下面的一段代码是实现选中DropDownList中的在ListBox中显示，我的怎么实现不了，大家帮忙看看。谢谢
using System;
using System.Data;
using System.Configuration;
using System.Data.SqlClient;
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 _Default : System.Web.UI.Page 
{

    private string myConnectionString = ConfigurationManager.ConnectionStrings["sqlconnectionstring"].ConnectionString;

    private static DataSet ds;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            //TestDataAdapter();
            InitFilterList();
            ds = GetDataSet();
        }
    }

    private void InitFilterList()
    {
        FilterList.Items.Clear();
        char aChar = 'a';
        FilterList.Items.Add(new ListItem("all","-1"));
        for (int i = 0; i < 26; i++)
        {
            FilterList.Items.Add(new ListItem(((char)((int)aChar+i)).ToString(),i.ToString()));//FilterList  DropdownList;UserList1 ListBox
        }
        FilterList.SelectedIndex = 0;
    }


    private DataSet GetDataSet()
    {
        SqlConnection myConnection = new SqlConnection(myConnectionString);
        string strSql = "select * from employees";
        SqlDataAdapter myDa = new SqlDataAdapter(strSql,myConnection);
        DataSet ds = new DataSet("UserDataSet");
        try
        {
            myConnection.Open();
            myDa.Fill(ds, "employees");
            UserList1.DataSource = ds.Tables[0].DefaultView;
            UserList1.DataTextField = "lastname";
            UserList1.DataValueField = "employeeid";
            UserList1.DataBind();
        }
        catch (SqlException ex)
        {
            Response.Write(ex.Message + "<br>");
        }
        finally
        {
            myConnection.Close();
        }
        return ds;
    }
    protected void FilterList_SelectedIndexChanged(object sender, EventArgs e)
    {
        DataView dv = ds.Tables[0].DefaultView;
        if (FilterList.SelectedItem.Text == "a")
        {
            dv.RowFilter = "lastname like '*'";
            //dv = ds.Tables[0].DefaultView;
        }
        else
        {
            dv.RowFilter = "lastname like '" + this.FilterList.SelectedItem.Text + "'";
            //dv = ds.Tables[0].DefaultView;
        }
        UserList1.DataSource = dv;
        UserList1.DataTextField = "lastname";
        UserList1.DataValueField = "employeeid";
        UserList1.DataBind();
    }