PRB: ByRef Has No Effect on Form's Public Variables

ID: Q216481

The information in this article applies to:

Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

SYMPTOMS

When you have a subroutine that has a ByRef parameter, the subroutine changes the values of that parameter when it returns. However, if you pass a form's public variables to this subroutine, you find that the variable you passed in has not been changed.

CAUSE

Form Modules, Class Modules, and User Control Modules are object modules. The public variables defined in these modules are actually implemented as properties, as demonstrated in the following:


Public FormPubVar as Integer

is effectively implemented as:


Private m_FormPubVar as Integer

Public Property Get FormPubVar() as Integer
   FormPubVar = m_FormPubVar
End Property

Public Property Let FormPubVar(newVal as Integer)
   m_FormPubVar = newVal
End Property

When you pass a form's public variable into a subroutine with the syntax Form1.FormPubVar, the Property Get is called and the result is placed in a temporary variable. It is this temporary variable that is actually passed to the subroutine. When the subroutine returns, only the value stored in the temporary variable is changed. The same problem occurs when passing a String parameter ByVal.

RESOLUTION

Use a temporary variable as in the following code snippet to work around the problem:


Dim temp as Integer
temp = Form1.FormPubVar
ChangeVar temp ' where ChangeVar is the Sub receiving the ByRef parameter
Form1.FormPubVar = temp

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce Behavior

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

Add a CommandButton to Form1.

Paste the following code in the General Declarations section of Form1:


Option Explicit
Public FormPubVar As Integer

Private Sub Command1_Click()
Dim LocVar As Integer

LocVar = 10
ChangeVar LocVar
MsgBox "LocVar is changed from 10 to " & LocVar

ModulePubVar = 10
ChangeVar Module1.ModulePubVar
MsgBox "ModulePubVar is changed from 10 to " & ModulePubVar

FormPubVar = 10
ChangeVar FormPubVar
MsgBox "FormPubVar is changed from 10 to " & FormPubVar

FormPubVar = 10
ChangeVar Me.FormPubVar
MsgBox "Me.FormPubVar is changed from 10 to " & Me.FormPubVar
End Sub

On the Project menu, click Add module. Click the Open button in the Add Module dialog box.

Add the following code to the General Declarations section of Module1:


Option Explicit
Public ModulePubVar As Integer

Public Sub ChangeVar(ByRef v As Integer)
    v = v + 10
End Sub

Run the application and click Command1. Note the values displayed in each message box, clicking OK on each one. The last message box demonstrates that Me.FormPubVar has not been changed.

NOTE: When you use FormPubVar directly in the form's code window, Visual Basic passes in the internal representation of the FormPubVar instead of a temporary variable as is the case when you pass Me.FormPubVar.

Additional query words:

Keywords : kbVBp500 kbVBp600 kbGrpVB
Version : WINDOWS:5.0,6.0
Platform : WINDOWS
Issue type : kbprb