js在调用C#定义的pagetop类的时候总提示不存在怎么办？

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="pagetop.ascx.cs" Inherits="pagetop" %>
<script type="text/javascript">
window.status='郁闷了pagetop未定义是怎么回事啊？';
function addfavorite(url,title,topicid)
{
    external.AddFavorite(url,title)
    pagetop.Collection(topicid);
    return false;
}
</script>上面是axcs内的js代码，下面是类pagetop.ascx.cs
当js一执行的时候就提示pagetop未定义，请知道是那里的原因？
using System;
using System.IO;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Web.Services;
using System.Web.Services.Protocols;public partial class pagetop : System.Web.UI.UserControl
{
    protected void Page_Load(object sender, EventArgs e)
    {
        AjaxPro.Utility.RegisterTypeForAjax(typeof(_pagetop));
        //Cookie登陆
        if (Session.Contents["username"] == null && Request.Cookies["xxxxxx"] != null)
        {
            Session.Contents["username"] = Request.Cookies["xxxxxx"].Value;
            Response.Cookies["xxxxxx"].Value = Request.Cookies["xxxxxx"].Value;
            Response.Cookies["xxxxxx"].HttpOnly = false;
            Response.Cookies["xxxxxx"].Expires = DateTime.Now.AddYears(1);
            SqlStore.SqlUserCookie(Request.Cookies["xxxxxx"].Value);
        }
    }
    [AjaxPro.AjaxMethod]
    public string AddCookie(int topicid, string title)
    {
        SqlStore.SqlAjax(topicid, 5, "");
        System.Web.HttpCookie newcookie = HttpContext.Current.Request.Cookies["topicid"];
        if (newcookie == null) newcookie = HttpContext.Current.Response.Cookies["topicid"];
        if (newcookie.Values.Count >= 50)
            return "false";
        newcookie.Values[topicid.ToString()] = HttpContext.Current.Server.UrlEncode(title);
        newcookie.Expires = DateTime.Now.AddYears(1);
        HttpContext.Current.Response.AppendCookie(newcookie);
        return "true";
    }    
}