regarding gridview in asp.net

Question

hi i got error in coding of grid view in asp.net

here is my coding

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;

using System.Data.Sql;

using System.Data;

namespace adapter

{

    public partial class _Default : System.Web.UI.Page

    {

        public SqlConnection con = new SqlConnection();

        public SqlCommand cmd = new SqlCommand();

        public SqlDataAdapter sda = new SqlDataAdapter();

        public SqlDataReader sdr;

        public DataTable myDataTable = new DataTable();

        public String constr, cmdstr;

        protected void Page_Load(object sender, EventArgs e)

        {

            constr = @"Data Source=RAHUL-PC\SQLEXPRESS;Initial Catalog=a1;Integrated Security=True";

            con = new SqlConnection(constr); 

            

        }

        protected void Button1_Click(object sender, EventArgs e)

        {

            con.Open();

                cmd=new SqlCommand("select * from details",con);

            sda=new SqlDataAdapter(cmd);

            sda.Fill(myDataTable);

            GridView2.DataSource = myDataTable;

            GridView2.DataBind();

            con.Close();

        }

        

    }

}