C# .Net学习笔记—— 异步和多线程（await/async）

一、介绍

1、控制台测试await/async

2、C# 5.0? ?.Net framework4.5? CLR4.0?以后才有，本身是一种语法糖

二、基本测试

1、不加await测试。

        private async static Task TestAsync() 
        {
            Log.Info($"当前主线程id={Thread.CurrentThread.ManagedThreadId}");
            NoReturnNoAwait();
            for (int i = 0; i < 10; i++)
            {
                Thread.Sleep(300);
                Log.Info($"Main Thread Task ManagedThreadID = {Thread.CurrentThread.ManagedThreadId}");
            }
        }
       

     private static async void NoReturnNoAwait() 
        {
            Log.Info($"NoReturn Sleep before await,ThreadID={Thread.CurrentThread.ManagedThreadId}");
            TaskFactory taskFactory = new TaskFactory();
            Task task = taskFactory.StartNew(() =>
            {
                Log.Info($"NoReturn Sleep before,ThreadID={Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(3000);
                Log.Info($"NoReturn Sleep after,ThreadID={Thread.CurrentThread.ManagedThreadId}");
            });
            Log.Info($"NoReturn Sleep after await,ThreadID={Thread.CurrentThread.ManagedThreadId}");
        }

2、加await测试。

        private async static Task TestAsync() 
        {
            Log.Info($"当前主线程id={Thread.CurrentThread.ManagedThreadId}");
            NoReturn();
            //NoReturnNoAwait();
            for (int i = 0; i < 10; i++)
            {
                Thread.Sleep(300);
                Log.Info($"Main Thread Task ManagedThreadID = {Thread.CurrentThread.ManagedThreadId}");
            }
        }

        private static async void NoReturn() 
        {
            Log.Info($"NoReturn Sleep before await,ThreadID={Thread.CurrentThread.ManagedThreadId}");
            TaskFactory taskFactory = new TaskFactory();
            Task task = taskFactory.StartNew(() => 
            {
                Log.Info($"NoReturn Sleep before,ThreadID={Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(3000);
                Log.Info($"NoReturn Sleep after,ThreadID={Thread.CurrentThread.ManagedThreadId}");
            });
            await task;  //主线程到这里就返回了，执行主线程任务
            Log.Info($"NoReturn Sleep after await,ThreadID={Thread.CurrentThread.ManagedThreadId}");
        }

结果：从打印出来的两种执行顺序来看，加了await以后，主线程运行到await这行就会执行
回主线程任务。而await后面的代码将由主线程或其他线程来执行

原理：await后面的代码，会被封装成委托，在await task之后成为回调（编译器功能，状态机实现） ，这个回调的线程是不确定的，可能是主线程，可能是子线程也可能是其他线程。

它可以等价于task.ContinueWith(t=>{}) 这里包起来

??验证一下：

       private async void AwaitTest() 
        {
            //方法一：
            NoReturn();
            //方法二：
            TaskFactory taskFactory = new TaskFactory();
            Task task = taskFactory.StartNew(() =>
            {
                Log.Info($"NoReturn Sleep before,ThreadID={Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(3000);
                Log.Info($"NoReturn Sleep after,ThreadID={Thread.CurrentThread.ManagedThreadId}");
            });
            task.ContinueWith(t =>
            {
                callBack();
            });
            await task;
        }
        private static async void NoReturnNoAwait()
        {
            Log.Info($"NoReturn Sleep before await,ThreadID={Thread.CurrentThread.ManagedThreadId}");
            TaskFactory taskFactory = new TaskFactory();
            Task task = taskFactory.StartNew(() =>
            {
                Log.Info($"NoReturn Sleep before,ThreadID={Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(3000);
                Log.Info($"NoReturn Sleep after,ThreadID={Thread.CurrentThread.ManagedThreadId}");
            });
            Log.Info($"NoReturn Sleep after await,ThreadID={Thread.CurrentThread.ManagedThreadId}");
        }
        private void callBack()
        {
            Log.Info($"我是一个回调。。");
            Log.Info($"NoReturn Sleep after await,ThreadID={Thread.CurrentThread.ManagedThreadId}");
        }

?三、返回值问题

1、不需要传值的时候，我们返回值就写成Task或者void都可以

2、需要传值的话?，可以使用这种带泛型传值

四、小案例

       private void MainThread() 
        {
            Log.Info("主线程Start");
            Async();
            Log.Info($"aaa {Thread.CurrentThread.ManagedThreadId.ToString("00")}");
        }

        private async void Async()
        {
            Log.Info($"ddd {Thread.CurrentThread.ManagedThreadId.ToString("00")}");
            await Task.Run(() =>
            {
                Thread.Sleep(500);
                Log.Info($"bbb {Thread.CurrentThread.ManagedThreadId.ToString("00")}");
            });
            Log.Info($"ccc {Thread.CurrentThread.ManagedThreadId.ToString("00")}");
        }

猜测执行顺序：

1、打印主线程Start

2、主线程进入Async方法，打印ddd

3、碰到了await，主线程返回回去执行，打印aaa

4、子线程等待500毫秒后，并行打印bbb

5、最后回调打印ccc

公布结果：