how to create cascading dropdownlist without using postback?

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





   


   

   

   
   
  
       
          
                              ForeColor="Red" Width="100px">
                                 AppendDataBoundItems="true"  runat="server" Height="20px" Width="156px"
                   onselectedindexchanged="ddlcountry_SelectedIndexChanged" Font-Bold="True"
                   ForeColor="#FF9900">
           
       

     
      
     
  
       


       
    
                  ForeColor="Red" Width="100px">
                    AppendDataBoundItems ="true"  runat="server" Height="20px"
Width="155px" onselectedindexchanged="ddlstate_SelectedIndexChanged" Font-Bold="True"
             ForeColor="#993300">
      
    
    
       
       
    
       

      
        
                ForeColor="Red" Width="100px">  
                  runat="server" Height="20px" Width="155px" Font-Bold="True" ForeColor="#CCCC00">
     
  
  
    
 
   

   

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 Country_state_city
{
    public partial class _Default : System.Web.UI.Page
    {
        string strConnString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        string str;
        SqlCommand com;
        SqlDataAdapter sqlda;
        DataSet ds;

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                bindcountry();
            }
        }

        protected void ddlcountry_SelectedIndexChanged(object sender, EventArgs e)
        {
            bindstate();
        }

        protected void ddlstate_SelectedIndexChanged(object sender, EventArgs e)
        {
            bindcity();
        }


        public void bindcountry()
        {
            SqlConnection con = new SqlConnection(strConnString);
            con.Open();
            str = "select CountryId,CountryName from Country";
            com = new SqlCommand(str, con);
            sqlda = new SqlDataAdapter(com);
            ds = new DataSet();
            sqlda.Fill(ds, "Country");
            ddlcountry.Items.Clear();
            ddlcountry.Items.Add("--Select--");
            ddlcountry.DataValueField = "CountryId";
            ddlcountry.DataTextField = "CountryName";
            ddlcountry.DataSource = ds;
            ddlcountry.DataMember = "Country";
            ddlcountry.DataBind();
            con.Close();
        }
        public void bindstate()
        {
            SqlConnection con = new SqlConnection(strConnString);
            con.Open();
            str = "select StateId,StateName from State where Country_Id='" + ddlcountry.SelectedValue + "'";
            com = new SqlCommand(str, con);
            sqlda = new SqlDataAdapter(com);
            ds = new DataSet();
            sqlda.Fill(ds, "State");
            ddlstate.Items.Clear();
            ddlstate.Items.Add("--Select--");
            ddlstate.DataValueField = "StateId";
            ddlstate.DataTextField = "StateName";
            ddlstate.DataSource = ds;
            ddlstate.DataMember = "State";
            ddlstate.DataBind();
            con.Close();
        }
        public void bindcity()
        {
            SqlConnection con = new SqlConnection(strConnString);
            con.Open();
            str = "select CityId,CityName from City where State_Id ='" + ddlstate.SelectedValue + "'";
            com = new SqlCommand(str, con);
            sqlda = new SqlDataAdapter(com);
            ds = new DataSet();
            sqlda.Fill(ds, "City");
            ddlcity.Items.Clear();
            ddlcity.Items.Add("--Select--");
            ddlcity.DataValueField = "CityId";
            ddlcity.DataTextField = "CityName";
            ddlcity.DataSource = ds;
            ddlcity.DataMember = "City";
            ddlcity.DataBind();
            con.Close();
        }


    }
}