when i select country on drop down list...

Default.aspx code

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





   


   

   

   

    
                  onselectedindexchanged="DropDownList_country_SelectedIndexChanged">
   
       

   
   
   
   

   

Default.aspx.cs code

using System;
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;
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);
      
        if (!IsPostBack)
        {
            DropDownList_country.Items.Add("Choose country");
            con.Open();
            str = "select country from area group by country";
            com = new SqlCommand(str, con);
            SqlDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                DropDownList_country.Items.Add(reader["country"].ToString());
            }
            reader.Close();
            con.Close();
        }
        DropDownList_state.Items.Clear();
    
    }



    protected void DropDownList_country_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(strConnString);
        con.Open();
        str = "select * from area where country='" + DropDownList_country.SelectedItem.Text + "'";
        com = new SqlCommand(str, con);
        SqlDataReader reader = com.ExecuteReader();
        while (reader.Read())
        {
            DropDownList_state.Items.Add(reader["state"].ToString());
        }
        reader.Close();
        con.Close();
    }
}

You can write the code on country's dropdown selectedindexchanged like
select stateid,statename from statetable where countryid=countrydropdown.selectedvalue