ILookup<TKey, TElement>
Defines an indexer, size property, and Boolean search method for
data structures that map keys to IEnumerable<T>
sequences of values.
using System.Collections;
using System.Collections.Generic;
namespace Newtonsoft.Json.Utilities.LinqBridge
{
internal interface ILookup<TKey, TElement> : IEnumerable<IGrouping<TKey, TElement>>, IEnumerable
{
int Count { get; }
IEnumerable<TElement> this[TKey key] { get; }
bool Contains(TKey key);
}
}