[RESOLVED] Background worker cross-thread operation not valid

[schoemr]

Hi. I have a form that is performing many calculations. Sometimes this can take maybe 10 seconds or more. In this time I wish to show user that it is busy. So I have create a progress bar (Marquee) and have put it inside panel.

So when this form is busy calculate I show progress bar. My problem is a cross-thread error. Here is my code:

Public Class frmCountHazards

    Private Sub frmCountHazards_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        MyProgressBar.Visible = True
        BackgroundWorker1.RunWorkerAsync()
    End Sub

    'BACKGOUNDWORKER = START

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

        Dim con As New SqlClient.SqlConnection(My.Settings.RiskDBConnectionString)
        con.Open()

'this is calculation. there are many more but I only put one to demonstrate. 

        Dim cmd1 As New SqlClient.SqlCommand("Select COUNT(tblRiskAssessment.Hazard) as TOTALS_Risks FROM tblContext LEFT OUTER JOIN tblRiskAssessment ON tblContext.ContextID = tblRiskAssessment.ContextID WHERE (tblContext.ContextID LIKE @ContextID) AND (tblRiskAssessment.Hazard = 'Stairs')", con)
        cmd1.Parameters.AddWithValue("ContextID", MyGlobalVariableContextID)
        Dim cnt1 As Integer = CInt(cmd1.ExecuteScalar)

this is where I get error:
       
 Me.TextBox1.Text = cnt1
   
 End Sub

    Private Sub BackgroundWorker1_RunWorkerCompleted(sender As Object, e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted

        If e.Error IsNot Nothing Then
            '' if BackgroundWorker terminated due to error
            MessageBox.Show(e.Error.Message)


        ElseIf e.Cancelled Then
            '' otherwise if it was cancelled
            MessageBox.Show("Task cancelled!")

        Else
            'otherwise it completed normally
            MyProgressBar.Visible = False

            MsgBox("Done", vbInformation, "Done")
            Me.Enabled = True
        End If
    End Sub

    'BACKGOUNDWORKER = END
End Class

[passel]

Must not have read the documentation.
You need to ReportProgress from the backgroundworker and handle the ProgressChanged event to update GUI controls. You can't update GUI controls directly from another thread. There is an example that is updating a textbox at the above link.

Ack, wrong document. That is silverlight. Just a minute.

The winform version is still pretty much the same methods.
https://msdn.microsoft.com/en-us/lib...v=vs.110).aspx

[schoemr]

Hi I was reading that documentation and I have create example. In post #1 I have said that my progress bar is "Marquee" - for this means that I don't want to report progress. The progress bar must just run until all calculations are finish.

Anyhow, I did this in the example and still produce very same error.

Imports System.ComponentModel

Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    End Sub
    Public Sub New()
        InitializeComponent()
        BackgroundWorker1.WorkerReportsProgress = True
        BackgroundWorker1.WorkerSupportsCancellation = True
    End Sub

    Private Sub startAsyncButton_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles startAsyncButton.Click
        If BackgroundWorker1.IsBusy <> True Then
            ' Start the asynchronous operation.
            BackgroundWorker1.RunWorkerAsync()
        End If
    End Sub

    Private Sub cancelAsyncButton_Click(ByVal sender As System.Object,
    ByVal e As System.EventArgs) Handles cancelAsyncButton.Click
        If BackgroundWorker1.WorkerSupportsCancellation = True Then
            ' Cancel the asynchronous operation.
            BackgroundWorker1.CancelAsync()
        End If
    End Sub

    ' This event handler is where the time-consuming work is done.
    Private Sub backgroundWorker1_DoWork(ByVal sender As System.Object, ByVal e As DoWorkEventArgs) Handles BackgroundWorker1.DoWork

        Dim worker As BackgroundWorker = CType(sender, BackgroundWorker)
        Dim i As Integer

        For i = 1 To 10
            If (worker.CancellationPending = True) Then
                e.Cancel = True
                Exit For
            Else
                ' Perform a time consuming operation and report progress.
                ' This is where calculations is done

                Dim con As New SqlClient.SqlConnection(My.Settings.RiskDBConnectionString)
                con.Open()
                Dim cmd1 As New SqlClient.SqlCommand("Select COUNT(tblRiskAssessment.Hazard) as TOTALS_Risks FROM tblContext LEFT OUTER JOIN tblRiskAssessment ON tblContext.ContextID = tblRiskAssessment.ContextID WHERE (tblContext.ContextID LIKE @ContextID) AND (tblRiskAssessment.Hazard = 'Ablution Facilities')", con)
                cmd1.Parameters.AddWithValue("ContextID", MyGlobalVariableContextID)
                Dim cnt1 As Integer = CInt(cmd1.ExecuteScalar)
                Me.txt1.Text = cnt1
                If CInt(txt1.Text) > 0 Then
                    txt1.BackColor = Color.Gold
                End If

                System.Threading.Thread.Sleep(500)
                worker.ReportProgress(i * 10)
            End If
        Next
    End Sub

    ' This event handler updates the progress.
    Private Sub backgroundWorker1_ProgressChanged(ByVal sender As System.Object, ByVal e As ProgressChangedEventArgs) Handles BackgroundWorker1.ProgressChanged
        resultLabel.Text = (e.ProgressPercentage.ToString() + "%")
    End Sub

    ' This event handler deals with the results of the background operation.
    Private Sub backgroundWorker1_RunWorkerCompleted(ByVal sender As System.Object,
    ByVal e As RunWorkerCompletedEventArgs) Handles BackgroundWorker1.RunWorkerCompleted
        If e.Cancelled = True Then
            resultLabel.Text = "Canceled!"
        ElseIf e.Error IsNot Nothing Then
            resultLabel.Text = "Error: " & e.Error.Message
        Else
            resultLabel.Text = "Done!"
        End If
    End Sub

End Class

Attachment 157089

[jmcilhinney]

Of course you get the same error. This is in your DoWork event handler:

Me.txt1.Text = cnt1
If CInt(txt1.Text) > 0 Then
    txt1.BackColor = Color.Gold
End If

That's three times you try to manipulate a control on the background thread. It's very simple: DO NOT manipulate a control in the DoWork event handler or in a method called from the DoWork event handler. That method is executed on a secondary thread, which is the whole point. If you want to update the UI during the work then call ReportProgress and update the UI in the ProgressChanged event handler. Why would you have to do that to specifically update progress but not for any other UI manipulation? The documentation for the BackgroundWorker class says this:

You must be careful not to manipulate any user-interface objects in your DoWork event handler. Instead, communicate to the user interface through the ProgressChanged and RunWorkerCompleted events.

[schoemr]

DO NOT manipulate a control in the DoWork event handler or in a method called from the DoWork event handler

Awesome! Thanks John

It is working now. I make note of this not to forget.

[jmcilhinney]

Better to make a note to always read the documentation for the types and/or members you're using. I feel like I must have mentioned that before.