C#

<?xml version="1.0" encoding="utf-8" ?> <configuration>
    <configSections>
    </configSections>
    <connectionStrings>
        <add name="connectionStringTestDb"
            connectionString="Data Source=ATLELAG786576\SQLEXPRESS;Initial Catalog=D:\Microsoft SQL Server\MSSQL10.SQLEXPRESS\MSSQL\DATA\TEST.mdf;Integrated Security=True;Connect Timeout=30;User Instance=False"
            providerName="System.Data.SqlClient" />
    </connectionStrings> </configuration>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
namespace SQLServerConnectionDemo
{
    class dbConnection
    {
        public static SqlConnection newCon;
        public static string connectionStringTestDb = ConfigurationManager.ConnectionStrings["connectionStringTestDb"].ConnectionString;
        public static SqlConnection GetConnection()
        {
            newCon = new SqlConnection(connectionStringTestDb);
            return newCon;
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.Data.SqlClient;
namespace SQLServerConnectionDemo
{
    class dbAccess
    {
        SqlConnection conn;
        public dbAccess()
        {
            conn = dbConnection.GetConnection();
        }
        //Method insert new in tblEmployees
        public void addEmployee(string Id, string Name, string Email)
        {
            if (conn.State.ToString() == "Closed")
            {
                conn.Open();
            }
            SqlCommand newCmd = conn.CreateCommand();
            newCmd.Connection = conn;
            newCmd.CommandType = CommandType.Text;
            newCmd.CommandText = "INSERT INTO tblEmployees VALUES ('"+ Id +"','"+ Name +"','"+ Email +"')";
            newCmd.ExecuteNonQuery();
        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace SQLServerConnectionDemo
{
    public partial class formEmployeeAdd : Form
    {
        dbAccess access = new dbAccess();
        public formEmployeeAdd()
        {
            InitializeComponent();
        }
        private void btnInsert_Click(object sender, EventArgs e)
        {
            access.addEmployee(txtId.Text, txtName.Text, txtEmail.Text);
            MessageBox.Show("Data successfully added");
        }
    }
}