FibonacciInterfaceIterator.cs

FibonacciExtended.cs

public interface IReader<T> where T : notnull
{
    IReadOnlyCollection<T> GetElements();
    T GetElementAt(int index) => GetElements().ElementAtOrDefault(index);
}

public record FibonacciReader : IReader<int>
{
    public IReadOnlyCollection<int> GetElements() => new FibonacciSequence().ToList();
}

public record FibonacciSequence : IEnumerable<int>
{
    public IEnumerator<int> GetEnumerator()
    {
        var value = (Current: 0, Next: 1);
        bool flag = true;

        while (flag)
        {
            value = (value.Next, value.Current + value.Next);
            if (value.Next < 0)
            {
                flag = false;
            }
            yield return value.Current;
        }
    }

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

IReader<int> reader = new FibonacciReader();
(reader.GetElements() as List<int>).ForEach(number => Console.WriteLine(number));