public class WeiXinHttpHandler:IHttpHandler
  {
    /// <summary>
    /// 
    /// </summary>
    public bool IsReusable
    {
      get { return true; }
    }
    /// <summary>
    /// 处理请求
    /// </summary>
    /// <param name="context"></param>
    public void ProcessRequest(HttpContext context)
    {
      //由微信服务接收请求，具体处理请求
      WeiXinService wxService = new WeiXinService(context.Request);
      string responseMsg = wxService.Response();
      context.Response.Clear();
      context.Response.Charset = "UTF-8";
      context.Response.Write(responseMsg);
      context.Response.End();
    }
  }

<httpHandlers>
  <add verb="*" path="WXService.ashx" type="namespace.WeiXinHttpHandler,WXWeb" validate="true"/>
</httpHandlers>

private function checkSignature()
  {
    $signature = $_GET["signature"];
    $timestamp = $_GET["timestamp"];
    $nonce = $_GET["nonce"];  
        
    $token = TOKEN;
    $tmpArr = array($token, $timestamp, $nonce);
    sort($tmpArr);
    $tmpStr = implode( $tmpArr );
    $tmpStr = sha1( $tmpStr );
    
    if( $tmpStr == $signature ){
      return true;
    }else{
      return false;
    }
  }

/// <summary>
    /// 检查签名
    /// </summary>
    /// <param name="request"></param>
    /// <returns></returns>
    private bool CheckSignature()
    {
      string signature = Request.QueryString[SIGNATURE];
      string timestamp = Request.QueryString[TIMESTAMP];
      string nonce = Request.QueryString[NONCE];

      List<string> list = new List<string>();
      list.Add(TOKEN);
      list.Add(timestamp);
      list.Add(nonce);
      //排序
      list.Sort();
      //拼串
      string input = string.Empty;
      foreach (var item in list)
      {
        input += item;
      }
      //加密
      string new_signature = SecurityUtility.SHA1Encrypt(input);
      //验证
      if (new_signature == signature)
      {
        return true;
      }
      else
      {
        return false;
      }
    }

/// <summary>
    /// SHA1加密
    /// </summary>
    /// <param name="intput">输入字符串</param>
    /// <returns>加密后的字符串</returns>
    public static string SHA1Encrypt(string intput)
    {
      byte[] StrRes = Encoding.Default.GetBytes(intput);
      HashAlgorithm mySHA = new SHA1CryptoServiceProvider();
      StrRes = mySHA.ComputeHash(StrRes);
      StringBuilder EnText = new StringBuilder();
      foreach (byte Byte in StrRes)
      {
        EnText.AppendFormat("{0:x2}", Byte);
      }
      return EnText.ToString();
    }

/// <summary>
    /// 处理请求
    /// </summary>
    /// <returns></returns>
    private string ResponseMsg()
    {
      string requestXml = Common.ReadRequest(this.Request);
      IHandler handler = HandlerFactory.CreateHandler(requestXml);
      if (handler != null)
      {
        return handler.HandleRequest();
      }

      return string.Empty;
    }

/// <summary>
    /// 处理请求，产生响应
    /// </summary>
    /// <returns></returns>
    public string Response()
    {
      string method = Request.HttpMethod.ToUpper();
      //验证签名
      if (method == "GET")
      {
        if (CheckSignature())
        {
          return Request.QueryString[ECHOSTR];
        }
        else
        {
          return "error";
        }
      }

      //处理消息
      if (method == "POST")
      {
        return ResponseMsg();
      }
      else
      {
        return "无法处理";
      }
    }

class EventHandler : IHandler
  {
    /// <summary>
    /// 请求的xml
    /// </summary>
    private string RequestXml { get; set; }
    /// <summary>
    /// 构造函数
    /// </summary>
    /// <param name="requestXml"></param>
    public EventHandler(string requestXml)
    {
      this.RequestXml = requestXml;
    }
    /// <summary>
    /// 处理请求
    /// </summary>
    /// <returns></returns>
    public string HandleRequest()
    {
      string response = string.Empty;
      EventMessage em = EventMessage.LoadFromXml(RequestXml);
      if (em.Event == EventType.Subscribe)
      {
        //注册用户
        User user = new User();
        user.OpenID = em.FromUserName;
        UserManager.Regester(user);

        //回复欢迎消息
        TextMessage tm = new TextMessage();
        tm.ToUserName = em.FromUserName;
        tm.FromUserName = em.ToUserName;
        tm.CreateTime = Common.GetNowTime();
        tm.Content = "欢迎您关注xxx，我是小微。有什么我能帮助您的吗？";
        response = tm.GenerateContent();
      }

      return response;
    }
  }

context.Response.Clear();
context.Response.Charset = "UTF-8";
context.Response.Write(responseMsg);
context.Response.End();