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VB.Net      BackgroundWorker

Posted on 2013-01-04
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Last Modified: 2013-01-04
Hi Experts.  I'm learning to use the backgroundworker and have completed the module below.   Clicking BtnStart starts a backgroundworker which updates a progress bar and when completed enters "Process Completed" in a textbox.  So far everything seems to work correctly

Please would someone be kind enough to check my code to see if I have it right but also I dont seem to be able to get the thread to cancel on a command button,  I have tried entering  a line in the do_work sub that checkes if cancel is pending and then using a sub with BGWorker.CancelAsync().  Howver this does not work.

Also, when the thread is running, should I be able to see an additional process in the task manager/Processes?     MAny Thanks



Imports System.Threading
Module TestingBGWorker
    Public WithEvents BGWorker As System.ComponentModel.BackgroundWorker
    Delegate Sub UpdateTextBoxDelegate()
    Dim abc As Integer = 0
    Public Sub StartBGWorker()
        BGWorker = New System.ComponentModel.BackgroundWorker
        BGWorker.WorkerReportsProgress = True
        BGWorker.WorkerSupportsCancellation = True
        BGWorker.RunWorkerAsync()
    End Sub
    Private Sub TestWorker_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BGWorker.DoWork
        For i = 1 To 100
            Thread.Sleep(50)
            abc = i
            BGWorker.ReportProgress(Nothing)
        Next
    End Sub
    Private Sub TestWorker_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles BGWorker.ProgressChanged
        Form1.ProgressBar1.Visible = True
        Form1.ProgressBar1.Value = abc
    End Sub
    Private Sub TestWorker_RunWorkerCompleted(ByVal sender As System.Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles BGWorker.RunWorkerCompleted
        Form1.TextBox1.Text = "Process Complete"
        Form1.ProgressBar1.Visible = False
        Form1.btnCancel.Enabled = False
        Form1.BtnStart.Enabled = True
    End Sub
    Public Sub TestWorker_CancelAsync()
        Form1.btnCancel.Enabled = False
        Form1.BtnStart.Enabled = True
    End Sub
End Module
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Question by:PNRT
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4 Comments
 
LVL 70

Assisted Solution

by:Éric Moreau
Éric Moreau earned 200 total points
ID: 38744634
in your for...loop, you need to check for CancellationPending

check http://emoreau.com/Entries/Articles/2006/12/The-BackgroundWorker-component.aspx
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LVL 85

Expert Comment

by:Mike Tomlinson
ID: 38744875
Also, as shown in Eric's article, pass the progress value out through ReportProgress() instead of using a global variable.

Speaking of global variables, why did you decide to put this code into a Module?  Since you're making everything go back to Form1 through default instance, why not have the code in the Form itself?...
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LVL 2

Author Comment

by:PNRT
ID: 38745178
Thanks idle, thats the sort of guidance I was looking for.  I ultimately needed to get a lot of variables out from the do_work and wasnt sure how to do this without global variables.   Thats also why I was going for the module.   It needs to output to other than the ofrm.   Any ideas how I would handle multiple cariable from the do_work?  Thanks again for the reply
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LVL 85

Accepted Solution

by:
Mike Tomlinson earned 300 total points
ID: 38745206
You can pass out ANYTHING in the ReportProgress() method.  For instance, you can make a Class or Structure that holds all the values and pass an instance of that out.  You'd use the overload of ReportProgress() that accepts TWO parameters, and use the second param:
http://msdn.microsoft.com/en-us/library/a3zbdb1t.aspx

Then, in the ProgressChanged() event, you cast "e.UserState" back to whatever was passed out in ReportProgress() and you've got all your values.
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