query words: kbdss kbDSupport kbVBp Access Violation MDAC 2.0 kbMDAC200 kbCompiler kbVBp500bug kbVBp600fix

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Keywords          : kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB500
Version           : :5.0
Issue type        : kbbug
Solution Type     : kbfix

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