CODEWITHSUNDEEP
.netvb.netmultithreadingasynchronoustask
ElektroStudios
ElektroStudios

Reputation: 20464

Unable to re-run a cancelled Task

I'm trying to run an asynchronous task, cancel it, and run it again, but when I cancel it for first time I'm not able anymore to run it again, what I'm doing wrong?.

Private TypeTask As Threading.Tasks.Task
Private TypeTaskCTS As New Threading.CancellationTokenSource
Private TypeTaskCT As Threading.CancellationToken = TypeTaskCTS.Token
Private RequestCancel As Boolean = True

Private Sub TypeWritter(ByVal CancellationToken As Threading.CancellationToken,
                        ByVal [Text] As String,
                        ByVal TypeSpeed As Integer,
                        ByVal PauseSpeed As Integer)

    ' For each Character in text to type...
    For Each c As Char In [Text]

        ' If not want to cancel then...
        If Not CancellationToken.IsCancellationRequested Then

            ' Type the character.
            Console.Write(CStr(c))

            ' Type-Wait.
            Threading.Thread.Sleep(TypeSpeed)

            If ".,;:".Contains(c) Then
                ' Pause-Wait.
                Threading.Thread.Sleep(PauseSpeed)
            End If

        Else ' want to cancel.

            ' Reset the request cancellation.
            RequestCancel = False

            ' Exit iteration.
            Exit For

        End If ' CancellationToken.IsCancellationRequested

    Next c ' As Char In [Text]

End Sub

Public Sub TypeWritter(ByVal [Text] As String,
                       Optional ByVal TypeSpeed As Integer = 75,
                       Optional ByVal PauseSpeed As Integer = 400)

    ' Run the asynchronous Task.
    TypeTask = Threading.Tasks.
               Task.Factory.StartNew(Sub()
                                         TypeWritter(TypeTaskCT, [Text], TypeSpeed, PauseSpeed)
                                     End Sub, TypeTaskCT)

    ' Until Task is not completed or is not cancelled, do...
    Do Until TypeTask.IsCompleted OrElse TypeTask.IsCanceled

        If RequestCancel Then

            If Not TypeTaskCTS.IsCancellationRequested Then
                TypeTaskCTS.Cancel
            End If

            RequestCancel = False
            Exit Do

        End If

    Loop ' TypeTask.IsCompleted OrElse TypeTask.IsCanceled

End Sub

Public Sub TypeWritterLine(ByVal [Text] As String,
                           Optional ByVal TypeSpeed As Integer = 75,
                           Optional ByVal PauseSpeed As Integer = 400)

    TypeWritter([Text] & vbCrLf, TypeSpeed, PauseSpeed)
    Console.WriteLine()

End Sub

Notice the variable:

Private RequestCancel As Boolean = True

Which is set to True to cancel the task when used for first time (just for make things faster to test what happens when I try to call the Task a second time, where I'm expecting the error).

The usage that I'm trying is this:

Sub Main()

    RequestCancel = True ' This should cancel this task:
    TypeWritterLine("Some text")

    ' And this task should run as normally, but it doesn't, I get an empty line:
    TypeWritterLine("Some other text")

End Sub

Upvotes: 0

Views: 161

Answers (1)

Dennis Kassel
Dennis Kassel

Reputation: 2786

This is by design. A task is always executed once, even if not begin canceled before. This means that you must pay attention to not call a task's Start method multiple times. For "rerunning" a task you will have to call the Task.Factory.StartNew method again.

Upvotes: 1

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