c# - 将取消 token 提供给任务什么都不做？

Question

我有两个例子，直接来自微软，这些例子似乎与取消 token 无关，因为我可以删除提供给任务的 token ，结果是一样的。所以我的问题是:什么是取消 token ，为什么是糟糕的例子？我错过了什么..？ :)

using System;
using System.Threading;
using System.Threading.Tasks;
namespace Chapter1.Threads
{
    public class Program
    {
        static void Main()
        {
            CancellationTokenSource cancellationTokenSource =
                new CancellationTokenSource();
            CancellationToken token = cancellationTokenSource.Token;
            Task task = Task.Run(() =>
            {
                while (!token.IsCancellationRequested)
                {
                    Console.Write(“*”);
                    Thread.Sleep(1000);
                }
                token.ThrowIfCancellationRequested();
            }, token);
            try
            {
                Console.WriteLine(“Press enter to stop the task”);
                Console.ReadLine();
                cancellationTokenSource.Cancel();
                task.Wait();
            }  
            catch (AggregateException e)
            {
                Console.WriteLine(e.InnerExceptions[0].Message);
            }
            Console.WriteLine(“Press enter to end the application”);
            Console.ReadLine();
        }
    }
}

代码示例2: https://msdn.microsoft.com/en-us/library/system.threading.cancellationtoken(v=vs.110).aspx

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

public class Example
{
   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();
      }
   }
}

Answer 1

因为 .Net 中的取消是通过协作将 CancellationToken 传递到 Task.Run 中的，例如不足以确保任务被取消。

将 token 作为参数传递只会将 token 与任务相关联。只有在 token 被取消之前它没有机会开始运行它才能取消任务。例如:

var token = new CancellationToken(true); // creates a cancelled token
Task.Run(() => {}, token);

要“中途”取消任务，您需要任务本身观察 token 并在发出取消信号时抛出，类似于:

Task.Run(() => 
{
    while (true)
    {
        token.ThrowIfCancellationRequested();
        // do something
    }
}, token);

此外，简单地从任务内部抛出异常只会将任务标记为Faulted。要将其标记为已取消 TaskCanceledException.CancellationToken 需要匹配传递给 Task.Run 的 token 。