Main

double feldElement;
            tb_Ne.SelectAll();

            if (e.Key == Key.Enter)

                if (index >= MAX)
                {
                    MessageBox.Show("Voll","Fehler");
                }
                else
                {
                        feldElement = Convert.ToDouble(tb_Ne.Text);
                        feld[index++] = feldElement;
                        tb_Ne.Clear();
                }