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TaskTask taskA = new Task( () => Console.WriteLine("From taskA."));// StarttaskA.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 tasktaskA.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 queryTask<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} } }