c# async await的使用方式及为啥要用它

c# 5的语法（.net framework 4.5+ 版本）引入了 async await的概念，再加上前端开发也经常出现这个概念。用的少，理解也不深，看到了一篇很容易入门的文章，所以在此写一些自己的理解：

本文来源于知乎的一篇文章，但是添加了一些自己的测试代码。知乎文章的链接如下

https://zhuanlan.zhihu.com/p/138805565

先来看一下顺序执行的例子

        internal static void Test()
        {
            Stopwatch sw = new Stopwatch();
            sw.Start();
            Task task = new Task(CallMethod);
            //Task task = new Task(CallMethodAsync);
            task.Start();
            task.Wait();


            sw.Stop();
            Console.WriteLine("程序运行共 耗时:" + sw.ElapsedMilliseconds);
            Console.ReadLine();
        }

        static async void CallMethod()
        {
            Console.WriteLine(" Other Work 1111");
            string filePath = "d:\\sampleFile.txt";
            int a = ReadFile(filePath);

            Console.WriteLine(" Other Work 1");
            Thread.Sleep(1000);
            Console.WriteLine(" Other Work 2");
            Thread.Sleep(1000);
            Console.WriteLine(" Other Work 3");

            int length = a;
            Console.WriteLine(" Total length: " + length);

            Console.WriteLine(" After work 1");
            Console.WriteLine(" After work 2");
        }


        static int ReadFile(string file)
        {
            int length = 0;

            Console.WriteLine(" File reading is stating");
            using (StreamReader reader = new StreamReader(file))
            {
                // Reads all characters from the current position to the end of the stream asynchronously   
                // and returns them as one string.   
                string s = reader.ReadToEnd();

                length = s.Length;
            }
            Thread.Sleep(2000);
            //Console.WriteLine("Hello world");
            Console.WriteLine(" File reading is completed");
            return length;
        }

程序执行以后，输出如下：

可以看到，因为我在ReadFile 里面进行了延时，所以ReadFile内的函数耗时 + 函数外的耗时 = 4秒左右

再看一下使用async await异步执行的例子

        internal static void Test()
        {
            Stopwatch sw = new Stopwatch();
            sw.Start();
            //Task task = new Task(CallMethod);
            Task task = new Task(CallMethodAsync);
            task.Start();
            task.Wait();


            sw.Stop();
            Console.WriteLine("程序运行共 耗时:" + sw.ElapsedMilliseconds);
            Console.ReadLine();
        }

        static async void CallMethodAsync()
        {
            Console.WriteLine(" Other Work 1111");
            string filePath = "d:\\sampleFile.txt";
            Task<int> task = ReadFileAsync(filePath);

            Console.WriteLine(" Other Work 1");
            Thread.Sleep(1000);
            Console.WriteLine(" Other Work 2");
            Thread.Sleep(1000);
            Console.WriteLine(" Other Work 3");

            int length = await task;
            Console.WriteLine(" Total length: " + length);

            Console.WriteLine(" After work 1");
            Console.WriteLine(" After work 2");
        }


        static async Task<int> ReadFileAsync(string file)
        {
            int length = 0;

            Console.WriteLine(" File reading is stating");
            using (StreamReader reader = new StreamReader(file))
            {
                // Reads all characters from the current position to the end of the stream asynchronously   
                // and returns them as one string.   
                string s = await reader.ReadToEndAsync();

                length = s.Length;
            }
            Thread.Sleep(2000);
            Console.WriteLine(" File reading is completed");
            return length;
        }