regarding gridview in asp.net

check your connection string it will be like
constr = @"Data Source=RAHUL-PC;Initial Catalog=a1;Integrated Security=True";
 all code is okay.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Data.SqlClient;

using System.Data;

 

public partial class adapter : 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;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();

    }   

}

Try This..

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;


namespace adapter
{
public partial class _Default : System.Web.UI.Page
    {
        public SqlConnection con ;
        public SqlCommand cmd ;
        public SqlDataAdapter sda;
        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();
           
            sda = new SqlDataAdapter("select * from details", con);
            DataSet ds = new DataSet();
            sda.Fill(ds);
            GridView2.DataSource = ds;
            GridView2.DataBind();
            con.Close();


        }




    }

}

one Example :
http://www.aspdotnet-suresh.com/2012/05/bind-data-to-dropdownlist-in-gridview.html