Question

I have a background worker which calls a function within a separate class. This process may be required to be canceled at any time via a button click from the front end. I have tried using CancelAsync() but this has no effect. The cofunds_downloadfiles is the function which i am calling. How do i go about canceling the process?

TIA.

Private Sub btnProcessdld_Click(sender As System.Object, e As System.EventArgs) Handles btnProcessdld.Click

    Dim cfHelper As New CoFundsHelper

    If btnProcessdld.Text = "Process" Then
        btnProcessdld.Text = "Cancel"

        If chkDailyFiles.Checked = False And chkWeeklyFiles.Checked = False Then
            MessageBox.Show("Please select which files you want to download")
        Else

            lblProgress.Text = "Processing...if you are downloading weekly files this may take a few minutes"
            uaWaitdld.AnimationEnabled = True
            uaWaitdld.AnimationSpeed = 50
            uaWaitdld.MarqueeAnimationStyle = MarqueeAnimationStyle.Continuous
            uaWaitdld.MarqueeMarkerWidth = 60

            _backGroundWorkerdld = New BackgroundWorker
            _backGroundWorkerdld.WorkerSupportsCancellation = True
            _backGroundWorkerdld.RunWorkerAsync()

        End If

    ElseIf btnProcessdld.Text = "Cancel" Then
        btnProcessdld.Text = "Process"
        _backGroundWorkerdld.CancelAsync()
        uaWaitdld.AnimationEnabled = False

    End If

Private Sub StartProcessdld(ByVal sender As Object, _
    ByVal e As System.ComponentModel.DoWorkEventArgs) Handles _backGroundWorkerdld.DoWork

    Dim cfHelper As New CoFundsHelper
    cfHelper.ConnString = PremiumConnectionString
    Dim dateValue As String

    Dim weekly As Boolean = False
    Dim daily As Boolean = False

    If dtePicker.Value IsNot Nothing Then
        dateValue = Format(dtePicker.Value, "yyyyMMdd")

        If chkWeeklyFiles.Checked = True Then
            weekly = True
        End If
        If chkDailyFiles.Checked = True Then
            daily = True
        End If

        cfHelper.Cofunds_DownloadFiles(dateValue, weekly, daily)

    Else
        Throw (New Exception("Date Field is empty"))
    End If
End Sub

Answer 1

Basically you can do the following:

• in the DoWork sub, test for cancellationpending property
• if it’s true then you simply do not call that function and maybe put e.Cancelled = true then check this in the RunWorkerCompleted and decide what you have to do.

• if you need to cancel it, simply make a Stop() sub in your class that does exactly that – stops the procedure. Then, you simply need to invoke it like

  Me.Invoke(Sub()
            myClass.Stop()
         End Sub)
  

• you may need to suspend the background worker until the call from your main thread has returned. You do this using semaphores: Private chk As New Semaphore(1,1,"checking1") You put this as a global variable to both your main thread and the background worker.

• in the backgroundworker_doWork you use the semaphore like chk.WaitOne() AFTER the line that need to execute.
• in the method of your class, when it has finished with computing you put a.Release

The semaphore is only needed if you need to make sure you wait for a result. It kind of defeats the purpose of multithreading but you can perform other actions in the worker before waiting for the main thread (like starting another thread with something else etc).

Other than that invoking a stop method should be enough. Sorry that i haven’t had time to analyze your code but i hope i put you in the right direction.