private void button_Click(object sender, RoutedEventArgs e) { Regex x = new Regex(@"^[\w.]+@\w+[.]\w+$"); if (login.Text == "login" || password.Password == "password" || password.Password != repeat_password.Password || datepicker.SelectedDate == null || email.Text == "*****@*****.**" || !x.IsMatch(email.Text)) { MessageBox.Show("Something wrong!", "Error!"); return; } Evennote.Registration(login.Text, password.Password, datepicker.SelectedDate.Value, email.Text, image); (Application.Current.MainWindow as MainWindow).ChangePage("pages/login_page.xaml"); }