windowedMyApp/WindowedMyApp/RegWind.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

namespace WindowedMyApp
{
    /// <summary>
    /// Логика взаимодействия для RegWind.xaml
    /// </summary>
    public partial class RegWind : Window
    {
        public RegWind()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            using (ModelClass db = new ModelClass())
            {
                int HashPasswrd = surnameText.Password.GetHashCode();
                Klients klient = db.Klients
                         .FirstOrDefault(x => x.Login == nameText.Text && x.Patronymic == PatronymText.Text && x.Password == HashPasswrd);

                if (klient == null)
                {
                    Klients Newklient = new Klients() { Login = nameText.Text, Patronymic = PatronymText.Text, Password = HashPasswrd, ID_Group = 1};

                    db.Klients.Add(Newklient);

                    db.SaveChanges();

                    MessageBox.Show("Вы успешно зарегестрировались");
                    MainWindow wind = new MainWindow();
                    wind.Show();
                    Close();
            }
                else
            {
                MessageBox.Show("Такой пользователь уже существует");
            }
        }
        }

        private void toLoginButton_Click(object sender, RoutedEventArgs e)
        {
            MainWindow wind = new MainWindow();
            wind.Show();
            Close();
        }
    }
}