private void Btn_Click(object sender, RoutedEventArgs e)
        {
            win_rentals.rentals_window rw = new win_rentals.rentals_window();

            this.Close();
            rw.ShowDialog();
        }
        private void Btn_login_Click(object sender, RoutedEventArgs e)
        {
            connection.openConnection();
            connection.sql             = "SELECT COUNT(1) FROM UserTb WHERE login=@login AND password=@password";
            connection.cmd.CommandType = CommandType.Text;
            connection.cmd.CommandText = connection.sql;
            connection.cmd.Parameters.AddWithValue("@login", txt_login.Text);
            connection.cmd.Parameters.AddWithValue("@password", txt_password.Password);

            int count = Convert.ToInt32(connection.cmd.ExecuteScalar());

            if (count == 1)
            {
                win_rentals.rentals_window rw = new win_rentals.rentals_window();

                this.Close();
                rw.ShowDialog();
            }
            else
            {
                MessageBox.Show("Podany login lub hasło jest błędne. Spróbuj ponownie", "Komunikat.");
            }
            connection.closeConnection();
        }