C#图解教程（第四版)_20章异步编程：例子-使用异步的情形

using System;
using System.Net;
using System.Diagnostics;
using System.Threading.Tasks;

namespace Chapter20AsyncSample_UsingAsyncP394
{
    class MyDownloadingString
    {
        Stopwatch sw = new Stopwatch();//类System.Diagnostic.Stopwatch: Provides a set of methods and properties
                                       //that you can use to accurately measure elapsed time.

        public void DoRun()
        {
            const int LargeNumber = 6000000;
            sw.Start();
            //下面是保存结果的对象

            Task<int> t1 = CountCharactersAsync(1, "http://www.baidu.com");
            Task<int> t2 = CountCharactersAsync(2, "http://www.youkuaiyun.com");
            CountToALargeNumber(1, LargeNumber); CountToALargeNumber(2, LargeNumber);
            CountToALargeNumber(3, LargeNumber); CountToALargeNumber(4, LargeNumber);

            Console.WriteLine("Chars in http://www.baidu.com   :{0}", t1.Result);//获取结果
            Console.WriteLine("Chars in http://www.youkuaiyun.com    :{0}", t2.Result);//获取结果
        }
        private async Task <int> CountCharactersAsync(int id, string uriString)
        {
            WebClient wc1 = new WebClient();//类System.Net.WebClient 提供通用的方法用于从资源处（用URI标识）发送和接收数据。
            Console.WriteLine("Starting call {0}:{1,4:N0} ms", id, sw.Elapsed.TotalMilliseconds);
            //下面的await是上下文关键字
            string result = await wc1.DownloadStringTaskAsync(new Uri(uriString));//将资源以字符串形式下载下来。返回一个字符串。
                                                                                  //上面是使用task对象，指定为异步操作
            Console.WriteLine("  Call {0} completed:{1,4:N0} ms", id, sw.Elapsed.TotalMilliseconds);
            //Console.WriteLine("{0}", result);
            return result.Length;

        }
        private void CountToALargeNumber(int id, int value)
        {
            for (long i = 0; i < value; i++)
                ;
            Console.WriteLine("    End counting {0}:{1,4:N0} ms", id, sw.Elapsed.TotalMilliseconds);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            MyDownloadingString ds = new MyDownloadingString();
            ds.DoRun();
            Console.WriteLine("Hello World!");
        }
    }
}