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;
using System.Data.SqlClient;
using System.Configuration;

namespace WindowsFormsApplication6
{
    public partial class authorization : Form
    {
        public authorization()
        {
            InitializeComponent();
        }

        private void button_exit_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void button_login_Click(object sender, EventArgs e)
        {
            var txtUser = textBox_login;
            var txtPasswd = textBox_password;
            var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
            var connectionStringsSection = (ConnectionStringsSection)config.GetSection("connectionStrings");

            connectionStringsSection.ConnectionStrings["WindowsFormsApplication6.Properties.Settings.РесторанConnectionString"].ConnectionString = @"Data Source=USER-ПК;Initial Catalog=Ресторан;Integrated Security=True; Persist Security Info=True;" +
                            "user ID=" + txtUser + ";" +
                            " Password=" + txtPasswd + ";";
            config.Save();


ConfigurationManager.RefreshSection("connectionStrings");
string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["WindowsFormsApplication6.Properties.Settings.РесторанConnectionString"].ConnectionString;
            using (var connection = new SqlConnection(connectionString))
            {
                try
                {
                    connection.Open();
                    MessageBox.Show(@"Подключение к базе данных " + connection.Database);
                    MessageBox.Show(@"Подключение успешно");

                    Hide();
                    var ss = new mainmenu();
                    ss.Show();
                }
                catch (Exception exception)
                {
                    MessageBox.Show(exception.Message);
                }
            }
        }

        private void authorization_Load(object sender, EventArgs e)
        {

        }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
    </configSections>
    <connectionStrings>
        <add name="WindowsFormsApplication6.Properties.Settings.РесторанConnectionString"
            connectionString="Data Source=USER-ПК;Initial Catalog=Ресторан;Integrated Security=True"
            providerName="System.Data.SqlClient" />
    </connectionStrings>
</configuration>