[RESOLVED] Cancellation of task

I am trying to setup a task and when I click the stop button it should cancel. Everything I have read states I could use

Code:

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cts.Token.ThrowIfCancellationRequested()

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But when I do I get an error stating: System.OperationCanceledException: 'The operation was canceled.' and sends me into a break screen. I do have it in a try catch scenario. I am not sure why it is not being caught in my try and catch. Any idea? Did I miss somethign really stupid? I'm trying to grasp this task scheduling so I might no have done it correctly.

Code:

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Try

            ' Is the Background Worker do some work?

            If CmboDir.SelectedItem = "" Then
                CmboDir.SelectedItem = "Common"
            End If

            If CmboDept.SelectedItem = "County_Shares" Then
                ' dPath = SharedDirPath
            ElseIf CmboDept.SelectedItem = "All Depts." And CmboDir.SelectedItem = "H Drive" Then
                AlldeptU = True
                ' dPath = DeptDirPath
            ElseIf CmboDept.SelectedItem = "All Depts." And CmboDir.SelectedItem = "Common" Then
                AlldeptC = True
                dPath = DeptDirPath

            Else

                If CmboDir.SelectedItem = "H Drive" Then
                    dPath = UserDirPath & "\" & CmboDept.SelectedItem & "\Users\"
                Else
                    dPath = DeptDirPath & "\" & CmboDept.SelectedItem & "\Common\"
                End If


            End If
         

            'End If

            cts = New CancellationTokenSource
         
                SelectedDept = CmboDept.SelectedItem.ToString
         
            If BtnSearch.Text = "Stop!!" Then
                BtnSearch.Text = "Search"
                dCancel = True
                cts.Cancel()
                Exit Sub

            End If
            If tab1 = True Then

                Label4.Text = "Finding directories that '" & TxtBxGroupName.Text & "' is associated with under the '" & SelectedDept & "' " & CmboDir.SelectedItem & " Dir"


            ElseIf tab2 = True And ChBxBrokenGUID.Checked = True Then
                Label4.Text = "Finding broken GUIDs " & intDepthLimit & " levels deep under the '" & SelectedDept & "' " & CmboDir.SelectedItem & " Dir"

            Else
                Label4.Text = "Finding folders with broken Inheritance " & intDepthLimit & " levels under the '" & SelectedDept & "' " & CmboDir.SelectedItem & " Dir"
            End If

            searchLabel.Text = "Searching..."
         
         

                Dim t As New Task(Sub()

                                      For i = 0 To UBound(DeptNames)

                                          If CBool(dCancel) Or cts.Token.IsCancellationRequested = True Then
                                              ' Clean up here, then...
                                              Console.WriteLine("Cancelling per user request.")
                                              cts.Token.ThrowIfCancellationRequested()
                                              ' Throw New OperationCanceledException()
                                           
                                              Exit For
                                          End If

                                          dPath = DeptDirPath & "\" & DeptNames(i).ToString & "\Common\"
                                          RecurseDirectory(dPath, intDepthLimit, intCurrentDepth, cts.Token)

                                      Next
                                  End Sub)


                tasks.Add(t)
                t.Start()
             

                If cts.Token.IsCancellationRequested Then
                    ' Clean up here, then...
                    cts.Token.ThrowIfCancellationRequested()

                End If         


            BtnSave2Text.Enabled = False
            BtnSearch.Text = "Stop!!"
            start_time = Now

        Catch ex As OperationCanceledException
            BtnSearch.Text = "Search"
            MessageBox.Show("Search has been cancelled.")
            cancelNotification()
        Catch ex As Exception
            MessageBox.Show("We Have An Error:" + vbCrLf + vbCrLf + ex.Message)
            'Debug.WriteLine(ex.Message)
        End Try

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Make sure you don't have a check on Common Language runtime exceptions under Exception settings in Visual Studio.

See the following example for a basic example that will not break on a cancellation unless Common Language runtime exceptions is checked.

Thanks Karen, I will check that out on Monday when I'm back at work.

A short example,

Code:

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    Private CTSource As New Threading.CancellationTokenSource()
    Private Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Button1.Enabled = False
        Button2.Enabled = True
        'setup the task
        Dim someTask As Task
        Dim CT As Threading.CancellationToken = CTSource.Token

        someTask = New Task(Sub()
                                While True
                                    If CT.IsCancellationRequested Then
                                        ' Clean up here, then...
                                        ' this causes an exception that says it is unhandled
                                        ' but it IS handled on the thread that started this task, just continue
                                        CT.ThrowIfCancellationRequested()
                                    End If
                                    Threading.Thread.Sleep(100)
                                End While
                            End Sub, CTSource.Token)


        Try
            someTask.Start() 'start the task
            Await someTask 'wait for it
        Catch ex As Exception
            If someTask.Status = TaskStatus.Canceled Then
                Stop
                'blah, blah
            End If
        Finally
            Stop
            CTSource.Dispose()
            someTask.Dispose()
        End Try
    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        CTSource.Cancel() 'cancel
    End Sub

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Quote:

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Originally Posted by phpman

Thanks Karen, I will check that out on Monday when I'm back at work.

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See my GitHub repository for various code samples.

Ok, so I unchecked the System.OperationCanceledException under Exceptions settings. That fixed the breaking, still throws the error in the console, but it also does not go to the " Catch ex As OperationCanceledException" line. It just stops. When I put a break on that line so I can step into it, its brings up a file location box and looking for file "CancellationToken.cs". I think my code is correct, I think my .NET is messed up? I am running it on 4.7.2.

Thanks debasnett, I have looked a few examples, including Karen's, thank you, and I believe I am doing it correctly. I just can't figure out why my catches aren't working for this exception.

Quote:

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Originally Posted by phpman

... Thanks debasnett, I have looked a few examples, including Karen's, thank you, and I believe I am doing it correctly. I just can't figure out why my catches aren't working for this exception.

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Looking at yours and mine I can see structurally they are not the same.

If I make my code like yours, well as close as I can as I have a for next loop in mine, and I can't use the "Await" line as it tells me that "Await operator can only be used within an Async method" which it wants to change my button.click code to a Async function or sub. How does yours work but not mine? Is it the way I'm doing the for next loop and a recursive sub?

Ok so I made some changes and I have it working now. I was missing the cts.token in the task sub. And after I changed it around to be more like yours, dbasnet, thinks started to align and I could tell where my issues were. I think the biggest one was I was not using a cancel button, just renaming the search button and checking for name. Using a cancel button seemed to improve on the process.

Thank you both for pointing me in the right direction