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);

        }

    }

}