ILookup<TKey, TElement> interface ILookup<TKey, TElement> : IEnumerable<IGrouping<TKey, TElement>>, IEnumerable Defines an indexer, size property, and Boolean search method for data structures that map keys to IEnumerable<T> sequences of values. Documentation Code int Count { get; } IEnumerable<TElement> this[TKey key] { get; } bool Contains(TKey key)