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How to: Cancel a Task and Its Children

These examples show how to perform the following tasks:

  1. Create and start a cancelable task.

  2. Pass a cancellation token to your user delegate and optionally to the task instance.

  3. Notice and respond to the cancellation request in your user delegate.

  4. Optionally notice on the calling thread that the task was canceled.

The calling thread does not forcibly end the task; it only signals that cancellation is requested. If the task is already running, it is up to the user delegate to notice the request and respond appropriately. If cancellation is requested before the task runs, then the user delegate is never executed and the task object transitions into the Canceled state.

This example shows how to terminate a Task and its children in response to a cancellation request. It also shows that when a user delegate terminates by throwing an OperationCanceledException, the calling thread can optionally use the Wait method or WaitAll method to wait for the tasks to finish. In this case, the delegate must use a try catch block to handle the exceptions on the calling thread.



namespace CancellationWithOCE
{
    using System;
    using System.Threading;
    using System.Threading.Tasks;

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Press any key to start. Press 'c' to cancel.");
            Console.ReadKey();

            var tokenSource = new CancellationTokenSource();
            var token = tokenSource.Token;

            // Store references to the tasks so that we can wait on them and
            // observe their status after cancellation.
            Task[] tasks = new Task[10];

            // Request cancellation of a single task when the token source is canceled.
            // Pass the token to the user delegate, and also to the task so it can 
            // handle the exception correctly.
            tasks[0] = Task.Factory.StartNew(() => DoSomeWork(1, token), token);

            // Request cancellation of a task and its children. Note the token is passed
            // to (1) the user delegate and (2) as the second argument to StartNew, so 
            // that the task instance can correctly handle the OperationCanceledException.
            tasks[1] = Task.Factory.StartNew(() =>
            {
                // Create some cancelable child tasks.
                for (int i = 2; i < 10; i++)
                {
                    // For each child task, pass the same token
                    // to each user delegate and to StartNew.
                    tasks[i] = Task.Factory.StartNew(iteration =>
                                DoSomeWork((int)iteration, token), i, token);
                }
                // Passing the same token again to do work on the parent task. 
                // All will be signaled by the call to tokenSource.Cancel below.
                DoSomeWork(2, token);
            }, token);

            // Give the tasks a second to start.
            Thread.Sleep(1000);

            // Request cancellation from the UI thread.
            if (Console.ReadKey().KeyChar == 'c')
            {
                tokenSource.Cancel();
                Console.WriteLine("\nTask cancellation requested.");

                // Optional: Observe the change in the Status property on the task.
                // It is not necessary to wait on tasks that have canceled. However,
                // if you do wait, you must enclose the call in a try-catch block to
                // catch the OperationCanceledExceptions that are thrown. If you do 
                // not wait, no OCE is thrown if the token that was passed to the 
                // StartNew method is the same token that requested the cancellation.

                #region Optional_WaitOnTasksToComplete
                try
                {
                    Task.WaitAll(tasks);
                }
                catch (AggregateException e)
                {
                    // For demonstration purposes, show the OCE message.
                    foreach (var v in e.InnerExceptions)
                        Console.WriteLine("msg: " + v.Message);
                }

                // Prove that the tasks are now all in a canceled state.
                for (int i = 0; i < tasks.Length; i++)
                    Console.WriteLine("task[{0}] status is now {1}", i, tasks[i].Status);
                #endregion
            }

            // Keep the console window open while the
            // task completes its output.
            Console.ReadLine();
        }

        static void DoSomeWork(int taskNum, CancellationToken ct)
        {
            // Was cancellation already requested?
            if (ct.IsCancellationRequested)
            {
                Console.WriteLine("We were cancelled before we got started.");
                Console.WriteLine("Press Enter to quit.");
                ct.ThrowIfCancellationRequested();
            }
            int maxIterations = 1000;

            // NOTE!!! A benign "OperationCanceledException was unhandled
            // by user code" error might be raised here. Press F5 to continue. Or,
            //  to avoid the error, uncheck the "Enable Just My Code"
            // option under Tools > Options > Debugging.
            for (int i = 0; i < maxIterations; i++)
            {
                // Do a bit of work. Not too much.
                var sw = new SpinWait();
                for (int j = 0; j < 3000; j++) sw.SpinOnce();
                Console.WriteLine("...{0} ", taskNum);
                if (ct.IsCancellationRequested)
                {
                    Console.WriteLine("bye from {0}.", taskNum);
                    Console.WriteLine("\nPress Enter to quit.");

                    ct.ThrowIfCancellationRequested();
                }
            }
        }
    }
}


The System.Threading.Tasks.Task class is fully integrated with the cancellation model that is based on the System.Threading.CancellationTokenSource and System.Threading.CancellationToken types. For more information, see Cancellation and Task Cancellation.

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