Immutable Linked List

LList.cs

using System.Collections;
using System.Collections.Generic;

namespace Immutable
{
    public sealed class LList<T> : IReadOnlyCollection<T>
    {
        public T Head { get; }
        public LList<T> Tail { get; }
        public bool IsEmpty { get; }
        public int Count { get; }

        public static LList<T> Empty { get; } = new LList<T>();

        private LList()
        {
            Count = 0;
            IsEmpty = true;
        }

        private LList(T head, LList<T> tail)
        {
            Head = head;
            Tail = tail;
            Count = tail.Count + 1;
        }

        public LList<T> Add(T value) => new LList<T>(value, this);

        public IEnumerator<T> GetEnumerator()
        {
            if (!IsEmpty && Head != null)
                yield return Head;
            var rest = Tail;
            while (rest != null && !rest.IsEmpty)
            {
                yield return rest.Head;
                rest = rest.Tail;
            }
        }

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

    public static class LList
    {
        public static LList<T> Cons<T>(T value, LList<T> list) => list.Add(value);
    }
}