Range.cs

class Range : IEnumerable
{
    int _start;
    int _end;
    int _step;

    public Range(int start, int end = int.MaxValue)
    {
        _start = start;
        _end = end;
        _step = start < end ? 1 : -1;
    }

    public IEnumerator GetEnumerator()
    {
        int current = _start;
        while (_step > 0 ? current <= _end : current >= _end)
        {
            yield return current;
            current += _step;
        }
    }

}