cookie为什么清除不掉？？？

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;





namespace CookieTest

{

    public partial class _Default : System.Web.UI.Page

    {

        protected void Page_Load(object sender, EventArgs e)

        {

            if (!IsPostBack) 

            {

                ReadCookie();

            }

        }



        //设置cookie

        protected void btnSetCookie_Click(object sender, EventArgs e)

        {

            Response.Cookies["userName"].Value = "admin";

            ReadCookie();

        }



        //清除cookie

        protected void btnClearCookie_Click(object sender, EventArgs e)

        {

            if (Request.Cookies["userName"] != null)

            {

                HttpCookie cookie = new HttpCookie("userName");                

                cookie.Expires = DateTime.Now.AddHours(-1);

                Response.Cookies.Add(cookie);

            }

            ReadCookie();

        }



        //读取cookie

        private void ReadCookie() 

        {

            string userName = "";

            if (Request.Cookies["userName"] != null)

            {

                userName = "userName=" + Server.HtmlEncode(Request.Cookies["userName"].Value);//读取cookie

            }

            else 

            {

                userName = "cookie不存在";

            }

            userName = "<script>alert('" + userName + "');</script>";

            this.Page.ClientScript.RegisterStartupScript(this.GetType(), "", userName);

        }

    }

}