MultiValueDictionary<TKey, TValue>

MultiValueDictionary.cs

namespace System.Collections.ObjectModel
{
  using System.Collections.Generic;
  using System.Linq;

  public class MultiValueDictionary<TKey, TElement>
  : Collection<TElement>, ILookup<TKey, TElement>
  {
    public MultiValueDictionary(Func<TElement, TKey> keyExtractor)
      : base(new Collection(keyExtractor))
    {
    }

    new Collection Items => (Collection)base.Items;

    public IEnumerable<TElement> this[TKey key] => Items[key];
    public bool Contains(TKey key) => Items.Contains(key);
    IEnumerator<IGrouping<TKey, TElement>>
      IEnumerable<IGrouping<TKey, TElement>>.GetEnumerator() => Items.GetEnumerator();

    class Collection
    : KeyedCollection<TKey, Grouping>, IList<TElement>
    {
      Func<TElement, TKey> KeyExtractor { get; }

      public Collection(Func<TElement, TKey> keyExtractor) => KeyExtractor = keyExtractor;
      protected override TKey GetKeyForItem(Grouping item) => item.Key;

      public void Add(TElement item)
      {
        var key = KeyExtractor(item);
        if (Dictionary != null && Dictionary.TryGetValue(key, out var collection))
          collection.Add(item);
        else
          Add(new Grouping(key) { item });        
      }

      public bool Remove(TElement item)
      {
        var key = KeyExtractor(item);
        if (Dictionary != null && Dictionary.TryGetValue(key, out var collection)
          && collection.Remove(item))
        {
          if (collection.Count == 0)
            Remove(key);
          return true;
        }
        return false;
      }

      IEnumerator<TElement> IEnumerable<TElement>.GetEnumerator()
      {
        foreach (var group in base.Items)
          foreach (var item in group)
            yield return item;
      }

      Exception IndexError() => new NotSupportedException("Indexing not supported.");
      public int IndexOf(TElement item) => throw IndexError();
      public void Insert(int index, TElement item) => Add(item);
      public bool Contains(TElement item) => Items.Contains(item);
      public void CopyTo(TElement[] array, int arrayIndex) => throw IndexError();
      new IEnumerable<TElement> Items => this;
      public bool IsReadOnly => false;
      TElement IList<TElement>.this[int index]
      {
        get => throw IndexError();
        set => throw IndexError();
      }
    }

    class Grouping : Collection<TElement>, IGrouping<TKey, TElement>
    {
      public Grouping(TKey key) => Key = key;
      public TKey Key { get; }
    }
  }
}