update panel for dropdownlist

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="Ajax_dropdown._Default" %>





   


   

   

   
   
   
   
          
                              ForeColor="Red" Width="100px">

                                 AppendDataBoundItems="true"  runat="server" Height="20px" Width="156px"
                   onselectedindexchanged="DropDownList1_SelectedIndexChanged" Font-Bold="True"
                   ForeColor="#FF9900">
           

         
              
              
       

     
      
     
  
   

   

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
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;
namespace Ajax_dropdown
{
    public partial class _Default : System.Web.UI.Page
    {
        string strConnString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        string str;
        SqlCommand com;

        protected void Page_Load(object sender, EventArgs e)
        {
            //SqlConnection con = new SqlConnection(strConnString);

            DropDownList1.AutoPostBack = true;
            SqlConnection con = new SqlConnection(strConnString);

            if (!IsPostBack)
            {
                DropDownList1.Items.Add("Choose City");
                con.Open();
                str = "select * from area ";
                com = new SqlCommand(str, con);
                SqlDataReader reader = com.ExecuteReader();
                while (reader.Read())
                {
                    DropDownList1.Items.Add(reader["city"].ToString());
                }
                reader.Close();
                con.Close();
            }
            DropDownList2.Items.Clear();

         
           
        }

        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
           

            DropDownList1.AutoPostBack = true;
            SqlConnection con = new SqlConnection(strConnString);
            con.Open();
            str = "select * from area where city='" + DropDownList1.SelectedValue + "'";
            com = new SqlCommand(str, con);
            SqlDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                DropDownList2.Items.Add(reader["stdcode"].ToString());
            }
            reader.Close();
            con.Close();
        }
    }
}