private void BtnTestProductIntList_Click(object sender, RoutedEventArgs e)
        {
            List <int> getallen = new List <int>()
            {
                3, 4, 5, 6, 7
            };

            lblResultaat.Content = Rekenen.Product(getallen);
        }
 private void BtnTestProductIntArray_Click(object sender, RoutedEventArgs e)
 {
     int[] getallen = { 4, 5, 7, 8, 12 };
     lblResultaat.Content = Rekenen.Product(getallen);
 }