<PackageReference Include="Newtonsoft.Json" Version="12.0.1" />

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.
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); } }