C#异步编程之async/await

1. 通过async/await这两个关键字编写出来的代码，代码长（zhang）的像同步编程，但实际上是异步编程。也就是说，async/await能够以同步编程的形式实现异步编程。
2. async/await是C#5.0引入的。
3. 关键字async用在方法定义前面，关键字await只能用在async标记的方法中。
4. async修饰的方法的返回值类型只能是void或者Task或者Task<T>这三种类型，且方法的参数能用ref/out修饰。
5. await异步等待的地方，await后面的代码和前面的代码执行的线程可能不一样（取决于await时任务是否完成）。

参考资料：

https://www.cnblogs.com/liqingwen/p/5831951.html#!comments

https://www.c-sharpcorner.com/article/async-and-await-in-c-sharp/

以下为本人调试时的代码：

代码1：

using System;
using System.Threading;
using System.Threading.Tasks;

namespace 简单用法
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " 主线程ThreadId = " + Thread.CurrentThread.ManagedThreadId);
            AsyncTest();

            Console.ReadLine();
        }

        static Task<int> Print()
        {
            var tcs = new TaskCompletionSource<int>();
            var thread = new Task(() =>
            {
                Console.WriteLine(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " 子线程Id = " + Thread.CurrentThread.ManagedThreadId);
                for (int i = 0; i < 3; i++)
                {
                    Console.WriteLine(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " =====等待===== " + i);
                    Thread.Sleep(1000);
                }

                tcs.SetResult(99);
            });

            thread.Start();

            return tcs.Task;
        }

        async static void AsyncTest()
        {
            Console.WriteLine(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " *****Start***** ThreadId = " + Thread.CurrentThread.ManagedThreadId);

            Task<int> taskA = Print();

            Console.WriteLine(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + " *****Middle***** ThreadId = " + Thread.CurrentThread.ManagedThreadId);

            await taskA;//await后面的代码和前面的代码执行的而线程可能不一样（这个应该是涉及“出让执行权”）

            Console.WriteLine(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + "*****End***** ThreadId = " + Thread.CurrentThread.ManagedThreadId);
        }
    }
}

运行结果：

代码2：

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

//In this example,we are going to take two methods,
//which are not dependent on each other.
namespace 简单用法2
{
    class Program
    {
        static void Main(string[] args)
        {
            Method1();
            Method2();
            Console.ReadLine();
        }

        public static async Task Method1()
        {
            await Task.Run(() =>
            {
                for (int i = 0; i < 100; i++)
                {
                    Console.WriteLine(" Method 1 ");
                    Thread.Sleep(100);
                }
            });
        }

        public static void Method2()
        {
            for (int i = 0; i < 25; i++)
            {
                Console.WriteLine(" Method 2 ");
                Thread.Sleep(500);
            }
        }
    }
}

运行结果：

代码3：

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

//In this example,Method1 is retirning total length as an integer value and we are passing a parameter
//as a length in a Method3,Which is coming from Method1.
namespace 简单用法3
{
    class Program
    {
        static void Main(string[] args)
        {
            callMethod();
            Console.ReadLine();
        }

        public static async void callMethod()
        {
            Task<int> task = Method1();
            Method2();
            int count = await task;
            Method3(count);
        }

        public static async Task<int> Method1()
        {
            int count = 0;
            await Task.Run(() =>
            {
                for (int i = 0; i < 100; i++)
                {
                    Console.WriteLine(" Method 1 ");
                    count++;
                    Thread.Sleep(100);
                }
            });
            return count;
        }

        public static void Method2()
        {
            for (int i = 0; i < 25; i++)
            {
                Console.WriteLine(" Method 2 ");
                Thread.Sleep(200);
            }
        }

        public static void Method3(int count)
        {
            Console.WriteLine("Total count is " + count);
        }
    }
}

运行结果：

代码4：

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

//In this example,we are going to read all the character from a
//large text file asynchronously and get the total length of all the characters.
namespace 简单用法4
{
    class Program
    {
        static void Main(string[] args)
        {
            Task task = new Task(CallMethod);
            task.Start();
            task.Wait();
            Console.ReadLine();
        }

        static async void CallMethod()
        {
            string filePath = @"C:\Io.dos";
            Task<int> task = ReadFile(filePath);

            Console.WriteLine(" other work 1");
            Console.WriteLine(" other work 2");
            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> ReadFile(string file)
        {
            int length = 0;
            Console.WriteLine(" File reading is starting ");
            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;
            }
            Console.WriteLine(" File reading is completed ");
            return length;
        }
    }
}

运行结果：