Original tail recursive version of the Sum method to sum integers in a list.

Sum.cs

private static int Sum(List<int> numbers, int result = 0)
{
  int size = numbers.Count();
  if (size == 0)
  {
    return result;
  }
  int last = size - 1;
  int n = numbers[last];
  numbers.RemoveAt(last);
  return Sum(numbers, n + result);
}