IEnumerable<T> memoization (based on http://www.kodefuguru.com/post/2012/08/13/Caching-LINQ-Results.aspx)

Memoize.cs

    public static class Sequence
    {
        public static IEnumerable<T> Memoize<T>(this IEnumerable<T> sequence)
        {
            return new MemoizedSequence<T>(sequence);
        }
    }

    // This code is for illustration and is not production-ready.

    internal class MemoizedSequence<T> : IEnumerable<T>
    {
        private readonly IEnumerable<T> _source;
        private readonly List<T> _cache;
        private int _lastCachedIndex;
        private IEnumerator<T> _inner;
        private bool _disposed;

        public MemoizedSequence(IEnumerable<T> source)
        {
            _source = source;
            _cache = new List<T>();
            _lastCachedIndex = -1;
        }

        #region Implementation of IEnumerable

        public IEnumerator<T> GetEnumerator()
        {
            if (_inner == null)
                _inner = _source.GetEnumerator();

            for (int i = 0; i <= _lastCachedIndex; i++)
            {
                yield return _cache[i];
            }

            while (!_disposed && _inner.MoveNext())
            {
                _cache.Add(_inner.Current);
                _lastCachedIndex++;
                yield return _inner.Current;
            }
            _inner.Dispose();
            _disposed = true;
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        #endregion
    }