ajax从javascript传参至asp.net，接收不到变量

HTML code

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeBehind="location.aspx.cs" Inherits="WebRole1.location" %><asp:Content ID="BodyContent" ContentPlaceHolderID="MainContent" runat="server">    <script type="text/javascript">        var xmlHttp;        function createXMLHttpRequest() {            if (window.ActiveXObject) {                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");            }            else if (window.XMLHttpRequest) {                xmlHttp = new XMLHttpRequest();            }            else {                alert("your browser doesn't support AJAX");            }        }        function startRequest(url){            createXMLHttpRequest();            xmlHttp.open("GET", url, true);            xmlHttp.onreadystatechange =callBack ;            xmlHttp.send(null);        }        function callBack() {            if (xmlHttp.readyState == 4) {                if (xmlHttp.status == 200) {                    var res = xmlHttp.responseText;                    window.alert(res);                } else {                    window.alert("error");                }            }        }          function getLocation() {            if (navigator.geolocation) {                navigator.geolocation.getCurrentPosition(getPostion);            }        }        function getPostion(position) {            var lat = position.coords.latitude;            var lon = position.coords.longitude;            var url = "location.aspx?lat=" + lat + "&lon=" + lon;            startRequest(url);        }    </script>    <asp:TextBox ID="text" runat="server" CssClass="text" MaxLength="100" Columns="40" TextMode="MultiLine"></asp:TextBox>    <asp:Button ID="textSubmit" runat="server" CssClass="textSubmit" Text="submit" OnClientClick="getLocation()" OnClick="Post"/>     <hr /></asp:Content>[code=C#]using System;using System.Collections.Generic;using System.Linq;using System.Web;using System.Web.UI;using System.Web.UI.WebControls;namespace WebRole1{    public partial class location : System.Web.UI.Page    {        protected void Post(object sender, EventArgs e)        {            Response.Write("<script>alert(" + Request.QueryString["lat"] + "); </script>");        }    }}alert出来的值一直是undifined，求解啊！~ 在线等

[/code]

[解决办法]
protected void Post(object sender, EventArgs e)
{
Response.Write("<script>alert(" + Request.QueryString["lat"] + "); </script>");
}
改成

protected void Page_Load(object sender, EventArgs e)
{
Response.Clear();
Response.Write("Request.QueryString["lat"]);
Response.End();
}