Cancelling a Task Using a Cancellation Token

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
    /// <summary>
    /// The following example uses a random number generator to emulate a data
    /// collection application that reads 10 integral values from eleven different
    /// instruments. A value of zero indicates that the measurement has failed
    /// for one instrument, in which case the operation should be cancelled
    /// and no overall mean should be computed.
    ///
    /// To handle the possible cancellation of the operation, the example
    /// instantiates a CancellationTokenSource object that generates a
    /// cancellation token which is passed to a TaskFactory object.
    /// The TaskFactory object in turn passes the cancellation token to
    /// each of the tasks responsible for collecting readings for a particular
    /// instrument. The TaskFactory.ContinueWhenAll&lt;TAntecedentResult,TResult>(Task&lt;TAntecedentResult>[], Func&lt;Task&lt;TAntecedentResult&gt;[],TResult&gt;, CancellationToken)
    /// method is called to ensure that the mean is computed only after all readings have been gathered successfully.
    /// If a task has not because it has been cancelled, the call to the
    /// TaskFactory.ContinueWhenAll method throws an exception.
    /// </summary>
    /// <remarks>https://docs.microsoft.com/en-us/dotnet/api/system.threading.cancellationtokensource?view=netframework-4.8#examples</remarks>
    public static void Main()
    {
        // Define the cancellation token.
        CancellationTokenSource source = new CancellationTokenSource();
        CancellationToken token = source.Token;

        Random rnd = new Random();
        Object lockObj = new Object();

        List<Task<int[]>> tasks = new List<Task<int[]>>();
        TaskFactory factory = new TaskFactory(token);

        for (int taskCtr = 0; taskCtr <= 10; taskCtr++)
        {
            int iteration = taskCtr + 1;

            tasks.Add(factory.StartNew( () =>
            {
                int value;
                int[] values = new int[10];
                for (int ctr = 1; ctr <= 10; ctr++)
                {
                    lock (lockObj)
                    {
                        value = rnd.Next(0, 101);
                    }
                    if (value == 0)
                    {
                        source.Cancel();
                        Console.WriteLine("Cancelling at task {0}", iteration);
                        break;
                    }
                    values[ctr - 1] = value;
                }

                return values;
            }, token));
        }

        try
        {
            Task<double> fTask = factory.ContinueWhenAll(tasks.ToArray(), (results) =>
            {
                Console.WriteLine("Calculating overall mean...");
                long sum = 0;
                int n = 0;
                foreach (var t in results)
                {
                    foreach (var r in t.Result)
                    {
                        sum += r;
                        n++;
                    }
                }
                return sum / (double) n;
            } , token);

            Console.WriteLine("The mean is {0}.", fTask.Result);
        }
        catch (AggregateException ae)
        {
            foreach (Exception e in ae.InnerExceptions)
            {
                if (e is TaskCanceledException)
                {
                    Console.WriteLine("Unable to compute mean: {0}", ((TaskCanceledException) e).Message);
                }
                else
                {
                    Console.WriteLine("Exception: " + e.GetType().Name);
                }
            }
        }
        finally
        {
            source.Dispose();
        }
    }
}
// Repeated execution of the example produces output like the following:
//       Cancelling at task 5
//       Unable to compute mean: A task was canceled.
//
//       Cancelling at task 10
//       Unable to compute mean: A task was canceled.
//
//       Calculating overall mean...
//       The mean is 5.29545454545455.
//
//       Cancelling at task 4
//       Unable to compute mean: A task was canceled.
//
//       Cancelling at task 5
//       Unable to compute mean: A task was canceled.
//
//       Cancelling at task 6
//       Unable to compute mean: A task was canceled.
//
//       Calculating overall mean...
//       The mean is 4.97363636363636.
//
//       Cancelling at task 4
//       Unable to compute mean: A task was canceled.
//
//       Cancelling at task 5
//       Unable to compute mean: A task was canceled.
//
//       Cancelling at task 4
//       Unable to compute mean: A task was canceled.
//
//       Calculating overall mean...
//       The mean is 4.86545454545455.