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

Question

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?

Answer 1

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 :)