gistfile1.txt

readonly struct Foo
{
    private readonly int[] _arr;
    public Foo(int length) => _arr = new int[length];
    public int Length => _arr?.Length ?? 0;

    public ref int this[int index] => ref _arr[index];
    public FooEnumerator GetEnumerator() => new FooEnumerator(_arr);

    public struct FooEnumerator
    {
        private readonly int[] _arr;
        private readonly int _length;
        private int _index;
        public FooEnumerator(int[] arr)
        {
            _arr = arr;
            _length = arr.Length;
            _index = -1;
        }
        public bool MoveNext() => ++_index < _length;
        public ref int Current => ref _arr[_index];
    }

    static void Main()
    {
        var obj = new Foo(8);
        for (int i = 0; i < obj.Length; i++)
            obj[i] = i;

        foreach (ref int value in obj)
        {
            System.Console.WriteLine(value);
            value *= 2;
        }
        System.Console.WriteLine("====");
        foreach (int value in obj)
            System.Console.WriteLine(value);
    }
}