.Net 多线程 异步编程 Await、Async和Task

await和async简介

  await和async是在C#5中引入，并且在.NetFramewor4.5以及.NetCore中进行了支持。主要是解决性能瓶颈，并且增强系统的响应能力。

msdn关于await以及async运行机理的描述

以上demo详细的描述了使用await时程序的运行机制，需要注意的以下几点：

• 第五步：执行完成后如果发现下一步还有await即设置了中断点，此时命令会直接跳出此方法即执行6，换句话说，await表达式执行时暂停并不构成方法退出，只会导致 finally 代码块不运行。
• 第七步：当执行当getStringTask返回结果（第七步）及await等待方法执行完成开始执行后续操作时（8）此时的线程已经和第四步执行的线程不同编号了，但是此时4所在线程上下文中的AsyncLocal变量会被复制到await返回后的线程上下文中。详细参照https://www.cnblogs.com/liyong-blackStone/p/10270526.html
• async 和 await 关键字不会创建其他线程。 因为异步方法不会在其自身线程上运行，因此它不需要多线程.

Task简介

Task类的表示单个操作不会返回一个值，通常以异步方式执行Task对象是一种的中心思想基于任务的异步模式。首次引入.NET Framework 4 中。

Task可以通过以下几种方式开始执行：

            Task.Run(() =>
            {
                Console.WriteLine("无返回值委托");
            });
            Task.Run<int>(() =>
            {
                Console.WriteLine("带返回值委托");
                return 1;
            });
            Task t = new Task(() =>
            {
                Console.WriteLine("声明一个Task可以后续通过t.run执行");
            });
            TaskFactory factory = new TaskFactory();
            factory.StartNew<int>(() =>
            {
                Console.WriteLine("通过TaskFactory执行");
                return 1;
            });
            Task t4 = new Task(() =>
            {
                Console.WriteLine("同步执行");
            });
            // 同步执行
            t4.RunSynchronously();

task的执行方式这里不多做介绍，下面主要说下Task的异常处理。

异步方法内部的异常处理

.Net中异步方法由 async修饰符标记，通常包含一个或多个await表达式或语句。await表达式将await运算符应用于 Task 或Task。由于在异步方法中通常会存在多个线程，而子线程中的异常信息不能自动的抛到主线程上，所以要想在主线程上获得子线程的异常信息，需要借助于返回的task这个对象。以下为Msdn给出的一段示例代码:

public async Task DoSomethingAsync()
{
    Task<string> theTask = DelayAsync();

    try
    {
        string result = await theTask;
        Debug.WriteLine("Result: " + result);
    }
    catch (Exception ex)
    {
        Debug.WriteLine("Exception Message: " + ex.Message);
    }
    Debug.WriteLine("Task IsCanceled: " + theTask.IsCanceled);
    Debug.WriteLine("Task IsFaulted:  " + theTask.IsFaulted);
    if (theTask.Exception != null)
    {
        Debug.WriteLine("Task Exception Message: "
            + theTask.Exception.Message);
        Debug.WriteLine("Task Inner Exception Message: "
            + theTask.Exception.InnerException.Message);
    }
}

private async Task<string> DelayAsync()
{
    await Task.Delay(100);

    // Uncomment each of the following lines to
    // demonstrate exception handling.

    //throw new OperationCanceledException("canceled");
    //throw new Exception("Something happened.");
    return "Done";
}

// Output when no exception is thrown in the awaited method:
//   Result: Done
//   Task IsCanceled: False
//   Task IsFaulted:  False

// Output when an Exception is thrown in the awaited method:
//   Exception Message: Something happened.
//   Task IsCanceled: False
//   Task IsFaulted:  True
//   Task Exception Message: One or more errors occurred.
//   Task Inner Exception Message: Something happened.

// Output when a OperationCanceledException or TaskCanceledException
// is thrown in the awaited method:
//   Exception Message: canceled
//   Task IsCanceled: True
//   Task IsFaulted:  False

对于异步线程的执行结果，最多有三种情况

• 正常执行结束：可以通过IsCompletedSuccessfully 属性表示任务正常结束，并成功执行。特别说明：IsCompleted为true只能表示执行完成，当任务处于三种最终状态之一： RanToCompletion， Faulted，或Canceled。时他都为true
• 发生异常：发生异常时，可以通过返回的task对象中的IsFaulted属性判断，为true时表示发生异常，如果要获取具体的异常信息，可以通过以下以下方式获取

    task.Exception.InnerException.Message

• 异步线程被取消:异步线程被取消时task中的IsCanceled属性会被设置为true
  
  了解取消任务可以参考msdn示例:

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

public class Example
{
   public static void Main()
   {
      // Create a cancellation token and cancel it.
      var source1 = new CancellationTokenSource();
      var token1 = source1.Token;
      source1.Cancel();
      // Create a cancellation token for later cancellation.
      var source2 = new CancellationTokenSource();
      var token2 = source2.Token;

      // Create a series of tasks that will complete, be cancelled,
      // timeout, or throw an exception.
      Task[] tasks = new Task[12];
      for (int i = 0; i < 12; i++)
      {
          switch (i % 4)
          {
             // Task should run to completion.
             case 0:
                tasks[i] = Task.Run(() => Thread.Sleep(2000));
                break;
             // Task should be set to canceled state.
             case 1:
                tasks[i] = Task.Run( () => Thread.Sleep(2000),
                         token1);
                break;
             case 2:
                // Task should throw an exception.
                tasks[i] = Task.Run( () => { throw new NotSupportedException(); } );
                break;
             case 3:
                // Task should examine cancellation token.
                tasks[i] = Task.Run( () => { Thread.Sleep(2000);
                                             if (token2.IsCancellationRequested)
                                                token2.ThrowIfCancellationRequested();
                                             Thread.Sleep(500); }, token2);
                break;
          }
      }
      Thread.Sleep(250);
      source2.Cancel();

      try {
         Task.WaitAll(tasks);
      }
      catch (AggregateException ae) {
          Console.WriteLine("One or more exceptions occurred:");
          foreach (var ex in ae.InnerExceptions)
             Console.WriteLine("   {0}: {1}", ex.GetType().Name, ex.Message);
       }   

      Console.WriteLine("\nStatus of tasks:");
      foreach (var t in tasks) {
         Console.WriteLine("   Task #{0}: {1}", t.Id, t.Status);
         if (t.Exception != null) {
            foreach (var ex in t.Exception.InnerExceptions)
               Console.WriteLine("      {0}: {1}", ex.GetType().Name,
                                 ex.Message);
         }
      }
   }
}
// The example displays output like the following:
//   One or more exceptions occurred:
//      TaskCanceledException: A task was canceled.
//      NotSupportedException: Specified method is not supported.
//      TaskCanceledException: A task was canceled.
//      TaskCanceledException: A task was canceled.
//      NotSupportedException: Specified method is not supported.
//      TaskCanceledException: A task was canceled.
//      TaskCanceledException: A task was canceled.
//      NotSupportedException: Specified method is not supported.
//      TaskCanceledException: A task was canceled.
//
//   Status of tasks:
//      Task #13: RanToCompletion
//      Task #1: Canceled
//      Task #3: Faulted
//         NotSupportedException: Specified method is not supported.
//      Task #8: Canceled
//      Task #14: RanToCompletion
//      Task #4: Canceled
//      Task #6: Faulted
//         NotSupportedException: Specified method is not supported.
//      Task #7: Canceled
//      Task #15: RanToCompletion
//      Task #9: Canceled
//      Task #11: Faulted
//         NotSupportedException: Specified method is not supported.
//      Task #12: Canceled