Cross-Thread operation not valid VB.NET

Question

I looked around the site and the questions I found relating to this subject were for C# (the application that I am maintaining is written in VB.NET), so I apologize if I overlooked one.

Here is where I am calling my thread:

Private Sub saveBtn_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles saveBtn.Click
    If Not LoadedFilePath = String.Empty Then
        Dim oTrd = New Threading.Thread(AddressOf SaveData)
        oTrd.Start()
    End If
End Sub

And here are the methods:

Private Sub SaveData()
    SaveData(LoadedFilePath)
End Sub
Private Sub SaveData(ByVal filepath As String)
    If InvokeRequired Then
        Me.Invoke(New MethodInvoker(AddressOf SaveData))
    End If
    Try
        Me.Cursor = Cursors.WaitCursor
        Dim oSettings As New SettingsClass(filepath)
        Dim oEnc As New AES
        With oSettings
            //' Code removed for brevity
        End With
        oEnc = Nothing
        oSettings.SaveSettings()
        savedLbl.Visible = True
        If SavedTimeout IsNot Nothing Then
            Try
                SavedTimeout.StopEvent()
            Catch
            End Try
        End If
        SavedTimeout = New TimedEvent(Now.AddSeconds(5))
        SavedTimeout.StartEvent()
        Me.Cursor = Cursors.Default
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
End Sub

The save function works just fine, but I get the cross-thread error when the program tries to switch the cursor back to default. What can I do to fix this problem?

Answer 1

I'm sure I've seen this happen before, when we were building a multithreaded messaging app. I'll check and get back to you. IIRC it was an easy fix.

Answer 2

You started a helper thread that is not allowed to call into the user interface. You are only allowed to set the cursor from the UI-Thread itself.

To achieve this you have to fire an event from your helper thread, that tells the UI-thread that your work has finished and that it can set the cursor back or alternatively do this via Invoking into the UI-Thread:

private void ResetCursor()
{
    this.Cursor = Cursor.Default;
}

private delegate void UpdateCursor();
private void SaveData()
{
    //Do your work here
    if(this.InvokeRequired)
    {
        this.Invoke(new UpdateCurcor(ResetCursor));
    }
    else
    {
        ResetCursor();
    }
}

Answer 3

Your way of invoking the method in the owner (GUI) thread is wrong. If invocation is required, you should not execute the rest of the code in the method. If you do, you will be executing it both in the GUI thread and the background thread, and when you try to access the GUI elements from the background thread you get the cross-thread error.

The invocation should look like this:

Private Sub SaveData(ByVal filepath As String)
   If InvokeRequired Then
      Me.Invoke(New MethodInvoker(AddressOf SaveData))
   Else
      ... the actual code
   End If
End Sub

But why are you starting the method in a background thread, when it has to invoke itself in the GUI thread anyway?

Answer 4

In a multi-threaded Windows Forms application, it's illegal to call a method or property on a control from any thread other than the one that created it. All cross-thread calls must be explicitly marshalled to the thread that created the control (usually the main thread), using the Control.Invoke or Control.BeginInvoke method.

Here is a web page you can use to assist you in solving this problem:

http://www.dreamincode.net/forums/showtopic35616.htm C#

http://www.codeproject.com/KB/vb/ISinchronizedInvoke.aspx VB.NET

Answer 5

This web page has the stripped-down code needed to access a form control from a separate thread: http://www.databatrix.com/2009/09/cross-thread-operation-not-valid-net-cross-threading-to-update-form-control/