ReadOnlyList

public sealed class ReadOnlyList<T>
{
    private readonly T[] _backend;

    private readonly int _length;

    public ReadOnlyList([NotNull] T[] backend)
        :
        this(backend, backend.Length)
    {
    }

    public ReadOnlyList([NotNull] T[] backend, int length)
    {
        ThrowIf.Argument.IsNull(backend, nameof(backend));
        ThrowIf.Argument.OutOfRange(length, 0, backend.Length, nameof(length));

        _backend = backend;

        _length = length;
    }

    public ReadOnlyList([NotNull] IEnumerable<T> source)
    {
        ThrowIf.Argument.IsNull(source, nameof(source));

        _backend = source.ToArray();

        _length = _backend.Length;
    }

    [Pure]
    public ArrayEnumerator<T> GetEnumerator()
    {
        return new ArrayEnumerator<T>(_backend, _length);
    }

    [Pure]
    public ReadOnlySpan<T> Access()
    {
        return _backend.AsSpan(0, _length);
    }
}