singletonList-transient.cs

class SingletonList<T> : IEnumerable<T>, IEnumerator<T>
    {
        [ThreadStatic]
        static readonly SingletonList<T> TransientInstance = new SingletonList<T>();

        public static SingletonList<T> Create(T value) => new SingletonList<T> { Value = value };
        
        public static SingletonList<T> Transient(T value)
        {
            TransientInstance.Value = value;
            return TransientInstance; // dirty hacks to avoid allocating lists every time
        }

        T Value { get; set; }

        public IEnumerator<T> GetEnumerator() {
            Reset();
            return this; // it's it's own enumerator!
        } 

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

        public T Current => Value;

        object IEnumerator.Current => Current;

        public void Dispose() { }

        private bool m_moveNextResult = true;

        public bool MoveNext()
        {
            var result = m_moveNextResult;
            m_moveNextResult = false; // the next time someone calls us we will return false;
            return result;
        }

        public void Reset() => m_moveNextResult = true;
    }