C graphical tutorial (Fourth Edition)_ Chapter 20 asynchronous programming: examples - using asynchronous

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

namespace Chapter20AsyncSample_UsingAsyncP394
{
    class MyDownloadingString
    {
        Stopwatch sw = new Stopwatch();// class 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();
            // The following is the object to save the results 

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

            Console.WriteLine("Chars in http://www.baidu.com   :{0}", t1.Result);// To get the results 
            Console.WriteLine("Chars in http://www.csdn.net    :{0}", t2.Result);// To get the results 
        }
        private async Task <int> CountCharactersAsync(int id, string uriString)
        {
            WebClient wc1 = new WebClient();// class System.Net.WebClient  Provide a common method for obtaining information from resources （ use URI identification ） Send and receive data .
            Console.WriteLine("Starting call {0}:{1,4:N0} ms", id, sw.Elapsed.TotalMilliseconds);
            // Below await It's a contextual keyword 
            string result = await wc1.DownloadStringTaskAsync(new Uri(uriString));// Download the resource as a string . Returns a string .
                                                                                  // Above is the use of task object , Specify as asynchronous operation 
            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!");
        }
    }
}