Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

How to cancel a task that is waiting with a timeout without exceptions being thrown

When canceling a task that has a timeout (before the timeout has ended) using a cancel token an exception is thrown. Example:

mytask.start();
bool didTaskRunInTime = mytask.wait(5 mins, _cancelToken);

Which means I cannot go on like below.

//was the task cancelled
if (_cancelToken.IsCancelRequested)
{
    // log cancel from user to file etc
}

if (didTaskRunInTime )
{
    int taskResult = myTask.Result;
    // log result to file
}
else if (!_cancelToken.IsCancelRequested)
{
    // Tell user task timed out , log a message etc
}

I will have to do all this in my catch block and my code is looking messy. What is the correct way to do this?

like image 701
Gullu Avatar asked Jul 13 '11 16:07

Gullu


People also ask

How do I terminate async task?

A task can be cancelled at any time by invoking cancel(boolean). Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(Object), instead of onPostExecute(Object) will be invoked after doInBackground(Object[]) returns.

What is a task canceled exception?

TaskCanceledException(String, Exception, CancellationToken) Initializes a new instance of the TaskCanceledException class with a specified error message, a reference to the inner exception that is the cause of this exception, and the CancellationToken that triggered the cancellation.

Which of the following option should be used to implement cancellation for a long running task?

Here is the sample code to cancel a task, CancellationTokenSource mCancellationTokenSource = new CancellationTokenSource();


1 Answers

You could call Task.WaitAny with an array of just that task. Then you can act on the status of the task, however the method returns. Sample code:

using System;
using System.Threading;
using System.Threading.Tasks;

class Test
{
    static void Main()
    {
        Task sleeper = Task.Factory.StartNew(() => Thread.Sleep(100000));

        int index = Task.WaitAny(new[] { sleeper },
                                 TimeSpan.FromSeconds(0.5));
        Console.WriteLine(index); // Prints -1, timeout

        var cts = new CancellationTokenSource();

        // Just a simple wait of getting a cancellable task
        Task cancellable = sleeper.ContinueWith(ignored => {}, cts.Token);

        // It doesn't matter that we cancel before the wait
        cts.Cancel();

        index = Task.WaitAny(new[] { cancellable },
                             TimeSpan.FromSeconds(0.5));
        Console.WriteLine(index); // 0 - task 0  has completed (ish :)
        Console.WriteLine(cancellable.Status); // Cancelled
    }
}

Note that if the task is faulted, you should "observe" the exception in order to avoid it going bang when it's finalized :)

like image 60
Jon Skeet Avatar answered Sep 27 '22 19:09

Jon Skeet