How to fetch data in a textbox from dropdownlist when selected any listitem of dropdown.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Data;
using System.Data.SqlClient;
using System.Web.UI.WebControls;
using Fobbs.Logic;
using System.Globalization;
using System.Threading.Tasks;

namespace Fobbs
{
    public partial class Eng1 : System.Web.UI.Page
    {

        IndexLogic il = new IndexLogic();
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                Aieebind();   
            }
        }

        void Aieebind()
        {
            DataSet ds = new DataSet();
            ds = il.FetchData(1);
            DDLAieee.DataSource = ds.Tables[0];
            DDLAieee.DataBind();
            DDLAieee.Items.Insert(0, new ListItem("Select", "Select"));
        }


        protected void DDLAieee_SelectedIndexChanged(object sender, EventArgs e)
        {
            DataSet ds = new DataSet();
            ds = il.FetchDataById(2, Convert.ToInt32(DDLAieee.SelectedItem.Text));
            if (ds.Tables[0].Rows.Count > 0)
            {
                txtdescription.Text = ds.Tables[0].Rows[0][1].ToString();
                
            }
        }
    }
}