C#Async,await异步简单介绍

C# 5.0 引入了async/await，.net framework4.5开始支持该用法

使用：

由async标识的方法必须带有await，如果不带await，方法将被同步执行

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static void Main(string[] args) {     fun22();     Console.WriteLine($"Main await ThreadId {Thread.CurrentThread.ManagedThreadId}");     Console.ReadKey(); }   private async static void fun22() {     //sync同步(相对于调用函数的线程)     Console.WriteLine($"pre await ThreadId {Thread.CurrentThread.ManagedThreadId}");     await HelloWorld();     //async异步(相对于调用函数的线程)     Console.WriteLine($"after await ThreadId {Thread.CurrentThread.ManagedThreadId}"); }   private static Task HelloWorld() {     //sync同步(相对于调用函数的线程)     Console.WriteLine($"pre Task ThreadId {Thread.CurrentThread.ManagedThreadId}");       //async异步(相对于调用函数的线程)     return Task.Factory.StartNew(() =>     {<br>　　　　　　　　　　Thread.Sleep(1000);         Console.WriteLine($"Task ThreadId {Thread.CurrentThread.ManagedThreadId}");     }); }

运行结果：

结果说明：

只有执行到Task异步的时候，才使真正的异步 

 

微软的例子：

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using System; using System.IO;   namespace ConsoleApplication {     class Program     {         static async Task Main()         {             await ReadCharacters();         }           static async Task ReadCharacters()         {             String result;             using (StreamReader reader = File.OpenText("existingfile.txt"))             {                 Console.WriteLine("Opened file.");                 result = await reader.ReadToEndAsync();                 Console.WriteLine("Contains: " + result);             }         }     } }

　　查看ReadToEndAsync源码，发现最后是TaskFactory<Int32>.FromAsyncTrim，真正的异步由这个函数执行，也就是真正的异步是由Task的线程池执行的。

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[HostProtection(ExternalThreading = true)] [ComVisible(false)] public Task<int> ReadAsync(Byte[] buffer, int offset, int count) {     return ReadAsync(buffer, offset, count, CancellationToken.None); }   [HostProtection(ExternalThreading = true)] [ComVisible(false)] public virtual Task<int> ReadAsync(Byte[] buffer, int offset, int count, CancellationToken cancellationToken) {     // If cancellation was requested, bail early with an already completed task.     // Otherwise, return a task that represents the Begin/End methods.     return cancellationToken.IsCancellationRequested                 ? Task.FromCancellation<int>(cancellationToken)                 : BeginEndReadAsync(buffer, offset, count); }   private Task<Int32> BeginEndReadAsync(Byte[] buffer, Int32 offset, Int32 count) {                return TaskFactory<Int32>.FromAsyncTrim(                 this, new ReadWriteParameters { Buffer = buffer, Offset = offset, Count = count },                 (stream, args, callback, state) => stream.BeginRead(args.Buffer, args.Offset, args.Count, callback, state), // cached by compiler                 (stream, asyncResult) => stream.EndRead(asyncResult)); // cached by compiler }