Task Parallel Library - How do you get a Continuation with TaskContinuationOptions.OnlyOnCanceled to Fire?

问题

I'm experimenting with the Task support in .NET 4.0 - specifically with the continuation support. What I'm perplexed about is that I can't figure out how to get a continuation with the TaskContinuationOptions.OnlyOnCanceled flag set to execute. If I do a ThrowIfCancellationRequested in my worker routine, it only seems to propagate out of the continuation as a fault instead of a cancel operation. For instance, given this code:

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

namespace TaskExp1
{
    class Program
    {
        static void Main()
        {
            var n = 10000;

            DumpThreadId("main method");

            var cts = new CancellationTokenSource();
            var task = Task.Factory.StartNew<int>(_ => Sum(cts.Token, n), 
                                                  cts.Token);

            task.ContinueWith(t =>
            {
                DumpThreadId("ContinueWith Completed, ", newline:false);
                Console.WriteLine("The result is " + t.Result);
            }, TaskContinuationOptions.OnlyOnRanToCompletion);

            task.ContinueWith(t =>
            {
                DumpThreadId("ContinueWith Faulted, ", newline: false);
                Console.WriteLine(t.Exception.InnerExceptions[0].Message);
            }, TaskContinuationOptions.OnlyOnFaulted);

            task.ContinueWith(_ =>
            {
                DumpThreadId("ContinueWith Cancelled, ");
            }, TaskContinuationOptions.OnlyOnCanceled);

            Console.WriteLine("Computing sum of " + n + " ...");
            Thread.SpinWait(100000);
            cts.Cancel();

            Console.WriteLine("Done.");
            Console.ReadLine();
        }

        static int Sum(CancellationToken cancelToken, int n)
        {
            DumpThreadId("from Sum method");
            int sum = 0;
            for (; n > 0; n--)
            {
                Thread.SpinWait(500000);
                if (n == 10000) cancelToken.ThrowIfCancellationRequested();
                checked { sum += n; }
            }
            return sum;
        }

        static void DumpThreadId(string msg = "", bool newline = true)
        {
            var formattedMsg = String.Format("ThreadId: {0} {1}", 
                                  Thread.CurrentThread.ManagedThreadId, msg);
            if (newline) formattedMsg += "\n";
            Console.Write(formattedMsg);
        }
    }
}

This outputs:

ThreadId: 9 main method
Computing sum of 10000 ...
Done.
ThreadId: 10 from Sum method
ThreadId: 10 ContinueWith Faulted, The operation was canceled.

How do I exit my worker (Sum) method such that the OnlyOnCanceled continuation gets fired?

回答1:

When you are using the _ => lambda expression you are using the

Func<Object, TResult> function, Object state

overload. If you change the Factory.StartNew to

Task.Factory.StartNew<int>(() => Sum(cts.Token, n), cts.Token);

it will call the "ContinueWith Cancelled".

来源：https://stackoverflow.com/questions/3956055/task-parallel-library-how-do-you-get-a-continuation-with-taskcontinuationoptio

标签

task-parallel-library

parallel-extensions