ParallelForCancellation.cs

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ComputePi
{
    public class ParallelForCancellation
    {
        // Demonstrated features:
        //		CancellationTokenSource
        // 		Parallel.For()
        //		ParallelOptions
        //		ParallelLoopResult
        // Expected results:
        // 		An iteration for each argument value (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) is executed.
        //		The order of execution of the iterations is undefined.
        //		The iteration when i=2 cancels the loop.
        //		Some iterations may bail out or not start at all; because they are temporally executed in unpredictable order, 
        //          it is impossible to say which will start/complete and which won't.
        //		At the end, an OperationCancelledException is surfaced.
        // Documentation:
        //		http://msdn.microsoft.com/en-us/library/system.threading.cancellationtokensource(VS.100).aspx
        public void CancelDemo()
        {
            CancellationTokenSource cancellationSource = new CancellationTokenSource();
            ParallelOptions options = new ParallelOptions();
            options.CancellationToken = cancellationSource.Token;

            try
            {
                ParallelLoopResult loopResult = Parallel.For(
                        0,
                        10,
                        options,
                        (i, loopState) =>
                        {
                            Console.WriteLine("Start Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i);

                            // Simulate a cancellation of the loop when i=5
                            if (i == 5)
                            {
                                cancellationSource.Cancel();
                            }

                            // Simulates a long execution
                            for (int j = 0; j < 10; j++)
                            {
                                Thread.Sleep(1 * 200);

                                // check to see whether or not to continue
                                if (loopState.ShouldExitCurrentIteration) return;
                            }

                            Console.WriteLine("Finish Thread={0}, i={1}", Thread.CurrentThread.ManagedThreadId, i);
                        }
                    );

                if (loopResult.IsCompleted)
                {
                    Console.WriteLine("All iterations completed successfully. THIS WAS NOT EXPECTED.\n");
                }
            }
            // No exception is expected in this example, but if one is still thrown from a task,
            // it will be wrapped in AggregateException and propagated to the main thread.
            catch (AggregateException e)
            {
                Console.WriteLine("Parallel.For has thrown an AggregateException. THIS WAS NOT EXPECTED.\n{0}\n", e);
            }
            // Catching the cancellation exception
            catch (OperationCanceledException e)
            {
                Console.WriteLine("An iteration has triggered a cancellation. THIS WAS EXPECTED.\n{0}\n", e.ToString());
            }
        }
    }
}