IAsyncResult  BeginGetResponse(AsyncCallback callback, object state)
WebResponse  EndGetResponse(IAsyncResult asyncResult)

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Net.Http;
namespace Test
{
    class Program
    {
        static void Main(string[] args)
       {
         var asyncMethod =  AccessTheWebAsync();
         Console.WriteLine("go on ...... "+ Thread.CurrentThread.ManagedThreadId );
         // 等待异步线程处理完毕,没有以下句子，await使控制回到调用方，主线程即终止。
         asyncMethod.Wait();
       }
      public static async Task<int> AccessTheWebAsync()
      {
          HttpClient client = new HttpClient();
          // GetStringAsync returns a Task<string>.
          // That means that when you await the task you'll get a string (urlContents).
          Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");
          // You can do work here that doesn't rely on the string from GetStringAsync.
         DoIndependentWork();

        // The await operator suspends AccessTheWebAsync.
        // - AccessTheWebAsync can't continue until getStringTask is complete.
        // - Meanwhile, control returns to the caller of AccessTheWebAsync.
        // - Control resumes here when getStringTask is complete.
        // - The await operator then retrieves the string result from getStringTask.

         string urlContents = await getStringTask;
         Console.WriteLine(urlContents.Length+"....... "+Thread.CurrentThread.ManagedThreadId );
         // The return statement specifies an integer result.
         // Any methods that are awaiting AccessTheWebAsync retrieve the length value.
         return urlContents.Length;
       }
        public static void DoIndependentWork()
        {
            Console.WriteLine("work ......"+Thread.CurrentThread.ManagedThreadId);
        }
    }
}