Bind 2 DropDownList using C#

This code snippet for Bind 2 DropDownList using C#.

Create 2 or 3 tables in SQL,

Assign a primary key in first table(e.g. Country table).

Primary & foreign key in second table,(e.g. State Table) etc.

Add a connection string in web.config.

Create a class file (e.g. code).

using System.Linq;
using System.Web;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
namespace WebApplication1 {
    public class code {
        SqlConnection con = new SqlConnection(ConfigurationManager.AppSettings["con"].ToString());
        public DataSet select(string s) {
            DataSet ds = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter(s, con);
            da.Fill(ds);
            return ds;
        }
        public int insert(string s) {
            int i;
            SqlCommand cmd = new SqlCommand(s, con);
            con.Open();
            i = cmd.ExecuteNonQuery();
            con.Close();
            return i;
        }
    }
}

Code starts here:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
namespace WebApplication1 {
    public partial class DDList: System.Web.UI.Page {
        DataSet ds = new DataSet();
        code gs = new code();
        string s;
        protected void Page_Load(object sender, EventArgs e) {
            if (!IsPostBack) {
                s = "Select * from tbl_Country"; //  Table 1
                ds = gs.select(s);
                if (ds.Tables[0].Rows.Count > 0) {
                    DropDownList1.DataTextField = "country_name"; // Country name = Column name
                    DropDownList1.DataValueField = "country_id"; // Primary Key
                    DropDownList1.DataSource = ds;
                    DropDownList1.DataBind();
                    // DropDownList1.Items.Insert(0, "Select Country");
                }
            }
        }
        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e) {
            s = "select * from tbl_State where country_id = '" + DropDownList1.SelectedValue + "'"; // tbl_State = Table 2
            ds = gs.select(s);
            if (ds.Tables[0].Rows.Count > 0) {
                DropDownList2.DataTextField = "state_name";
                DropDownList2.DataValueField = "state_id";
                DropDownList2.DataSource = ds;
                DropDownList2.DataBind();
                // DropDownList2.Items.Insert(0, "Select State");
            }
        }
        protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e) {
            s = "select * from tbl_city where state_id = '" + DropDownList2.SelectedValue + "'"; // tbl_city = Table 3
            ds = gs.select(s);
            if (ds.Tables[0].Rows.Count > 0) {
                DropDownList3.DataTextField = "city_name";
                DropDownList3.DataValueField = "city_id";
                DropDownList3.DataSource = ds;
                DropDownList3.DataBind();
                // DropDownList3.Items.Insert(0, "Select City");
            }
        }
    }
}

Note: Don't forget to set AutoPostBack="true".