Example of duck typing with Ranges from Nick Chapsas

FunWithRangeDuckTyping.cs

// Full credit to Nick Chapsas - See: https://youtu.be/jmmz1cInNow

foreach (var i in 0..5)
{
    Console.WriteLine(i);
}

public static class Extensions
{
    public static CustomIntEnumerator GetEnumerator(this Range range) => new CustomIntEnumerator(range);

    public static CustomIntEnumerator GetEnumerator(this int i) => new CustomIntEnumerator(new Range(0, i));
}

public ref struct CustomIntEnumerator
{
    private int _current;
    private readonly int _end;

    public CustomIntEnumerator(Range range)
    {
        if (range.End.IsFromEnd)
            throw new NotSupportedException();

        _current = range.Start.Value - 1;
        _end = range.End.Value;
    }

    public int Current => _current;

    public bool MoveNext() => ++_current <= _end;
}