Connection String in c#.net

Connection string in App.config file.

in App.Config file write down below coding.

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <startup>
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>

<connectionStrings>
<add name="con" connectionString="Data Source=.;Initial Catalog=vdm;Integrated Security=True;user id=sa password=******"
providerName="System.Data.SqlClient" />
</connectionStrings>
</configuration>

2. Create a new class file.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Configuration;

namespace vidhyamandir
{
  public  class dbConnection
    {
      public string con = ConfigurationManager.ConnectionStrings["con"].ConnectionString;
    }
}

3.I have a login page in this I created a connection see below

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace vidhyamandir
{
    public partial class Login : Form
    {
        public Login()
        {
            InitializeComponent();
        }
        dbConnection db = new dbConnection();
        SqlConnection con;
        private void button1_Click(object sender, EventArgs e)
        {
            con = new SqlConnection(db.con);
            SqlCommand cmd = new SqlCommand("select username , password from login where username='"+textBox1.Text+"' and password='"+textBox2.Text+"'",con);
            con.Open();
            SqlDataReader dr = cmd.ExecuteReader();
         
            if (dr.HasRows)
            {
                frmmainmenu m = new frmmainmenu();
                this.Hide();
                m.Show();
             
            }
            con.Close();

        }
    }
}