asp.net ajax

reg.htm
function checkName(){
if(document.getElementById('LoginName').value =='')
{
document.getElementById('LoginErr').innerHTML='请输入用户名。';
return false;
}
document.getElementById('LoginErr').innerHTML='正在检测。。';
var xmlObj = null;
if(window.XMLHttpRequest){
xmlObj = new XMLHttpRequest();
} else if(window.ActiveXObject){
xmlObj = new ActiveXObject("Microsoft.XMLHTTP");
} else {
return;
}
xmlObj.onreadystatechange = function(){
if(xmlObj.readyState == 4){
updateObj(xmlObj.responseText);
}
}
xmlObj.open ('GET', 'reg.aspx?loginname='+document.getElementById('LoginName').value, false);
xmlObj.send ('');
}
function updateObj(data)
{
document.getElementById('LoginErr').innerHTML = data;
}
<input name="LoginName" id="LoginName" onblur="checkName();">

reg.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="reg.aspx.cs" Inherits="reg" %>

reg.aspx.cs
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
if (Request.QueryString["loginname"] != null)
{
string strXML = "";
try
{
string strSQL = "select count(1) from Bigshop_member where LoginName='" + Request.QueryString["loginname"] + "'";

object obj = ObjDB.DeleteAndGetBySql(strSQL, GetConnectionString(), false, "判断用户是否存在", ReturnType.Object);

if (obj.GetType().Name != "MessageBox")
{
int memberId = (Int32)obj;

if (memberId != 0)
{
strXML = " 该用户名已被注册，请换一个再试试！ ";
}
else
{
strXML = " 恭喜，该用户名可以注册！ ";
}
}
}
catch
{
strXML = strXML + " 恭喜，该用户名可以注册！ ";
}

Response.Write(strXML);
}
else
{
Response.Write(" 请输入用户名！ ");
}