gridview筛选后竟然没有显示？怎么回事？


<%@ Page Language="C#" AutoEventWireup="true" CodeFile="geidviewdropdoenlist.aspx.cs" Inherits="geidviewdropdoenlist" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>无标题页</title>
</head>
<body>
    <form id="form1" runat="server">
     <div>
        &nbsp;<asp:GridView ID="GridView1" 
        runat="server" 
        BackColor="White"
        BorderColor="#DEDFDE" 
        BorderStyle="None" 
        BorderWidth="1px" 
        CellPadding="4" 
        ForeColor="Black"
        GridLines="Vertical"
        OnRowCreated="GridView1_RowCreated">
            <Columns>
                <asp:TemplateField>
                 <HeaderTemplate>
                  <asp:DropDownList 
                   runat="server" 
                   ID="dropdownlist1"
                   BackColor="NavajoWhite"
                   AutoPostBack="true"
                   
                   >
                  <asp:ListItem Value="0" Text=" "/>
                  <asp:ListItem Value="1" Text="讲师"/>
                  <asp:ListItem Value="2" Text="教授"/>
                  <asp:ListItem Value="3" Text="管理员"/>
                  </asp:DropDownList>
                 </HeaderTemplate>
                </asp:TemplateField>
            </Columns>
            <FooterStyle BackColor="#CCCC99" />
            <RowStyle BackColor="#F7F7DE" />
            <SelectedRowStyle BackColor="#CE5D5A" Font-Bold="True" ForeColor="White" />
            <PagerStyle BackColor="#F7F7DE" ForeColor="Black" HorizontalAlign="Right" />
            <HeaderStyle BackColor="#6B696B" Font-Bold="True" ForeColor="White" />
            <AlternatingRowStyle BackColor="White" />
        </asp:GridView>
       </div>
    </form>
</body>
</html>
后台代码：using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;[code=C#]
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;public partial class geidviewdropdoenlist : System.Web.UI.Page
{
    SqlConnection conn = new SqlConnection(ConfigurationManager.AppSettings["ConnectionString1"]);
    DataView dv = new DataView();
    DropDownList ddl;
    public  void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            BindData();        }
    }
    void BindData()
    {
        string sql = "select top 20 TeaID,Roles,TeaJob from TeacherInfo";
        conn.Open();
        SqlDataAdapter myda = new SqlDataAdapter(sql, conn);
        DataSet ds = new DataSet();
        myda.Fill(ds, "RusTable");
        dv = ds.Tables["RusTable"].DefaultView;
        GridView1.DataSource = dv;
        GridView1.DataBind();
        conn.Close();
    }    protected void dropdownlist1_SelectedIndexChanged(object sender, EventArgs e)
    {
        Response.Write("<script language=javascript>alert('删除成功!')</script>");
        DropDownList ddl = sender as DropDownList;
        string selectedText = ddl.SelectedValue;
        BindData();
        switch (ddl.SelectedItem.Text)
        {
            case "讲师":
                dv.RowFilter = "TeaJob='" + ddl.SelectedItem.Value + "'";
                GridView1.DataSource = dv;
                GridView1.DataBind();
                break;
            case "教授":
                dv.RowFilter = "TeaJob='" + ddl.SelectedItem.Value + "'";
                GridView1.DataSource = dv;
                GridView1.DataBind();
                break;
            case "管理员":
                dv.RowFilter = "TeaJob='" + ddl.SelectedItem.Value + "'";
                GridView1.DataSource = dv;
                GridView1.DataBind();
                break;
            default:
                BindData();
                break;
        }
    }    protected void GridView1_RowCreated(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.Header)
        {
            DropDownList ddl = (DropDownList)e.Row.FindControl("dropdownlist1");
            ddl.SelectedIndexChanged += new EventHandler(dropdownlist1_SelectedIndexChanged);
        }
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            e.Row.Cells[0].BackColor = System.Drawing.Color.White;
        }
    }
}[/code]