C# Task详解

1、Task产生背景

Task出现之前，微软的多线程处理方式有：Thread→ThreadPool→委托的异步调用，虽然也可以基本业务需要的多线程场景，但它们在多个线程的等待处理方面、资源占用方面、线程延续和阻塞方面、线程的取消方面等都显得比较笨拙，在面对复杂的业务场景下，显得有点捉襟见肘了。

ThreadPool相比Thread来说具备了很多优势，但是ThreadPool却又存在一些使用上的不方便。比如：

• ThreadPool不支持线程的取消、完成、失败通知等交互性操作；
• ThreadPool不支持线程执行的先后次序；

正是在这种背景下，Task应运而生。Task是微软在.Net 4.0时代推出来的，也是微软极力推荐的一种多线程的处理方式，Task看起来像一个Thread，实际上，它是在ThreadPool的基础上进行的封装，Task的控制和扩展性很强，在线程的延续、阻塞、取消、超时等方面远胜于Thread和ThreadPool。以下是一个简单的任务示例：

static void Main(string[] args)
{Task t = new Task(() =>{Console.WriteLine("任务开始工作……");Thread.Sleep(5000);  //模拟工作过程});t.Start();t.ContinueWith(task =>{Console.WriteLine("任务完成，完成时候的状态为：");Console.WriteLine("IsCanceled={0}\tIsCompleted={1}\tIsFaulted={2}", task.IsCanceled, task.IsCompleted, task.IsFaulted);});Console.ReadKey();
}

2、Task使用方法 

2.1　创建和启动任务

2.1.1 无返回值的方式

方式1:调用Start方法

var t1 = new Task(() => TaskMethod("Task 1"));
t1.Start();
Task.WaitAll(t1);//等待所有任务结束 

任务的状态：Start之前为Created，之后为WaitingToRun 

方式2:静态方法Run

Task.Run(() => TaskMethod("Task 2"));

方式3:TaskFactory工厂

// 方法1. TaskFactory工厂
TaskFactory taskFactory = new TaskFactory() ;
taskFactory.StartNew(() => TaskMethod("Task 3"));
// 方法2. Task.Factory属性
Task.Factory.StartNew(() => TaskMethod("Task 3"));
//或者
var t3=Task.Factory.StartNew(() => TaskMethod("Task 3"));
Task.WaitAll(t3);

 任务的状态：Start之前为Running，之后为Running

static void Main(string[] args)
{var t1 = new Task(() => TaskMethod("Task 1"));var t2 = new Task(() => TaskMethod("Task 2"));t2.Start();t1.Start();Task.WaitAll(t1, t2);Task.Run(() => TaskMethod("Task 3"));Task.Factory.StartNew(() => TaskMethod("Task 4"));//标记为长时间运行任务,则任务不会使用线程池,而在单独的线程中运行。Task.Factory.StartNew(() => TaskMethod("Task 5"), TaskCreationOptions.LongRunning);#region 常规的使用方式Console.WriteLine("主线程执行业务处理.");//创建任务Task task = new Task(() =>{Console.WriteLine("使用`System.Threading.Tasks.Task`执行异步操作.");for (int i = 0; i < 10; i++){Console.WriteLine(i);}});//启动任务,并安排到当前任务队列线程中执行任务(System.Threading.Tasks.TaskScheduler)task.Start();Console.WriteLine("主线程执行其他处理");task.Wait();#endregionThread.Sleep(TimeSpan.FromSeconds(1));Console.ReadLine();
}static void TaskMethod(string name)
{Console.WriteLine("Task {0} is running on a thread id {1}. Is thread pool thread: {2}",name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);
}

方式4:RunSynchronously同步启动

Task实例化后调用同步方法RunSynchronously，进行线程启动。(PS: 类似委托开启线程，BeginInvoke是异步，而Invoke是同步)

var task = new Task(() => TaskMethod("Task 1"));
task.RunSynchronously();

async/await的实现方式

using System;
using System.Threading;
using System.Threading.Tasks;namespace ConsoleApp1
{class Program{async static void AsyncFunction(){await Task.Delay(1);Console.WriteLine("使用`System.Threading.Tasks.Task`执行异步操作.");for (int i = 0; i < 10; i++){Console.WriteLine(string.Format("AsyncFunction:i={0}", i));}}public static void Main(){Console.WriteLine("主线程执行业务处理.");AsyncFunction();Console.WriteLine("主线程执行其他处理");for (int i = 0; i < 10; i++){Console.WriteLine(string.Format("Main:i={0}", i));}Console.ReadLine();}}
}

2.1.2 带返回值的方式

方式4:

Task<int> task = CreateTask("Task 1");
task.Start(); 
int result = task.Result;

using System;
using System.Threading;
using System.Threading.Tasks;namespace ConsoleApp1
{class Program{static Task<int> CreateTask(string name){return new Task<int>(() => TaskMethod(name));}static void Main(string[] args){TaskMethod("Main Thread Task");Task<int> task = CreateTask("Task 1");task.Start();int result = task.Result;Console.WriteLine("Task 1 Result is: {0}", result);task = CreateTask("Task 2"); task.RunSynchronously(); //该任务会运行在主线程中result = task.Result;Console.WriteLine("Task 2 Result is: {0}", result);task = CreateTask("Task 3");Console.WriteLine(task.Status);task.Start();while (!task.IsCompleted){Console.WriteLine(task.Status);Thread.Sleep(TimeSpan.FromSeconds(0.5));}Console.WriteLine(task.Status);result = task.Result;Console.WriteLine("Task 3 Result is: {0}", result);#region 常规使用方式//创建任务Task<int> getsumtask = new Task<int>(() => Getsum());//启动任务,并安排到当前任务队列线程中执行任务(System.Threading.Tasks.TaskScheduler)getsumtask.Start();Console.WriteLine("主线程执行其他处理");getsumtask.Wait(); //等待任务的完成执行过程Console.WriteLine("任务执行结果：{0}", getsumtask.Result.ToString());//获得任务的执行结果#endregion}static int TaskMethod(string name){Console.WriteLine("Task {0} is running on a thread id {1}. Is thread pool thread: {2}",name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);Thread.Sleep(TimeSpan.FromSeconds(2));return 42;}static int Getsum(){int sum = 0;Console.WriteLine("使用`Task`执行异步操作.");for (int i = 0; i < 100; i++){sum += i;}return sum;}}
}

using System;
using System.Threading.Tasks;namespace ConsoleApp1
{class Program{public static void Main(){var ret1 = AsyncGetsum();Console.WriteLine("主线程执行其他处理");for (int i = 1; i <= 3; i++)Console.WriteLine("Call Main()");int result = ret1.Result;     //阻塞主线程Console.WriteLine("任务执行结果：{0}", result);}async static Task<int> AsyncGetsum(){await Task.Delay(1);int sum = 0;Console.WriteLine("使用`Task`执行异步操作.");for (int i = 0; i < 100; i++){sum += i;}return sum;}}
}

2.2　线程等待、延续和组合

2.2.1 Task的线程等待和延续主要以下几类：

• Wait：针对单个Task的实例，可以task1.wait进行线程等待
• WaitAny：线程列表中任何一个线程执行完毕即可执行（阻塞主线程）
• WaitAll：线程列表中所有线程执行完毕方可执行（阻塞主线程）
• WhenAny：与ContinueWith配合,线程列表中任何一个执行完毕，则继续ContinueWith中的任务（开启新线程，不阻塞主线程）
• WhenAll：与ContinueWith配合,线程列表中所有线程执行完毕，则继续ContinueWith中的任务（开启新线程，不阻塞主线程）
• ContinueWith：与WhenAny或WhenAll配合使用
• ContinueWhenAny：等价于Task的WhenAny+ContinueWith
• ContinueWhenAll：等价于Task的WhenAll+ContinueWith

  public static void Main()
  {//创建一个任务Task<int> task = new Task<int>(() =>{int sum = 0;Console.WriteLine("使用`Task`执行异步操作.");for (int i = 0; i < 100; i++){sum += i;}return sum;});//启动任务,并安排到当前任务队列线程中执行任务(System.Threading.Tasks.TaskScheduler)task.Start();Console.WriteLine("主线程执行其他处理");//任务完成时执行处理。Task cwt = task.ContinueWith(t =>{Console.WriteLine("任务完成后的执行结果：{0}", t.Result.ToString());});task.Wait();cwt.Wait();Action<string,int> log = (name,time) =>{Console.WriteLine($"{name}任务开始...");Thread.Sleep(time);Console.WriteLine($"{name}任务结束!");};List<Task> tasks = new List<Task>{Task.Run(() => log("张三",3000)),Task.Run(() => log("李四",1000)),Task.Run(() => log("王五",2000))};//以下语句逐个测试效果Task.WaitAny(tasks.ToArray());Task.WaitAll(tasks.ToArray());Task.WhenAny(tasks.ToArray()).ContinueWith(x => Console.WriteLine("某个Task执行完毕"));Task.WhenAll(tasks.ToArray()).ContinueWith(x => Console.WriteLine("所有Task执行完毕"));Task.Factory.ContinueWhenAny(tasks.ToArray(), x => Console.WriteLine("某个Task执行完毕"));Task.Factory.ContinueWhenAll(tasks.ToArray(), x => Console.WriteLine("所有Task执行完毕"));Console.Read();
  }

  2.2.2 任务的串行

  static void Main(string[] args)
  {ConcurrentStack<int> stack = new ConcurrentStack<int>();//t1先串行var t1 = Task.Factory.StartNew(() =>{stack.Push(1);stack.Push(2);});//t2,t3并行执行var t2 = t1.ContinueWith(t =>{int result;stack.TryPop(out result);Console.WriteLine("Task t2 result={0},Thread id {1}", result, Thread.CurrentThread.ManagedThreadId);});//t2,t3并行执行var t3 = t1.ContinueWith(t =>{int result;stack.TryPop(out result);Console.WriteLine("Task t3 result={0},Thread id {1}", result, Thread.CurrentThread.ManagedThreadId);});//等待t2和t3执行完Task.WaitAll(t2, t3);//t4串行执行var t4 = Task.Factory.StartNew(() =>{Console.WriteLine("当前集合元素个数：{0},Thread id {1}", stack.Count, Thread.CurrentThread.ManagedThreadId);});t4.Wait();
  }

  2.2.3 子任务

  public static void Main()
  {Task<string[]> parent = new Task<string[]>(state =>{Console.WriteLine(state);string[] result = new string[2];//创建并启动子任务new Task(() => { result[0] = "我是子任务1"; }, TaskCreationOptions.AttachedToParent).Start();new Task(() => { result[1] = "我是子任务2"; }, TaskCreationOptions.AttachedToParent).Start();return result;}, "我是父任务，并在我的处理过程中创建多个子任务，所有子任务完成以后我才会结束执行");//任务完成后执行parent.ContinueWith(t =>{Array.ForEach(t.Result, r => Console.WriteLine(r));});    parent.Start(); //启动父任务parent.Wait();//等待任务结束Wait只能等待父线程结束,没办法等到父线程的ContinueWith结束Console.ReadLine();
  }

  2.2.4 动态并行

  TaskCreationOptions.AttachedToParent父任务等待所有子任务完成后整个任务才算完成

  class Node
  {public Node Left { get; set; }public Node Right { get; set; }public string Text { get; set; }
  }class Program
  {static Node GetNode(){Node root = new Node{Left = new Node{Left = new Node{ Text = "L-L" },Right = new Node{ Text = "L-R" },Text = "L"},Right = new Node{Left = new Node{ Text = "R-L" },Right = new Node{ Text = "R-R" },Text = "R"},Text = "Root"};return root;}static void Main(string[] args){Node root = GetNode();DisplayTree(root);}static void DisplayTree(Node root){var task = Task.Factory.StartNew(() => DisplayNode(root),CancellationToken.None,TaskCreationOptions.None,TaskScheduler.Default);task.Wait();}static void DisplayNode(Node current){if (current.Left != null)Task.Factory.StartNew(() => DisplayNode(current.Left),CancellationToken.None,TaskCreationOptions.AttachedToParent,TaskScheduler.Default);if (current.Right != null)Task.Factory.StartNew(() => DisplayNode(current.Right),CancellationToken.None,TaskCreationOptions.AttachedToParent,TaskScheduler.Default);Console.WriteLine("当前节点的值为{0};处理的ThreadId={1}", current.Text, Thread.CurrentThread.ManagedThreadId);}
  }

  2.3　取消任务

  private static int TaskMethod(string name, int seconds, CancellationToken token)
  {Console.WriteLine("Task {0} 正在运行,当前线程id {1}. Is thread pool thread: {2}",name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);for (int i = 0; i < seconds; i++){Thread.Sleep(TimeSpan.FromSeconds(1));if (token.IsCancellationRequested) return -1;}return 42 * seconds;
  }private static void Main(string[] args)
  {var cts = new CancellationTokenSource();var longTask = new Task<int>(() => TaskMethod("Task 1", 10, cts.Token), cts.Token);Console.WriteLine(longTask.Status);cts.Cancel();Console.WriteLine(longTask.Status);Console.WriteLine("第一个任务在执行前已被取消");cts = new CancellationTokenSource();longTask = new Task<int>(() => TaskMethod("Task 2", 10, cts.Token), cts.Token);longTask.Start();for (int i = 0; i < 5; i++){Thread.Sleep(TimeSpan.FromSeconds(0.5));Console.WriteLine(longTask.Status);}cts.Cancel();for (int i = 0; i < 5; i++){Thread.Sleep(TimeSpan.FromSeconds(0.5));Console.WriteLine(longTask.Status);}Console.WriteLine("任务已完成,结果为 {0}.", longTask.Result);
  }

  2.4　处理异常

  2.4.1 单个任务

  static int TaskMethod(string name, int seconds)
  {Console.WriteLine("Task {0} is running on a thread id {1}. Is thread pool thread: {2}",name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);Thread.Sleep(TimeSpan.FromSeconds(seconds));throw new Exception("Boom!");return 42 * seconds;
  }static void Main(string[] args)
  {try{Task<int> task = Task.Run(() => TaskMethod("Task 2", 2));int result = task.GetAwaiter().GetResult();Console.WriteLine("Result: {0}", result);}catch (Exception ex){Console.WriteLine("Task 2 Exception caught: {0}", ex.Message);}Console.WriteLine("----------------------------------------------");Console.ReadLine();
  }

  2.4.2 多个任务

  static int TaskMethod(string name, int seconds)
  {Console.WriteLine("Task {0} is running on a thread id {1}. Is thread pool thread: {2}",name, Thread.CurrentThread.ManagedThreadId, Thread.CurrentThread.IsThreadPoolThread);Thread.Sleep(TimeSpan.FromSeconds(seconds));throw new Exception(string.Format("Task {0} Boom!", name));return 42 * seconds;
  }public static void Main(string[] args)
  {try{var t1 = new Task<int>(() => TaskMethod("Task 3", 3));var t2 = new Task<int>(() => TaskMethod("Task 4", 2));var complexTask = Task.WhenAll(t1, t2);var exceptionHandler = complexTask.ContinueWith(t =>Console.WriteLine("Result: {0}", t.Result),TaskContinuationOptions.OnlyOnFaulted);t1.Start();t2.Start();Task.WaitAll(t1, t2);}catch (AggregateException ex){ex.Handle(exception =>{Console.WriteLine(exception.Message);return true;});}
  }

  2.4.3 async/await的方式

  class Program
  {static async Task ThrowNotImplementedExceptionAsync(){throw new NotImplementedException();}static async Task ThrowInvalidOperationExceptionAsync(){throw new InvalidOperationException();}static async Task Normal(){await Fun();}static Task Fun(){return Task.Run(() =>{for (int i = 1; i <= 10; i++){Console.WriteLine("i={0}", i);Thread.Sleep(200);}});}static async Task ObserveOneExceptionAsync(){var task1 = ThrowNotImplementedExceptionAsync();var task2 = ThrowInvalidOperationExceptionAsync();var task3 = Normal();try{Task allTasks = Task.WhenAll(task1, task2, task3); //异步的方式await allTasks;//Task.WaitAll(task1, task2, task3); //同步的方式}catch (NotImplementedException ex){Console.WriteLine("task1 任务报错!");}catch (InvalidOperationException ex){Console.WriteLine("task2 任务报错!");}catch (Exception ex){Console.WriteLine("任务报错!");}}public static void Main(){Task task = ObserveOneExceptionAsync();Console.WriteLine("主线程继续运行........");task.Wait();}
  }

  2.5　Task.FromResult

  此方法创建一个Task<TResult>对象，该对象的Task<TResult>.Result属性为result，其Status属性为RanToCompletion。 当立即知道任务的返回值而不执行更长的代码路径时，通常使用方法。

  class Program
  {static IDictionary<string, string> cache = new Dictionary<string, string>(){{"0001","A"}, {"0002","B"}, {"0003","C"},{"0004","D"}, {"0005","E"}, {"0006","F"}};public static void Main(){Task<string> task = GetValueFromCache("0006");Console.WriteLine("主程序继续执行。。。。");string result = task.Result;Console.WriteLine("result={0}", result);}private static Task<string> GetValueFromCache(string key){Console.WriteLine("GetValueFromCache开始执行。。。。");string result = string.Empty;//Task.Delay(5000);Thread.Sleep(5000);Console.WriteLine("GetValueFromCache继续执行。。。。");if (cache.TryGetValue(key, out result)){return Task.FromResult(result);}return Task.FromResult("");}
  }

  2.6　Factory.FromAsync

  简APM模式（委托）转换为任务，BeginXXX和EndXXX

  2.6.1 带回调方式的

  class Program
  {private delegate string AsynchronousTask(string threadName);private static string Test(string threadName){Console.WriteLine("开始...");Console.WriteLine("线程池是线程吗: {0}", Thread.CurrentThread.IsThreadPoolThread);Thread.Sleep(TimeSpan.FromSeconds(2));Thread.CurrentThread.Name = threadName;return string.Format("线程名称: {0}", Thread.CurrentThread.Name);}private static void Callback(IAsyncResult ar){Console.WriteLine("开始一个回调...");Console.WriteLine("传递给callbak的状态: {0}", ar.AsyncState);Console.WriteLine("线程池是线程吗: {0}", Thread.CurrentThread.IsThreadPoolThread);Console.WriteLine("线程池工作线程id: {0}", Thread.CurrentThread.ManagedThreadId);}//执行的流程是:先执行Test--->Callback--->task.ContinueWithstatic void Main(string[] args){AsynchronousTask d = Test;Console.WriteLine("Option 1");Task<string> task = Task<string>.Factory.FromAsync(d.BeginInvoke("AsyncTaskThread", Callback, "委托异步调用"), d.EndInvoke);task.ContinueWith(t => Console.WriteLine("完成回调, 现在继续! Result: {0}", t.Result));while (!task.IsCompleted){Console.WriteLine(task.Status);Thread.Sleep(TimeSpan.FromSeconds(0.5));}Console.WriteLine(task.Status);}
  }

  2.6.2 不带回调方式的

  class Program
  {private delegate string AsynchronousTask(string threadName);private static string Test(string threadName){Console.WriteLine("开始...");Console.WriteLine("线程池是线程吗: {0}", Thread.CurrentThread.IsThreadPoolThread);Thread.Sleep(TimeSpan.FromSeconds(2));Thread.CurrentThread.Name = threadName;return string.Format("线程名称: {0}", Thread.CurrentThread.Name);}//执行的流程是:先执行Test--->task.ContinueWithstatic void Main(string[] args){AsynchronousTask d = Test;Task<string> task = Task<string>.Factory.FromAsync(d.BeginInvoke, d.EndInvoke, "AsyncTaskThread", "委托异步调用");task.ContinueWith(t => Console.WriteLine("任务完成，现在运行一个延续! Result: {0}", t.Result));while (!task.IsCompleted){Console.WriteLine(task.Status);Thread.Sleep(TimeSpan.FromSeconds(0.5));}Console.WriteLine(task.Status);}
  }

  2.6.3 Task启动带参数和返回值的函数任务

  方法1

  private int MyTest(object i)
  {this.Invoke(new Action(() =>{pictureBox1.Visible = true;}));System.Threading.Thread.Sleep(3000);MessageBox.Show("hello:" + i);this.Invoke(new Action(() =>{pictureBox1.Visible = false;}));return 0;
  }private void Call()
  {//Func<string, string> funcOne = delegate(string s){ return "fff"; };object i = 55;var t = Task<int>.Factory.StartNew(new Func<object, int>(MyTest), i);
  }

  方法2

  private async Task<int> MyTest(object i)
  {this.Invoke(new Action(() =>{pictureBox1.Visible = true;}));HttpClient client = new HttpClient();var a = await client.GetAsync("http://www.baidu.com");Task<string> s = a.Content.ReadAsStringAsync();MessageBox.Show (s.Result);this.Invoke(new Action(() =>{pictureBox1.Visible = false;}));return 0;
  }async private void Call()
  {object i = 55;var t = Task<Task<int>>.Factory.StartNew(new Func<object, Task<int>>(MyTest), i);
  }

  方法3

  private async void MyTest()
  {this.Invoke(new Action(() =>{pictureBox1.Visible = true;}));HttpClient client = new HttpClient();var a = await client.GetAsync("http://www.baidu.com");Task<string> s = a.Content.ReadAsStringAsync();MessageBox.Show (s.Result);this.Invoke(new Action(() =>{pictureBox1.Visible = false;}));
  }private void Call()
  {var t = Task.Run(new Action(MyTest));//相当于//Thread th= new Thread(new ThreadStart(MyTest));//th.Start();
  }

  2.7　使用IProgress

  IProgress<in T>只提供了一个方法void Report(T value)，通过Report方法把一个T类型的值报告给IProgress，然后IProgress<in T>的实现类Progress<in T>的构造函数接收类型为Action<T>的形参，通过这个委托让进度显示在UI界面中。

  class Program
  {static void DoProcessing(IProgress<int> progress){for (int i = 0; i <= 100; ++i){Thread.Sleep(100);if (progress != null){progress.Report(i);}}}static async Task Display(){//当前线程var progress = new Progress<int>(percent =>{Console.Clear();Console.Write("{0}%", percent);});//线程池线程await Task.Run(() => DoProcessing(progress));Console.WriteLine("");Console.WriteLine("结束");}public static void Main(){Task task = Display();task.Wait();}
  }