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