CODEWITHSUNDEEP
vb.net
ILYA20
ILYA20

Reputation: 31

vb.net backgroundworker CancelAsync not work

I have a stop problem in the backgroundworker, I use the following commands for backgroundworker:

update codes

Private Sub UnpackSystem_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles UnpackSystem.DoWork
    Dim worker As System.ComponentModel.BackgroundWorker = DirectCast(sender, System.ComponentModel.BackgroundWorker)
    For i As Integer = 1 To 100
        If worker.CancellationPending Then
            e.Cancel = True
            Exit For
        End If
        worker.ReportProgress(i, i & " iterations complete")
        Threading.Thread.Sleep(250)
        Dim oProcess As New Process()
        Dim oStartInfo As New ProcessStartInfo("cmd.exe", "/c bin\Imgtool\simg2img.exe tmp/system.img tmp/system.img.ext4")
        oStartInfo.WindowStyle = ProcessWindowStyle.Hidden
        oStartInfo.CreateNoWindow = True
        oStartInfo.UseShellExecute = False
        oStartInfo.RedirectStandardOutput = True
        oProcess.StartInfo = oStartInfo
        oProcess.Start()
        Dim sOutput As String
        Using oStreamReader As System.IO.StreamReader = oProcess.StandardOutput
            sOutput = oStreamReader.ReadToEnd()
        End Using
        TextBox8.Invoke(Sub() TextBox8.AppendText(Environment.NewLine & sOutput))

    Next i

End Sub

Private Sub UnpackSystem_ProgressChanged(ByVal sender As Object, ByVal e As System.ComponentModel.ProgressChangedEventArgs) Handles UnpackSystem.ProgressChanged
    Me.ProgressBar5.Value = e.ProgressPercentage
End Sub

Private Sub UnpackSystem_RunWorkerCompleted(ByVal sender As Object, ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles UnpackSystem.RunWorkerCompleted
    If e.Cancelled = True Then
        TextBox8.AppendText(Environment.NewLine & "Canceled!")
    ElseIf e.Error IsNot Nothing Then
        TextBox8.AppendText(Environment.NewLine & "Error: " & e.Error.Message)
    Else
        TextBox8.AppendText(Environment.NewLine & "Done!")
    End If
End Sub

and, I use the following code to stop backgroundworker proccess ...

Private Sub Button55_Click(ByVal sender As System.Object, ByVal e As EventArgs) Handles Button55.Click
    If Me.UnpackSystem.IsBusy Then
        Me.UnpackSystem.CancelAsync()
    End If
    Dim cmd() As Process
    cmd = Process.GetProcessesByName("cmd")
    If cmd.Count > 0 Then
        Process.GetProcessesByName("cmd")(0).Kill()
    End If
End Sub

But it not be canceled, Where is my problem?

Upvotes: 0

Views: 599

Answers (1)

jmcilhinney
jmcilhinney

Reputation: 54427

You are only testing whether a cancellation is pending when the DoWork event handler first starts, which of course it will not be at that stage. In order for the background task to be cancelled later on, you would have to actually test whether a cancellation is pending later on.

There's no magic here though. As I said, in order to cancel, you have to actually test whether a cancellation is pending within the DoWork event handler. That test can only come between other lines of code. Once you call that ReadToEnd method, you are going to read the stream to the end, whether the user has requested cancellation or not. You obviously can't test for a pending cancelation until that call returns

Upvotes: 1

Related Questions