Question

I am getting this error..

I was trying to do progress bar that process database insert method with more than 1000 record. I need the progress bar to load exactly the same process in threading..

I got the error in; cross-threading operation not valid; control ‘listview1’ accessed from a thread other than the thread it was created on.

    Imports System.IO
Public Class Form1
    Private WithEvents DT As New DataTable
    Private Delegate Sub AsyncDelegate(ByVal value As Integer)
    Private ProgressUpdater As New AsyncDelegate(AddressOf UpdateProgress)
    Private DataLoader As AsyncDelegate
    Private DataLoaderIAsync As IAsyncResult
    Private UpdateIncrement As Integer

    Private Sub LoadListAttendance()
        Dim txtLine As String = ""
        ListView1.Items.Clear()
        If File.Exists(strFileNameAndPath) = True Then
            Dim objReader As New StreamReader(strFileNameAndPath)

            Do While objReader.Peek() <> -1
                Dim strLine As String = objReader.ReadLine()
                If Split(strLine, " ")(0) = "" Then MessageBox.Show("Invalid file.", SysMsg, MessageBoxButtons.OK) : Exit Sub
                ListView1.Items.Add(Split(strLine, " ")(0))
                ListView1.Items(ListView1.Items.Count - 1).SubItems.Add(Mid(Split(strLine, " ")(1), 1, 8))
                ListView1.Items(ListView1.Items.Count - 1).SubItems.Add(Mid(Split(strLine, " ")(1), 9, 4))
                ListView1.Items(ListView1.Items.Count - 1).SubItems.Add(Mid(Split(strLine, " ")(1), 13, 2))
            Loop
        End If
    End Sub


    Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
        'Initialize the progress bar.

        'Run a scalar command to select the row count and set the progress maximum

        'Calculate a percentage for the Row Changed event.
        '  ie if total recs is 1,000,000 set the progress amount to 10% of that.
        '  *See DT_RowChanged

        UpdateIncrement = 100000

        'DeEnable any controls you may not want the user to touch until the 
        'load is complete.

        'Load the data Ascync
        DataLoader = New AsyncDelegate(AddressOf LoadData)
        DataLoaderIAsync = DataLoader.BeginInvoke(0, New AsyncCallback(AddressOf LoadData_Completed), Nothing)


    End Sub





    Private Sub LoadData()

        'Your Load data code.
        'This sample code loads 1,000,000 recs

        'Do not access any controls in the procedure
        If Not checkServer() = True Then frmServerSet.ShowDialog()
        For x As Integer = 0 To ListView1.Items.Count - 1
            'If RecExist_2("tbltxt", "date", ListView1.Items(x).SubItems(1).Text, "id", ListView1.Items(x).Text, False) = False Then
            sqlSTR = "insert into tbltxt (id,date,time,code) values"
            sqlSTR = sqlSTR & " ( '" & ListView1.Items(x).Text & "','" & ListView1.Items(x).SubItems(1).Text & "','" & ListView1.Items(x).SubItems(2).Text & "', '" & ListView1.Items(x).SubItems(3).Text & "')" ','" & t1 & "')"

            ExecuteSQLQuery(sqlSTR)

            'End If
        Next
        MsgBox("ok")
    End Sub

    Private Sub LoadData_Completed()

        'Async op has completed.
        'Call the endinvoke to the delegate and unlock the UI

        'Do not access any controls in this procedure

        DataLoader.EndInvoke(DataLoaderIAsync)

        Me.Invoke(New AsyncDelegate(AddressOf UnlockUI), New Object() {Nothing})

    End Sub

    Private Sub UnlockUI()
        'Do what you want to the UI Here.
        ProgressBar1.Visible = False

    End Sub

    Private Sub UpdateProgress(ByVal value As Integer)
        'Updates the progress bar from the thread it was
        'created on.
        If ProgressBar1.InvokeRequired Then
            ProgressBar1.Invoke(ProgressUpdater, New Object() {value})
        Else
            If value > ProgressBar1.Maximum Then value = ProgressBar1.Maximum
            ProgressBar1.Value = value
        End If

    End Sub

    Private Sub DT_RowChanged(ByVal sender As Object, ByVal e As System.Data.DataRowChangeEventArgs) Handles DT.RowChanged
        'Handle a row change
        'Do not access any controls from this procedure

        Static counter As Integer = 1

        'first make sure the row is being added
        If e.Action = DataRowAction.Add Then
            'next check for an increment.  In this case I have a hard coded
            '100,000 Which is 10% of the million records that I know I have.

            'Do not update the progress bar on every record

            If counter Mod UpdateIncrement = 0 Then
                UpdateProgress(counter)
            End If
            counter += 1
        End If

    End Sub
End Class

Answer 1

LoadData runs on a background thread; you cannot talk to the list-view (even to read values) from this thread. More commonly. You would on the UI thread build some model from the UI (I would have a list of some custom class that expresses each of the values as properties), then make that model available to the worker thread. Then in the background thread, you only access your model that is unrelated to the UI.

Also: don’t concatenate values to make SQL. That is suicide – it is a clear SQL injection risk. Use a parameterless query instead, or a tool that handles parameterization for you.