How to cancel a specific task by its ID

Question

I have an WPF application in which I am generating numbers of task for parallel processing. Now when my condition becomes true then I need to cancel particular that task. When I am applying CancellationTokenSource for cancel task then it's cancel whole bunch of task but I need to cancel only single.

Task t1 = Task.Run(() => Task.Delay(30000, token), token);
                test.Add(t1.Id, symbolName);
                lstTask.Add(t1);
                await t1;

here I am holding Task Id so I can know that which Task should I cancel ?

So please can you suggest me that how can I able to cancel only single Task by it's Id ?

After discussion about problem I have made some changes in code so please suggest me that should I change my logic ? because recently I aware that task Id is not unique so I prefer CTS directly.

public static async void startAddToPosition(System.Threading.CancellationTokenSource cts1, string symbolName)
{
          try
          {
               Task t1 = Task.Run(() => Task.Delay(30000, cts1.Token), cts1.Token);
                test.Add(symbolName, cts1);
                lstTask.Add(t1);
                lstCts.Add(cts1);
                await t1;
                generateRealPosition("QQQ"); //here I need symbol name when task will complete it's 30 seconds.
          }
          catch (OperationCanceledException ex)
          {
                var symbolName = test.FirstOrDefault(q => q.Value.Token == ex.CancellationToken).Key;
            }
}

however on the other side when condition become full fill then I am trying to cancel task like below :

CancellationTokenSource cts = Evaluation.parallelScripts.test.FirstOrDefault(q => q.Key == eventData.symbolName).Value;
  cts.Cancel();

So I am trying to get CancellationTokenSource for particular symbol from list and cancelling that CTS. So that task become cancel.

So my question is that I don't know that CTS token is unique or not. because when CTS cancelling Task then in exception I need Symbol Name according to that CTS token.

Answer 1

Each cancellation token source is a separate logical "cancel". As you've found, if you share cancellation tokens, then a that single "cancel" will cancel all of them.

To create multiple logical "cancel"s, you'll need multiple cancellation token sources:

CancellationTokenSource cts1 = new CancellationTokenSource();
Task t1 = Task.Run(() => Task.Delay(30000, cts1.Token), cts1.Token);
test.Add(t1.Id, symbolName, cts1);
lstTask.Add(t1);
lstCts.Add(cts1);
await t1;

can you suggest me that how can I able to cancel only single Task by it's Id ?

This is not possible.

For one, it's not possible to cancel a task from the "outside". All you can do is give it a token and cancel that token. It's up to the task to respond to that token appropriately. This is how cooperative cancellation works.

There's another reason this isn't possible: task ids are not unique. It appears you are using task ids in your code; I recommend you re-evaluate the design to ensure that it can handle non-unique task ids. Perhaps using the actual Task instance instead (which is unique).

Answer 2

You can do something like this:

class Program
{
    static void Main(string[] args)
    {
        Guid lastID = default(Guid);
        List<CancellableTask> cancellableTasks = new List<CancellableTask>();
        for (int i = 0; i < 10; i++)
        {
            CancellableTask task = new CancellableTask(() =>
            {
                Console.WriteLine("New task!");
                Thread.Sleep(3000);
            });
            cancellableTasks.Add(task);
            lastID = task.ID;
        }

        CancellableTask cancellableTask = cancellableTasks.FirstOrDefault(x => x.ID == lastID);
        if (cancellableTask != null)
        {
            cancellableTask.Cancel();
        }
    }
}

public class CancellableTask
{
    public Guid ID { get; private set; }
    private CancellationTokenSource CancellationTokenSource { get; set; }
    public Task Task { get; private set; }
    public CancellableTask(Action action)
    {
        CancellationTokenSource = new CancellationTokenSource();
        Task = Task.Run(action, CancellationTokenSource.Token);
        ID = Guid.NewGuid();
    }
    public void Cancel()
    {
        CancellationTokenSource.Cancel();
    }
}