private void login_Click(object sender, RoutedEventArgs e)
{
User logUser = null;
using (BookLibContext db = new BookLibContext())
{
logUser = db.Users.Where(b => b.Login == loginTextBox.Text && b.Password == passwordTextBox.Text).FirstOrDefault();
}
if (logUser != null)
{
MessageBox.Show("Login was successful");
}
//перехід на головну сторінку
}