VB Migration Partner


Previous | Index | Next 

[INFO] VB.NET property values passed to a ByRef parameter can be modified

Note: this article only applies to conversions to VB.NET.

Under VB6, passing a writable property is equivalent to passing the value returned by a method - more precisely, the property’s getter method -, therefore if you pass the value of a property to a ByRef argument the called method receives a copy of the property value; if the method modifies the parameter’s value, the new value isn’t assigned to the property.

Under the same circumstances, however, VB.NET does modify the property when the method exits. This detail can lead to various issues when the VB6 code is migrated, from minor behavioral differences to runtime errors. Consider the following VB6 code:

        Sub Form_Load()
            LogAction "Load form", Me.Caption
        End Sub

        Sub LogAction(action As String, detail As String)
            detail = UCase(detail)
            Debug.Print action & ": " & detail
        End Sub

This code works as intended under VB6, but the converted VB.NET code mistakenly converts the form’s caption to uppercase, because it is a writable property passed to a ByRef parameter and the parameter is modified inside the LogAction method. This issue is quite common, because ByRef is the default parameter passing mechanism under VB6 and lazy developers tend to omit the explicit ByVal keyword that would be more appropriate here.

There are two ways to fix this problem, but VB Migration Partner can’t apply these fixes automatically. By far, the most frequent case is parameters that use the by-reference semantics only because the VB6 developer omitted the ByVal keyword in the parameter’s definition: in such cases, adding the ByVal keyword before the migration solves the problem.

        Sub LogAction(action As String, ByVal detail As String)
            detail = UCase(detail)
            Debug.Print action & ": " & detail
        End Sub

If the method does require by-reference semantics, you must ensure that you pass the method a copy of the property’s value rather than the value itself. You can do this by enclosing between parenthesis the value being passed to the method:

        Sub Form_Load()
            LogAction "Load form", (Me.Caption)
        End Sub
Previous | Index | Next 

Follow Francesco Balena on VB6 migration’s group on


Read Microsoft Corp’s official case study of a VB6 conversion using VB Migration Partner.

Code Architects and its partners offers remote and onsite migration services.

More details

Subscribe to our free newsletter for useful VB6 migration tips and techniques.


To learn more about your VB6 applications, run VB6 Analyzer on your source code and send us the generated text file. You will receive a detailed report about your VB6 applications and how VB Migration Partner can help you to quickly and effectively migrate it to .NET.

Get free advice

A fully-working, time-limited Trial Edition of VB Migration Partner allows you to test it against your actual code

Get the Trial

The price of VB Migration Partner depends on the size of the VB6 application, the type of license, and other factors

Request a quote

Migrating a VB6 application in 10 easy steps

Comparing VB Migration Partner with Upgrade Wizard

Migration tools: Feature Comparison Table

All whitepapers