using System;
using System.Collections.Generic;
using System.Configuration;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;
namespace WebClientAsynProject
{
  public class Program
  {
    #region HttpWebRequest異步GET
    public static
 
 void AsyncGetWithWebRequest(string url)
    {
      var request = (HttpWebRequest) WebRequest.Create(new Uri(url));
      request.BeginGetResponse(new AsyncCallback(ReadCallback), request);
    }
    private static void ReadCallback(IAsyncResult asynchronousResult)
    {
      var request = (HttpWebRequest) asynchronousResult.AsyncState;
      
 
var response = (HttpWebResponse) request.EndGetResponse(asynchronousResult);
      using (var streamReader = new StreamReader(response.GetResponseStream()))
      {
        var resultString = streamReader.ReadToEnd();
        Console.WriteLine(resultString);
      }
    }
    #endregion
    #region
 
 WebClient異步GET
    public static void AsyncGetWithWebClient(string url)
    {
      var webClient = new WebClient();
      webClient.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);
      webClient.DownloadStringAsync(new Uri(url));
    }
    private static void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
      //Console.WriteLine(e.Cancelled);
      Console.WriteLine(e.Error != null ? "WebClient異步GET發生錯誤！" : e.Result);
    }
    #endregion
    #region WebClient的OpenReadAsync測試
    public static void TestGetWebResponseAsync(string url)
    {
      var webClient = new WebClient();
      webClient.OpenReadCompleted += new OpenReadCompletedEventHandler(webClient_OpenReadCompleted);
      webClient.OpenReadAsync(new Uri(url));
    }
    private static void webClient_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
    {
      if(e.Error == null)
      {
        var streamReader = new StreamReader(e.Result);
        var result = streamReader.ReadToEnd();
        Console.WriteLine(result);
      }
      else
      {
        Console.WriteLine("執行WebClient的OpenReadAsync出錯：" + e.Error);
      }
    }
    #endregion
    public static void Main(string[] args)
    {
      AsyncGetWithWebRequest("http://baidu.com");
      Console.WriteLine("hello");
      AsyncGetWithWebClient("http://baidu.com");
      Console.WriteLine("world");
      TestGetWebResponseAsync("http://baidu.com");
      Console.WriteLine("jxqlovejava");
      Console.Read();
    }
  }
}

C#實現異步GET的方法