Cancel Task and set status to "Cancelled"

Question

I want a task to finish when the 50 Milliseconds are over. The status of the task should then be set to "Cancelled", otherwise to "RunToCompletion".

The task creation is here:

CancellationTokenSource cts = new CancellationTokenSource(50);
CancellationToken ct = cts.Token;
Task test_task = Task.Run(async () =>
{
    try
    {
        tokenS.Token.Register(() =>
        {
            cts.Cancel();
            ct.ThrowIfCancellationRequested();
        });
        await NotifyDevice(BLEDevice);
    }
    catch (Exception e)
    {
    }
},ct);

All i get till now is an AggregateException, that does not get catched somehow by the try/catch-block.

Do you want a task to be cancelled after 50ms or do you want your operation to to be cancelled after 50ms? — Andrii Litvinov, May 11 '17 at 12:00
The task "test_task". What do you mean by operation? The `await NotifyDevice(BLEDevice)` ? — Laurence, May 11 '17 at 12:01
As far a I understand `NotifyDevice` can last longer than 50ms. Do you want it continue to run when `test_task` is cancelled after 50ms (easy solution)? — Andrii Litvinov, May 11 '17 at 12:06
Ok here is the thing. I actually want the `NotifyDevice`-Method to get cancelled. But so far i have not found a way for this. The method i used is a wrapper for `WriteClientCharacteristicConfigurationDescriptorAsync`. I put this on another thread here on [stackoverflow](http://stackoverflow.com/questions/43909288/how-to-stop-a-call-resulting-in-iasyncoperation), but nobody seemed to be able to help me. — Laurence, May 11 '17 at 12:09
I have posted an answer which will ensure the wrapper task will be cancelled in 50ms. I can also added comment in your other question. It should be possible to cancel `IAsyncOperation`. — Andrii Litvinov, May 11 '17 at 12:29

score 1 · Accepted Answer · edited May 23 '17 at 12:34

Here is similar question to yours: Is it possible to cancel a C# Task without a CancellationToken?. But the solution will not cancel the task in NotifyDevice method. That task can be cancelled only if underlying task supports cancellation. And based on the docs IAsyncInfo can be cancelled. I would go with the wrapper to ensure the task is cancelled in 50ms in case if cancelling underlying task takes more time:

CancellationTokenSource cts = new CancellationTokenSource(50);
await NotifyDevice(BLEDevice, cts.Token).WithCancellation(cts.Token);

EDIT: the extension method itself:

public static async Task<T> WithCancellation<T>(this Task<T> task, CancellationToken cancellationToken) 
{ 
    var tcs = new TaskCompletionSource<bool>(); 
    using(cancellationToken.Register(s => ((TaskCompletionSource<bool>)s).TrySetResult(true), tcs)) 
        if (task != await Task.WhenAny(task, tcs.Task)) 
            throw new OperationCanceledException(cancellationToken); 
    return await task; 
}

Very sorry but i cant make this extension-method work with me. The compiler just wouldnt recognize the method?! — Laurence, May 11 '17 at 12:59
Yes, you should implement it somewhere in your project. I will update my answer so that it includes it. — Andrii Litvinov, May 11 '17 at 13:02
Sounds weird. Try to put the method in the same class, and invoke directly: `await WithCancellation(NotifyDevice(BLEDevice, cts.Token), cts.Token)` — Andrii Litvinov, May 11 '17 at 13:15

Cancel Task and set status to "Cancelled"

1 Answers1