多线程---await async用法

1.概述

await/async:是个新语法，出现C#5.0,.NetFramework在4.5及以上（CLR4.0）是一个语法糖，不是一个全新的异步多线程使用方式.

 

语法糖：就是编译器提供的新功能

本身并不会产生新的线程，但是依托于Task而存在，所以程序执行时，也是有多线程的

 

async可以随便添加，可以不用await，但是await只能出现在task前面，但是方法必须声明async,不能单独出现

 

await/async之后，原本没有返回值的，可以返回Task，原本有返回值的，可以返回Task<TType>

2.基本用法

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading.Tasks;

namespace AsyncDemo1
{
    public class AwaitAsyncClass
    {
        public async Task DoSomething()
        {
            await Task.Run(()=>
            {
                Console.WriteLine("***********");
            });
        }
    }
}

3.await关键字后执行顺序

（1）不加async和await的执行

 public class AwaitAsyncClass
    {
        public void Show()
        {
            Console.WriteLine($"This Main Start {Thread.CurrentThread.ManagedThreadId} ");
            NoReturn();
            Console.WriteLine($"This Main End {Thread.CurrentThread.ManagedThreadId} ");
        }
        public void NoReturn()
        {
            Console.WriteLine($"This NoReturn Start {Thread.CurrentThread.ManagedThreadId} ");
            Task.Run(()=>
            {
                Console.WriteLine($"This NoReturn Task Start {Thread.CurrentThread.ManagedThreadId} ");
                Thread.Sleep(2000);
                Console.WriteLine($"This NoReturn Task End {Thread.CurrentThread.ManagedThreadId} ");
            });
            Console.WriteLine($"This NoReturn End {Thread.CurrentThread.ManagedThreadId} ");
        }
    }

      static void Main(string[] args)
        {
            AwaitAsyncClass awaitAsync = new AwaitAsyncClass();
            awaitAsync.Show();
            Console.ReadLine();
        }

执行结果：

 （2）加上await后

 public class AwaitAsyncClass
    {
        public void Show()
        {
            Console.WriteLine($"This Main Start {Thread.CurrentThread.ManagedThreadId} ");
            ReturnTask();
            Console.WriteLine($"This Main End {Thread.CurrentThread.ManagedThreadId} ");
        }
  
        public async Task ReturnTask()
        {
            Console.WriteLine($"This NoReturn Start {Thread.CurrentThread.ManagedThreadId} ");//调用线程执行（主线程）
            Task task = Task.Run(()=>
            {
                Console.WriteLine($"This NoReturn Task Start {Thread.CurrentThread.ManagedThreadId} ");//Task的子线程完成
                Thread.Sleep(2000);
                Console.WriteLine($"This NoReturn Task End {Thread.CurrentThread.ManagedThreadId} ");//Task的子线程完成
            });
            await task;//调用线程回去忙自己的事
            Console.WriteLine($"This NoReturn End {Thread.CurrentThread.ManagedThreadId} ");//Task的子线程完成

        }
    }

执行顺序：

 

 

（3） 加await的执行顺序，跟以下不加await的顺序一致

 public void Show()
        {
            Console.WriteLine($"This Main Start {Thread.CurrentThread.ManagedThreadId} ");
            NoReturn();
            Console.WriteLine($"This Main End {Thread.CurrentThread.ManagedThreadId} ");
        }
        public void NoReturn()
        {
            Console.WriteLine($"This NoReturn Start {Thread.CurrentThread.ManagedThreadId} ");
            Task task=Task.Run(() =>
            {
                Console.WriteLine($"This NoReturn Task Start {Thread.CurrentThread.ManagedThreadId} ");
                Thread.Sleep(2000);
                Console.WriteLine($"This NoReturn Task End {Thread.CurrentThread.ManagedThreadId} ");
            });
            //当task完成之后执行的回调动作
            task.ContinueWith(t=> Console.WriteLine($"This NoReturn End {Thread.CurrentThread.ManagedThreadId} "));
        }

 

 

 

总结：可以认为，加了await,就等同于将await后面的代码，包装成一个回调，其实回调的线程具备多种可能性

async await原理可以参考：https://www.cnblogs.com/heyuquan/archive/2012/11/30/async-and-await-faq.html