页面从aa.aspx跳转进bb.aspx后原来能正常显示的列表不能显示，请问怎么修改？

页面从aa.aspx跳转进bb.aspx后原来能正常显示的dropdownlist列表不能显示了，如果屏蔽bb.aspx中的Page_Load事件就可以显示，是不是与HttpContext.Current.Session["username"]有关，请问代码怎么修改？登录页面aa.aspx和内容页面bb.aspx两个页面，只能通过登录aa.aspx跳转到bb.aspx，并在bb.aspx的Lbsername上显示登录名，不允许直接输入地址就进入bb.aspx。网上一位朋友提供的代码如下：
aa.aspx：
<%@ Page Language="C#" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><script runat="server">    /// <summary>
    /// 登录
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    protected void Button1_Click(object sender, EventArgs e)
    {
        HttpContext.Current.Session["username"] = Txtusername.Text;
        Response.Redirect("bb.aspx");
    }
</script><html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>aa.aspx</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    
        <asp:TextBox ID="Txtusername" runat="server"></asp:TextBox>
        <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
    
    </div>
    </form>
</body>
</html>bb.aspx：
<%@ Page Language="C#" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><script runat="server">    protected void Page_Load(object sender, EventArgs e)
    {
        //判断登录
        if (HttpContext.Current.Session["username"] == null)
            Response.Redirect("aa.aspx");
        
        //判断页面
        if (Request.ServerVariables["HTTP_REFERER"].Equals("http://localhost:1736/aa.aspx", StringComparison.OrdinalIgnoreCase))
            Response.Write(Request.ServerVariables["HTTP_REFERER"]);
            //Response.Redirect("aa.aspx");        Lbsername.Text = HttpContext.Current.Session["username"].ToString();
    }
</script><html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>bb.aspx</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    
        <asp:Label ID="Lbsername" runat="server" Text=""></asp:Label>
    
    </div>
    </form>
</body>
</html>bb.aspx.cs:
using System;
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;public partial class bb : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!this.IsPostBack)
        {
            //绑定
            SqlConnection conn = DB.createConnection();
            conn.Open();
            SqlCommand cmdc = new SqlCommand("select * from tbcc", conn);
            SqlDataReader sdr = cmdc.ExecuteReader();
            this.ddlC.DataSource = sdr;
            this.ddlC.DataTextField = "cityName";
            this.ddlC.DataValueField = "cityID";
            this.ddlC.DataBind();
            sdr.Close();            // 绑定       
            SqlCommand cmdx = new SqlCommand("select * from tbdd where cityID=" + this.ddlC.SelectedValue, conn);
            SqlDataReader sdrx = cmdx.ExecuteReader();
            this.ddlXQ.DataSource = sdrx;
            this.ddlXQ.DataTextField = "xqName";
            this.ddlXQ.DataValueField = "xqID";
            this.ddlXQ.DataBind();
            sdrx.Close();
            //
            conn.Close();
        }
    }
}