Q166928: FIX: Public Properties of VB4 Class Are Passed by Reference

Article: Q166928
Product(s): Microsoft Visual Basic for Windows
Version(s): 4.0 5.0
Operating System(s): 
Keyword(s): kbVBp500 kbGrpDSVB
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Control Creation Edition for Windows, version 5.0 
- 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 
- Microsoft Visual Basic Professional Edition for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 4.0 
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SYMPTOMS
========

If you have a Visual Basic 4.0 class with a public property implemented like the
following:

    Public MyProp As Integer

Visual Basic 4.0 passes the property by reference (ByRef).

However, if your property is implemented in Visual Basic 4.0 using property
procedures (Property Get, Property Let, Property Set), it will be passed by
value (ByVal).

STATUS
======

This problem has been fixed in Visual Basic 5.0. All properties are now passed
by value (ByVal) for consistency.

In addition, all code internal to the class still has direct access to member
variable data:

    MyClass.CLS
    ===========
    Public MyProp As Integer

    Public Sub MyMethod()
      MyProp = 5  '// This has direct access to the class data
    End Sub
    ===========

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

Microsoft has acknowledged that this change in behavior may be an issue for some
developers porting Visual Basic 4.0 code to Visual Basic 5.0. Code that relies
on the ByRef functionality stated above will need to be modified.

One possible way to modify the Visual Basic 4.0 for porting to Visual Basic 5.0
is shown below:

Original Visual Basic 4.0 code:

     MyClass.CLS
     ===========
     Public MyProp As Integer
     ===========

     MyBas.BAS
     =========
     Sub Compute(ByRef iTarget As Integer)
       Dim iTemp as Integer
       '// implementation code
       iTarget = iTemp
     End Sub

     Sub Main()
       Dim MyObject As MyClass
       MyObject.MyProp = 97
       Call Compute(MyObject.MyProp)
     End Sub
     =========

Modified Sub Main() for Visual Basic 5.0:

     Sub Main()
       Dim MyObject As MyClass
       Dim iTtemp1 As Integer
       Dim iTemp2 As Integer
       MyObject.MyProp = 97
       iTemp1 = MyObject.MyProp
       iTemp2 = iTemp1
       Call Compute(iTemp2)
       If iTemp1 <> iTemp2 Then
         MyObject.MyProp = iTemp2
         iTemp1 = iTemp2
       End If
     End Sub

Exposing public variables is not a recommended programming technique. The
following demonstrates possible modifications to MyClass.CLS, illustrating a
data-hiding technique to preserve class member data:

     MyClass.CLS
     ===========
     Private m_MyProp As Integer

     Public Property Let MyProp(NewValue As Integer)
       '// implement validation code and data formatting
       m_MyProp = NewValue
     End Property

     Public Property Get MyProp() As Integer
       MyProp = m_MyProp
     End Property

Additional query words:

======================================================================
Keywords          : kbVBp500 kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVBA500Search kbVBA500 kbVB500 kbVB400Search kbVB400 kbZNotKeyword3
Version           : 4.0 5.0
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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