VB.Net (or C#) 2008 Multi Threaded Import

Question

I am looking to build a multi-threaded text import facility (generally CSV into SQL Server 2005) and would like to do this in VB.NET but I am not against C#. I have VS 2008 trial and just dont know where to begin. Can anyone point me in the direction of where I can look at and play with the source of a VERY simple multi-threaded application for VS 2008?

Thanks!

Answer 1

The referenced DevX article is from 2001 and .Net Framework 1.1, but today .Net Framework 2.0 provides the BackgroundWorker class. This is the recommended threading class if your application includes a foreground UI component.

From MSDN Threads and Threading:

If you need to run background threads that interact with the user interface, the .NET Framework version 2.0 provides a BackgroundWorker component that communicates using events, with cross-thread marshaling to the user-interface thread.

This example from MSDN BackgroundWorker Class shows a background task, progress %, and cancel option. (The example is longer than the DevX sample, but has a lot more functionality.)

Imports System.ComponentModel

Partial Public Class Page
    Inherits UserControl
    Private bw As BackgroundWorker = New BackgroundWorker

    Public Sub New()
        InitializeComponent()

        bw.WorkerReportsProgress = True
        bw.WorkerSupportsCancellation = True
        AddHandler bw.DoWork, AddressOf bw_DoWork
        AddHandler bw.ProgressChanged, AddressOf bw_ProgressChanged
        AddHandler bw.RunWorkerCompleted, AddressOf bw_RunWorkerCompleted

    End Sub
    Private Sub buttonStart_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)
        If Not bw.IsBusy = True Then
            bw.RunWorkerAsync()
        End If
    End Sub
    Private Sub buttonCancel_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)
        If bw.WorkerSupportsCancellation = True Then
            bw.CancelAsync()
        End If
    End Sub
    Private Sub bw_DoWork(ByVal sender As Object, ByVal e As DoWorkEventArgs)
        Dim worker As BackgroundWorker = CType(sender, BackgroundWorker)

        For i = 1 To 10
            If bw.CancellationPending = True Then
                e.Cancel = True
                Exit For
            Else
                ' Perform a time consuming operation and report progress.
                System.Threading.Thread.Sleep(500)
                bw.ReportProgress(i * 10)
            End If
        Next
    End Sub
    Private Sub bw_RunWorkerCompleted(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs)
        If e.Cancelled = True Then
            Me.tbProgress.Text = "Canceled!"
        ElseIf e.Error IsNot Nothing Then
            Me.tbProgress.Text = "Error: " & e.Error.Message
        Else
            Me.tbProgress.Text = "Done!"
        End If
    End Sub
    Private Sub bw_ProgressChanged(ByVal sender As Object, ByVal e As ProgressChangedEventArgs)
        Me.tbProgress.Text = e.ProgressPercentage.ToString() & "%"
    End Sub
End Class

Answer 2

About the best threading document I ever found was this http://www.albahari.com/threading/

If I may, the problem with simple examples is that that they're often too simple. Once you get past the counting or sort in background demos you generally need to update the UI or similar and there are some gotchas. Similarly you rarely have to deal with resource contention in simple examples and having threads degrade gracefully when a resource isn't available (such as a Db connection) requires thought.

Conceptually you need to decide how you're going to distribute your work across the threads and how many do you want. There's overhead associated with managing threads and some mechanisms use a shared thread pool that could be subject to resource contention itself (for example, any time you run a program that simply displays an empty form, how many threads do you see under task manager).

So for your case, you threads doing the actual uploading need to signal back if they've completed, if they've failed (and what the failure was). The controller needs to be able to deal with those and manage the start/stop processes and so on.

Finally (almost), assuming that making something multithread will increase performance doesn't always hold true. If for example, you chop a file up into segments but it has to travel across a low speed link (ADSL say), you're constrained by external forces and no amount of threading trickery is going to get around that. The same can apply for database updates, web requests, anything invloving large amounts of disk i/o and so on.

Despite all this, I'm not the prophet of doom. The references here are more than adequate to help you achieve what you want but be aware that one of the reasons threading seems complicated is because it can be :)

If you want more control than the BackgroundWorker/Threadpool but don't want to do everything yourself there are at least two very good freebie threading libraries knocking around the place (Wintellect & PowerThreading)

Cheers

Simon

Answer 3

This is a great article:

http://www.devx.com/DevX/10MinuteSolution/20365

In particular:

Dim t As Thread
t = New Thread(AddressOf Me.BackgroundProcess)
t.Start()

Private Sub BackgroundProcess()
   Dim i As Integer = 1
   Do While True
        ListBox1.Items.Add("Iterations: " + i)
        i += 1
        Thread.CurrentThread.Sleep(2000)
   Loop
End Sub