.NET (Core and Framework)

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.Net.Mail;
using System.IO;
using System.ComponentModel;
using System.Drawing;
using System.Web.SessionState;

namespace AccountManagement
{
    public partial class Account : System.Web.UI.Page
    {
        SqlConnection con;
       SqlDataAdapter da;
        SqlDataReader dr;
        //DataSet ds1;
       // String Sql;
        //int Count;
        DataTable dt;
        SqlCommand cmd;
        protected void Page_Load(object sender, EventArgs e)
        {
            string s = ConfigurationManager.ConnectionStrings["conn"].ConnectionString;
            con = new SqlConnection(s);
            //if (!IsPostBack)
            //{
            //    bindgrid();
            //    bindlst();
            //}
            con.Open();
        }

        //protected void Button1_Click(object sender, EventArgs e)
        //{
        //    bindgrid();
        //    GridView1.Visible = true;
            
        //}
        public void bindgrid()
        {
            cmd=new SqlCommand ("select distinct Fname,Lname from Client_registration_tbl",con);
            //ds1 = new DataSet();
            con.Open();
            dr = cmd.ExecuteReader();
            //da.Fill(ds1);
            GridView1.DataSource = dr;
            GridView1.DataBind();
            GridView1.Visible = true;
            con.Close();

        }

        public void bindlst()
        {
            da = new SqlDataAdapter("select distinct Fname,Lname from Client_registration_tbl", con);
            dt = new DataTable();
            da.Fill(dt);
            DropDownList1.DataSource = dt;
            DropDownList1.DataBind();
        }
        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {

            da = new SqlDataAdapter("select distinct Fname,Lname from Client_registration_tbl" + DropDownList1.SelectedItem.Text + "", con);
            dt = new DataTable();
            da.Fill(dt);
            GridView1.DataSource = dt;
            GridView1.DataBind();
           
            bindgrid();
            bindlst();
            //GridView1.Visible = true;
        }

      


    }
}