Cancel Task running under User Control

Question

In my project I have few User controls changed by navigation. One of controls runs Tasks. I do it like this:

public partial class uc_WorkingArea : UserControl
{
    CancellationTokenSource cts = new CancellationTokenSource();
    CancellationToken token;

    public uc_WorkingArea()
        {
            InitializeComponent();
            this.Unloaded += uc_WorkingArea_Unloaded;
            token = cts.Token;
            Task Printer1 = Task.Run(() => PrinterPooler(lst_PrinterStruct, 0), cts.Token);
        }


         private void uc_WorkingArea_Unloaded(object sender, RoutedEventArgs e)
        {
            cts.Cancel();
            if (token.CanBeCanceled)
            {
                MessageBox.Show("Can be canceled");
            }

            if (token.IsCancellationRequested)
            {
                MessageBox.Show("Canceled requested");
            }
            cts.Cancel();
            MessageBox.Show("Status: " + Printer1.Status.ToString());
        }
}

When I leave current user control and switching to another uc_WorkingArea_Unloaded executes. I see messages, that Task can be canceled and request to cancel accepted.

But, current status of Printer1 task still "IsRunning". So, If I return back to this user control, Task starts again and Application had two running similar tasks.

I tried run task under Factory, like this

Task Printer1 = Task.Factory.StartNew(() => PrinterPooler(lst_PrinterStruct, 0), cts.Token);

But without success. App still runs two similar tasks.

PrinterPooler method not async.

I can't understand where mistake was made. Your help guys needed.

score 1 · Accepted Answer · answered Apr 04 '19 at 10:59

1

You have to pass the token into the PrintPooler method, and there inside check if it should be cancelled.

for(int i = 0; i < 10000; i++)
{
   DoStuff();
   cancelToken.ThrowIfCancellationRequested(); // if tasks end with this exception, it knows the work has been cancelled
}

Canceling a Task does not stop the execution, it only gives signal to code inside that it should end and sets the task status to Cancelled/Faulted/RanToCompletion depending on how execution stops.

Note that you need to pass the same token to the Task and to the method that will throw it.

answered Apr 04 '19 at 10:59

Krzysztof Skowronek

2,796
1
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Yep! This works perfectly. But, another question. May be you can answer on it. I have few tasks, that run the same function, but with different arguments. Can I use the same cts for every task run? – Alex Apr 04 '19 at 11:20
they can share the cancellation token and will be cancelled all at once – Krzysztof Skowronek Apr 04 '19 at 11:21
1

Nice! Thank you very much for explanations. Today you saved my life :) – Alex Apr 04 '19 at 11:22

Mar Tin · Answer 2 · 2019-04-04T11:28:41.703

Regarding to this post How do I abort/cancel TPL Tasks?

You have to Implement your cancle condition by your self. For example:

public partial class uc_WorkingArea : UserControl
{
    public CancellationTokenSource cts = new CancellationTokenSource();
    public CancellationToken token;
    public Task Printer1;

    public uc_WorkingArea()
    {
        token = cts.Token;
        Printer1 = Task.Factory.StartNew(() =>
        {
            while (!token.IsCancellationRequested)
            {
                Console.WriteLine("run");
                Application.DoEvents();
            }
        }, token);
    }
}

Cancel Call:

    uc_WorkingArea gc = new uc_WorkingArea();

    for (int i = 0; i < 10; i++) //PASS SOME TIME
    {
        Application.DoEvents(); //CONSOLE SHOULD SPAM 'RUN' FROM TASK
        Thread.Sleep(1);
    }

    gc.cts.Cancel(); //CANCEL CALL, WHILE LOOP END
    if (gc.token.IsCancellationRequested)
    {
        Console.WriteLine("stop");
        MessageBox.Show("Canceled requested");

    }
    gc.cts.Dispose();
    gc.Printer1.Dispose();

Hope it helps.

Hi. Thank you for answer. I have decision already. But, I will place your answer into my code scratch book. May be I can apply your decision later. Anyway, thank you! — Alex, Apr 04 '19 at 11:25

Cancel Task running under User Control

2 Answers2