combobox multiple thread error

Question

I have a problem with my code. I keep getting Multiple thread Error with backgroundworker, because of the combobox item display. Please look at my code below its a very simple code which I am planning to use on big scale, all I want it to do is "If item "1" selected show item "1" in label1. I can only assume that problem exists because Combobox runs in different thread....

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    BackgroundWorker1.runworkerasync()
    BackgroundWorker1.WorkerReportsProgress = True
    Me.Cursor = Cursors.WaitCursor 'Cursor changes to wait
End Sub
Public Structure controlwithtext
    Public controlname As Control
    Public text As String
    Public Sub New(ByVal ctrl As Control, ByVal text As String)
        Me.controlname = ctrl
        Me.text = text

    End Sub
End Structure

Private Sub BackgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As DoWorkEventArgs) Handles BackgroundWorker1.DoWork

If comboBox1.SelectedItem = "1" then
        BackgroundWorker1.ReportProgress(5, New controlwithtext(Label1, ComboBox1.SelectedItem))

End If

End Sub

Private Sub SetBackgroundWorker_ProgressChanged(ByVal sender As Object, 
            ByVal e As ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged

    If TypeOf e.UserState Is controlwithtext Then
        Dim cwt As controlwithtext = CType(e.UserState, controlwithtext)
        cwt.controlname.Text = cwt.text
    End If

End Sub

What work is really supposed to go on in DoWork? Do the long running task in DoWork, dont talk to (or create) controls there. — Ňɏssa Pøngjǣrdenlarp, Oct 12 '15 at 02:08
Possible duplicate of [Crossthread operation not valid... - VB.NET](http://stackoverflow.com/questions/2240702/crossthread-operation-not-valid-vb-net) — Nathan Tuggy, Oct 12 '15 at 02:08
The thing that is wrong with that answer, @NathanTuggy, is that it has 2 upvoted answers advising `CheckForIllegalCrossThreadCalls = False` — Ňɏssa Pøngjǣrdenlarp, Oct 12 '15 at 02:18
Why are you trying to use background worker to do something so short-running? — Enigmativity, Oct 12 '15 at 03:21
Thanks for the help guys, the actual code is 500+ pages long, everything works fine, it just I decided to put background worker so program doesn't hang while .texts being populated, and run into dilemma with a cross-threads — Alek, Oct 13 '15 at 02:39

score 2 · Accepted Answer · answered Oct 12 '15 at 02:38

Here's an example of how to read from and write to controls from the BackgroundWorker thread:

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    BackgroundWorker1.RunWorkerAsync()
End Sub

Private Sub BackgroundWorker1_DoWork(sender As Object, e As DoWorkEventArgs) Handles BackgroundWorker1.DoWork
    While True
        System.Threading.Thread.Sleep(250)

        Dim selection As String = Me.Invoke(Function()
                                                If Not IsNothing(ComboBox1.SelectedItem) Then
                                                    Return ComboBox1.SelectedItem.ToString
                                                Else
                                                    Return String.Empty
                                                End If
                                            End Function).ToString

        If selection = "1" Then
            Me.Invoke(Sub()
                          Label1.Text = ComboBox1.SelectedItem.ToString
                      End Sub)
        Else
            Me.Invoke(Sub()
                          Label1.Text = "something else"
                      End Sub)
        End If
    End While
End Sub

Thanks a lot !! Everything populates now ( I have multiple labels) Now for some reason it hangs after populating all the texts... Only if I disable "While" it works fine, but I understand while needs to be there for refresh? Or am I confusing something? Also "On Error GoTo" not supported with invoke? What would you recommend to use instead? Is "Try" a really good alternative? Thanks — Alek, Oct 13 '15 at 04:13
The `While` was really just there to keep the worker thread alive and show that it was doing something. The **point** of the post was to show you how to read/write from the thread. Those two activities don't require a while loop. You need to figure out how to incorporate those examples into your existing code... — Idle_Mind, Oct 13 '15 at 05:18

combobox multiple thread error

1 Answers1