FIX: Public Properties of VB4 Class Are Passed by Reference

ID: Q166928

The information in this article applies to:

Microsoft Visual Basic Control Creation, Learning, Professional, and Enterprise Editions for Windows, version 5.0
Microsoft Visual Basic Professional and Enterprise Editions for Windows, version 4.0

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 kbGrpVB VB4WIN
Version : 4.0 5.0
Platform : WINDOWS
Issue type : kbbug