Question

I’m having some trouble getting cross-thread communication/field-updating working properly in my VB.NET 2010 program. I’m trying to update a field on my main form whenever a thread that I’ve started throws an event. Here’s a simplified version of my code:

My main form:

Public Class Main
    ' stuff

    ' Eventually, startProcessing gets called:
    Private Sub startProcessing()
        Dim processingClass = New MyProcessingClass("whatever")
        AddHandler processingClass.processStatusUpdate, AddressOf handleProcessStatusUpdate
        Dim processingThread = New Thread(AddressOf processingClass.process)
        processingThread.Start()
    End Sub

    Private Sub handleProcessStatusUpdate(statusUpdate As String)
        txtMainFormTextBox.Text = statusUpdate ' InvalidOperationException
        ' "Cross-threaded operation not valid: Control 'txtMainFormTextBox' accessed from a thread other than the thread it was created on"
    End Sub
End Class

The class which raises the event:

Public Class MyProcessingClass
    Private whatever As String    

    Public Event processStatusUpdate(status As String)

    Public Sub New(inWhatever As String)
        whatever = inWhatever
    End Sub

    Public Sub process()
        ' do some stuff
        RaiseEvent processStatusUpdate(whatever)
    End Sub
End Class

As you can see, the handler in my main class doesn’t have access to the TextBox I need since it was triggered by a different thread (I think). I’ve tried a number of other approaches to get this working, including:

1. Moving the event handler to MyProcessingClass, and passing txtMainFormTextBox by reference (ByRef) to the class.
2. Having the actual thread start inside of MyProcessingClass instead of Main.

None of these have worked. Clearly there’s a concept that I’m missing here. What’s the best way to get this done? Thanks!

Answer 1

You need to update the textbox on the UI thread by calling BeginInvoke.

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You should use the BackgroundWorker component, which does all of this for you.
Simply handle the DoWork and ProgressChanged events.