C# .Net並行(多核)編程 3 - Task的基本用法(取消Task的執行)

轉自:http://ggicci.blog.163.com/blog/static/210364096201272034821778/

Title :

• Pro .NET 4 Parallel Programming in C# (Adam Freeman) Task的基本用法
• Task 的取消

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Steps :

 

 

Quote :

You must also throw an instance of System.Threading.OperationCanceledException in your task body; this is how you acknowledge the cancellation, and if you forget, the status of your task will not be set correctly. [ From “Pro .Net 4 Parallel Programming in C#”]

你必須在你的Task的執行體里面拋出一個System.Threading.OperationCanceledException異常。具體有兩種形式 :

1. while(true) {
       if(token.IsCancellationRequested) {
           throw new OperationCanceledException(token);
       } else {
           //do your work
       }
   }

    

2. while(true) {
       token.ThrowIfCancellationRequested();
       //do your work
   }

Simple Sample :

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

namespace Parallel_TaskProgramming
{
    class Program
    {
        public static void Main(string[] args)
        {
            //step 1: create a instance of System.Threading.CancellationTokenSource
            CancellationTokenSource tokenSource = new CancellationTokenSource();
            //step 2: call the CancellationTokenSource.Token property to get a System.Threading.CancellationToken
            CancellationToken token = tokenSource.Token;

            //step 3: create a new Task or Task<T>
            Task task = new Task(() => {
                int i = 0;
                while (true)
                {
                    token.ThrowIfCancellationRequested();
                    Console.WriteLine(i++);
                }
            }, token);

            //step 4: start the task and then you can call CancellationTokenSource.Cancel() to cancel the task
            task.Start();
            //主線程睡眠 5 ms
            Thread.Sleep(5);
            tokenSource.Cancel();

            Console.WriteLine("Press enter to finish.");
            Console.ReadLine();
        }
    }
}

Result : 結果是不確定的，5 ms 能打印多少次數字取決於運行時 CPU 的忙碌程度

0
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Press enter to finish.

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Register : Monitoring Cancellation with a Delegate

CancellationToken 的 Register 方法可以為 token 登記一個 Action 或 Action<Object> 委托(代理)的實例。當 Task 被取消執行后，這個委托會被執行。

在上面 Simple Sample 的代碼中的 task.Start() 前加上如下代碼：

token.Register(() =>
{
    Console.WriteLine("Task was canceled and then I'm invoked.");
});

Result :

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Task was canceled and then I'm invoked.
Press enter to finish.

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WaitHandle : Monitoring Cancellation with a Wait Handle

CancellationToken 有個 WaitHandle 屬性，這個屬性是一個 WaitHandle 類的對象，WaitHandle 類的 WaitOne 方法的作用是：阻塞當前線程，直到當前的 WaitHandle 接收到一個信號。

Sample :

public static void Main(string[] args)
{
    CancellationTokenSource tokenSource = new CancellationTokenSource();
    CancellationToken token = tokenSource.Token;

    Task task = new Task(() =>
    {
        Console.WriteLine("Task running...");
        token.WaitHandle.WaitOne();
        Console.WriteLine("Hello");
    }, token);

    Console.WriteLine("Press enter to cancel the task.");
    task.Start();

    //按空格鍵取消任務
    Console.ReadLine();
    tokenSource.Cancel();

    Console.WriteLine("Press enter to finish.");
    Console.ReadLine();
}

Result :

Press enter to cancel the task.
Task running...

Hello
Press enter to finish.

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Canceling Several Tasks : 取消多個任務

很簡單，只需要在創建任務的時候用同一個 CancallationToken 的實例就可以了。