C#中的thread和task之Task

简介

Task是在.NET Framework 4中添加进来的，这是新的namespace：System.Threading.Tasks;它强调的是adding parallelism and concurrency to applications。

现在都是多核的CPU，在系统内，Task Parallel Library能更有效地利用CPU核心。TPL 会动态地按比例调节并发程度，以便最有效地使用所有可用的处理器。TPL 还处理工作分区、ThreadPool 上的线程调度、取消支持、状态管理以及其他低级别的细节操作。对比ThreadPool上的Thread并没有很好地支持Cancel的操作。

在语法上，和lambda表达式更好地结合。

创建Task

显式创建Task

先看一个简单的示例：

using System;using System.Threading;using System.Threading.Tasks;public class TaskExample1{   public static void Main()   {      Thread.CurrentThread.Name = "Main";        // 使用lambda方式，之间提供一个用户delegate，创建一个Task        Task taskA = new Task( () => Console.WriteLine("From taskA."));        // Start        taskA.Start();        // Output a message from the calling thread.        Console.WriteLine("From thread '{0}'.",                           Thread.CurrentThread.Name);        //等待task结束        taskA.Wait();   }}//       From thread 'Main'.//       From taskA.

任务对象Task提供可在任务的整个生存期内从调用线程访问的方法和属性。例如，可以随时访问任务的 Status 属性，以确定它是已开始运行、已完成运行、已取消还是引发了异常。状态由 TaskStatus 枚举表示。

简化创建并开始的Task操作版本–直接使用一个操作： System.Threading.Tasks.Task.Run()

using System;using System.Threading;using System.Threading.Tasks;public class TaskExample2{   public static void Main()   {      Thread.CurrentThread.Name = "Main";      // Define and run the task.      Task taskA = Task.Run( () => Console.WriteLine("From taskA."));      // Output a message from the calling thread.      Console.WriteLine("From thread '{0}'.",                           Thread.CurrentThread.Name);      taskA.Wait();   }}//       From thread 'Main'.//       From taskA.

TaskFactory创建Task

TPL提供了工厂类TaskFactory，也可用直接创建Task。 一个操作System.Threading.Tasks.TaskFactory.StartNew， 即可完成创建并开始一个Task.

Use this method when creation and scheduling do not have to be separated and you require additional task creaion options or the use of a specific scheduler, or when you need to pass additional state into the task through its AsyncState property, as shown in the following example.

using System;using System.Threading;using System.Threading.Tasks;public class TaskExample3{   public static void Main()   {      Thread.CurrentThread.Name = "Main";      // 创建task并启动      Task taskA = Task.Factory.StartNew(() => Console.WriteLine("From taskA."));      //       Console.WriteLine("From thread '{0}'.",                         Thread.CurrentThread.Name);      // wait task      taskA.Wait();                     }}//       Hello from thread 'Main'.//       Hello from taskA.

传入参数

来自MS的示例：

using System;using System.Threading;using System.Threading.Tasks;public class Example{   public static void Main()   {      Task[] taskArray = new Task[10];      for (int i = 0; i < taskArray.Length; i++) {         taskArray[i] = Task.Factory.StartNew( (int param) => {                                                 Console.WriteLine("Task #{0}.", param);                                               },                                              i );      }      Task.WaitAll(taskArray);        }}

Task返回值

如果一个Task执行方法是有返回值的，可用得到其值。在创建一个task时，其返回的值为Task，表示一个返回类型为T的Task；在taks完成执行后，可用通过Task的Result属性获取结果。

public class Task<TResult> : System.Threading.Tasks.Task

来自MS的示例：

using System;using System.Linq;using System.Threading.Tasks;class Program{    static void Main()    {        // lambda表达式创建一个task并执行，并返回值。        Task<int> task1 = Task<int>.Factory.StartNew(() => 1);        int i = task1.Result;        // 返回对象        Task<Test> task2 = Task<Test>.Factory.StartNew(() =>        {            string s = ".NET";            double d = 4.0;            return new Test { Name = s, Number = d };        });        Test test = task2.Result;        // Return an array produced by a PLINQ query        Task<string[]> task3 = Task<string[]>.Factory.StartNew(() =>        {            string path = @"C:\Users\Public\Pictures\Sample Pictures\";            string[] files = System.IO.Directory.GetFiles(path);            var result = (from file in files.AsParallel()                          let info = new System.IO.FileInfo(file)                          where info.Extension == ".jpg"                          select file).ToArray();            return result;        });        foreach (var name in task3.Result)            Console.WriteLine(name);    }    class Test    {        public string Name { get; set; }        public double Number { get; set; }    }}

Continue操作

如果一个Task开始执行时间依赖于其它的Task的完成，可以使用Continue系列方法。

来自MS的示例：

using System;using System.Threading.Tasks;public class Example{   public static void Main()   {                               var getData = Task.Factory.StartNew(() => {                                              Random rnd = new Random();                                              int[] values = new int[100];                                             for (int ctr = 0; ctr <= values.GetUpperBound(0); ctr++)                                                values[ctr] = rnd.Next();                                             return values;                                          } );        var processData = getData.ContinueWith((x) => {                                                int n = x.Result.Length;                                                long sum = 0;                                                double mean;                                                for (int ctr = 0; ctr <= x.Result.GetUpperBound(0); ctr++)                                                   sum += x.Result[ctr];                                                mean = sum / (double) n;                                                return Tuple.Create(n, sum, mean);                                             } );       var displayData = processData.ContinueWith((x) => {                                                    return String.Format("N={0:N0}, Total = {1:N0}, Mean = {2:N2}",                                                                         x.Result.Item1, x.Result.Item2,                                                                          x.Result.Item3);                                                 } );                               Console.WriteLine(displayData.Result);   }}// 输出：//    N=100, Total = 110,081,653,682, Mean = 1,100,816,536.82

而且，可以使用更简化的写法.

using System;using System.Threading.Tasks;public class Example{   public static void Main()   {                               var displayData = Task.Factory.StartNew(() => {                                                  Random rnd = new Random();                                                  int[] values = new int[100];                                                 for (int ctr = 0; ctr <= values.GetUpperBound(0); ctr++)                                                    values[ctr] = rnd.Next();                                                 return values;                                              } ).                          ContinueWith((x) => {                                        int n = x.Result.Length;                                        long sum = 0;                                        double mean;                                        for (int ctr = 0; ctr <= x.Result.GetUpperBound(0); ctr++)                                           sum += x.Result[ctr];                                        mean = sum / (double) n;                                        return Tuple.Create(n, sum, mean);                                     } ).                         ContinueWith((x) => {                                        return String.Format("N={0:N0}, Total = {1:N0}, Mean = {2:N2}",                                                             x.Result.Item1, x.Result.Item2,                                                              x.Result.Item3);                                     } );                               Console.WriteLine(displayData.Result);   }}

Cancel操作

在TPL中，为Task能cancel执行，提供了CancellationTokenSource和CancellationToken；
需要完成的工作是：在Task的action执行方法内，周期性地检查CancellationToken的IsCancellationRequested属性。
示例：

    private var tokenSource = new CancellationTokenSource();    public void Start()    {        var token = tokenSource.Token;        for (int i = 0; i<5; i++>)         {            Task t = Task.Factory.StartNew( () => DoSomeWork(i, token), token);            Console.WriteLine("Task {0} executing", t.Id);        }    }    public void Stop()    {        var token = tokenSource.Token;        tokenSource.Cancel();    }    void DoSomeWork(int taskNum, CancellationToken ct)    {      // 先检查，调度进入时，是否cancel了。      if (ct.IsCancellationRequested == true) {         Console.WriteLine("Task {0} was cancelled before it got started.",                           taskNum);         ct.ThrowIfCancellationRequested(); // 抛出异常-- 或者 return      }       // 正式开始任务。       int maxIterations = 100;      // NOTE!!! A "TaskCanceledException was unhandled       for (int i = 0; i <= maxIterations; i++) {         //          var sw = new SpinWait();         for (int j = 0; j <= 100; j++)            sw.SpinOnce();         if (ct.IsCancellationRequested) {            Console.WriteLine("Task {0} cancelled", taskNum);            ct.ThrowIfCancellationRequested(); //抛出异常-- 或者 return         }       }     }