Stashbox by Peter Csajtai

IImmutableArray<TKey, TValue>

interface IImmutableArray<TKey, TValue> : IEnumerable<TValue>, IEnumerable

IImmutableArray<TKey, TValue> Add(TKey key, TValue value)

IImmutableArray<TKey, TValue> AddOrUpdate(TKey key, TValue value, bool byRef, Func<TValue, TValue, TValue> update = null)

TValue GetOrDefault(TKey key, bool byRef)