enumerable-range.cs

public readonly struct EnumerableRange : IEnumerable<int>
{
    readonly Range _range;

    public EnumerableRange(Range range)
    {
        if (range.Start.IsFromEnd || range.End.IsFromEnd)
        {
            throw new ArgumentException(nameof(range));
        }
        _range = range;
    }

    public static implicit operator EnumerableRange(Range range) => new EnumerableRange(range);

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    IEnumerator<int> IEnumerable<int>.GetEnumerator() => GetEnumerator();

    // Important to return RangeEnumerator, not IEnumerable<int> here, or foreach would box the struct
    public RangeEnumerator GetEnumerator() => new RangeEnumerator(_range);

    public override string ToString() => _range.ToString();

    public struct RangeEnumerator : IEnumerator<int>, IEnumerator
    {
        public int Current => _range.Start.Value + (_isAscending ? _index : -_index);
        object IEnumerator.Current => Current;

        int _index;
        readonly Range _range;
        readonly bool _isAscending;

        internal RangeEnumerator(Range range)
        {
            _index = -1;
            _range = range;
            _isAscending = _range.Start.Value < _range.End.Value;
        }

        public bool MoveNext()
        {
            if (_index + 1 >= Math.Abs(_range.End.Value - _range.Start.Value))
            {
                return false;
            }
            _index++;
            return true;
        }

        public void Reset() => _index = 0;

        public void Dispose() { }
    }
}