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