await和async簡介

??await和async是在C#5中引入，并且在.NetFramewor4.5以及.NetCore中進行了支持。主要是解決性能瓶頸，并且增強系統的響應能力。

msdn關于await以及async運行機理的描述

以上demo詳細的描述了使用await時程序的運行機制，需要注意的以下幾點：

• 第五步：執行完成后如果發現下一步還有await即設置了中斷點，此時命令會直接跳出此方法即執行6，換句話說，await表達式執行時暫停并不構成方法退出，只會導致 finally 代碼塊不運行。
• 第七步：當執行當getStringTask返回結果（第七步）及await等待方法執行完成開始執行后續操作時（8）此時的線程已經和第四步執行的線程不同編號了，但是此時4所在線程上下文中的AsyncLocal變量會被復制到await返回后的線程上下文中。詳細參照https://www.cnblogs.com/liyong-blackStone/p/10270526.html
• async 和 await 關鍵字不會創建其他線程。 因為異步方法不會在其自身線程上運行，因此它不需要多線程.

Task簡介

Task類的表示單個操作不會返回一個值，通常以異步方式執行Task對象是一種的中心思想基于任務的異步模式。首次引入.NET Framework 4 中。
Task可以通過以下幾種方式開始執行：

Task.Run(() =>{Console.WriteLine("無返回值委托");});Task.Run<int>(() =>{Console.WriteLine("帶返回值委托");return 1;});Task t = new Task(() =>{Console.WriteLine("聲明一個Task可以后續通過t.run執行");});TaskFactory factory = new TaskFactory();factory.StartNew<int>(() =>{Console.WriteLine("通過TaskFactory執行");return 1;});Task t4 = new Task(() =>{Console.WriteLine("同步執行");});// 同步執行t4.RunSynchronously();

task的執行方式這里不多做介紹，下面主要說下Task的異常處理。

異步方法內部的異常處理

.Net中異步方法由 async修飾符標記，通常包含一個或多個await表達式或語句。await表達式將await運算符應用于 Task 或Task。由于在異步方法中通常會存在多個線程，而子線程中的異常信息不能自動的拋到主線程上，所以要想在主線程上獲得子線程的異常信息，需要借助于返回的task這個對象。以下為Msdn給出的一段示例代碼:

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

對于異步線程的執行結果，最多有三種情況

• 正常執行結束：可以通過IsCompletedSuccessfully 屬性表示任務正常結束，并成功執行。特別說明：IsCompleted為true只能表示執行完成，當任務處于三種最終狀態之一： RanToCompletion， Faulted，或Canceled。時他都為true
• 發生異常：發生異常時，可以通過返回的task對象中的IsFaulted屬性判斷，為true時表示發生異常，如果要獲取具體的異常信息，可以通過以下以下方式獲取

task.Exception.InnerException.Message

• 異步線程被取消:異步線程被取消時task中的IsCanceled屬性會被設置為true
  了解取消任務可以參考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

轉載于:https://www.cnblogs.com/liyong-blackStone/p/10276769.html

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