asp.net 中ajax验证用户名是否重复

这是js部分

function createXMLHttpRequest()
{
    if(window.XMLHttpRequest) return new XMLHttpRequest();
    try{return new ActiveXObject('MSXML2.XMLHTTP.4.0');}
    catch(e){try{return new ActiveXObject('MSXML2.XMLHTTP.3.0');}
    catch(e){try{return new ActiveXObject('MSXML2.XMLHTTP.2.6');}
    catch(e){try{return new ActiveXObject('MSXML2.XMLHTTP');}
    catch(e){try{return new ActiveXObject('Microsoft.XMLHTTP');}
    catch(e){return null;}}}}}
} 

function checktbxDescription() 
{
//需要验证的字段
 var tbxDescription=document.getElementById("tbxDescription").value;            
 chkXML = createXMLHttpRequest();
 if(chkXML==null){top.MessageBox.show("info","你的浏览器不支持XmlHttpRequest,请更新!",null,null);return false;}
 chkXML.open("GET","http://www.cnblogs.com/Ajax/AjaxChkDocDescription.ashx?Name=" + tbxDescription,true);
 chkXML.onreadystatechange = function()
 {
  if(chkXML.readyState == 4)
  {
   if(chkXML.status == 200)
   {
    if(chkXML.responseText == "1")
    {
      alert("[温馨提示]该名称已存在，请重新输入！");
      document.getElementById("tbxDescription").select();
      return false;
    }
   }
  }
 }
 chkXML.send(null);
 return true;
} 

下面是AjaxChkDocDescription.ashx里的代码部分：

using System;
using System.Data;
using System.Web;
using System.Collections;
using System.Web.Services;
using System.Web.Services.Protocols;

namespace HCOA.Net.OAModule.Ajax
{
    /**//// <summary>
    /// $codebehindclassname$ 的摘要说明
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    public class AjaxChkDocDescription : IHttpHandler
    {

        public void ProcessRequest(HttpContext context)
        {
            context.Response.ContentType = "text/plain";
            string description = context.Request.QueryString["Description"];
            BLL.m_doc_kind bmdockind = new BLL.m_doc_kind();
            DataSet ds = bmdockind.GetList("description='" + description+"'");
            if (ds.Tables[0].Rows.Count > 0)
                context.Response.Write("1");
            else
                context.Response.Write("0");
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}