Create connection file and fill datatable with data in asp.net

 Create connection file and fill data table with data:-----------

 file name : DataAccess.cs
 ======================


using System;
using System.Data;
using System.Configuration;
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.Web.Configuration;

namespace DataAccessNamespace
{
    /// <summary>
    /// Summary description for DataAccess
    /// </summary>

    public class DataAccess
    {
        //Variables & public properties ***********************************
        private string connectionString = "";
        private int recordCount = -1;

        /// <summary>
        /// Property: gets count of records retrieved or changed
        /// </summary>
        public int Count
        {
            get
            {
                return recordCount;
            }
        }

        //Class constructor is executed when object is initialized ***********
        /// <summary>
        /// Connection string name in web.config file is required to initialize DataAccess
        /// </summary>
        /// <param name="ConnectionName">Name of web.config connection string</param>
        public DataAccess()
        {
            //if (WebConfigurationManager.ConnectionStrings[ConnectionName] == null)
            //{
            //    throw new Exception("Cannot find connection string named '" +
            //       ConnectionName + "' in web.config");
            //}
            //Get connection string from web.config.
            //connectionString = WebConfigurationManager.ConnectionStrings[ConnectionName].ConnectionString;
            connectionString = @"Data Source=USF142\SQLEXPRESS;Initial Catalog=demo;Integrated Security=True;";
        }
        /// <summary>
        /// Executes SELECT statement and returns results in dataTable
        /// </summary>
        /// <param name="SQL">Select SQL statement</param>
        /// <returns></returns>
        public DataTable FillDataTable(string SQL)
        {
            SqlConnection _objConn = new SqlConnection(connectionString);
            SqlDataAdapter objAdapter = new SqlDataAdapter(SQL, _objConn);
            DataTable dt = new DataTable();
            try
            {
                objAdapter.Fill(dt);
            }
            catch (SqlException ex)
            {
                throw new Exception("Error in SQL:" + SQL, ex);
            }
            catch (Exception ex)
            {
                throw ex; //Bubbling exception up to parent class
            }
            finally
            {
                _objConn.Close();
            }

            recordCount = dt.Rows.Count;
            return dt;
        }

        /// <summary>
        /// Executes "non-query" SQL statements (insert, update, delete)
        /// </summary>
        /// <param name="SQL">insert, update or delete</param>
        /// <returns>Number of records affected</returns>
        public int ExecuteNonQuery(string SQL)
        {
            SqlConnection _objConn = new SqlConnection(connectionString);
            try
            {
                _objConn.Open();
                SqlCommand objCmd = new SqlCommand(SQL, _objConn);
                recordCount = objCmd.ExecuteNonQuery();
            }
            catch (SqlException ex)
            {
                throw new Exception("Error in SQL:" + SQL, ex);
            }
            catch (Exception ex)
            {
                throw new Exception(ex.Message); //Rethrowing exception up to parent class
            }
            finally { _objConn.Close(); }

            return recordCount;
        }

        public int ExecuteScalar(String SQL)
        {
            SqlConnection _objConn = new SqlConnection(connectionString);
            int intID;
            try
            {
                _objConn.Open();
                SqlCommand objCmd = new SqlCommand(SQL, _objConn);
                intID = Convert.ToInt32(objCmd.ExecuteScalar());
            }
            catch (SqlException ex)
            {
                throw new Exception("Error in SQL:" + SQL, ex);
            }
            catch (Exception ex)
            {
                throw new Exception(ex.Message); //Rethrowing exception up to parent class
            }
            finally { _objConn.Close(); }
            return intID;
        }


    }
}

==============================
create Default2.aspx+cs file

aspx.cs file


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 DataAccessNamespace;
public partial class demo_Default2 : System.Web.UI.Page
{
   
    protected void Page_Load(object sender, EventArgs e)
    { 

     
        DataAccess myDA = new DataAccess();

        //Populate dataTable and bind to GridView Control
        string strSQL = "Select * from employee";

        GridView1.DataSource = myDA.FillDataTable(strSQL);
        GridView1.DataBind();
    }
}

.aspx file:-----------

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="demo_Default2" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:GridView ID="GridView1" runat="server" >
        </asp:GridView>
     
    </div>
    </form>
</body>
</html>