Multi Value Dictionary?

Question

Anyone know of a good implementation of a MultiValueDictionary? Basically, I want something that allows multiple values per key. I want to be able to do somethi

Answer 1

Yet here's my attempt using ILookup and an internal KeyedCollection. Make sure the key property is immutable.
Cross posted here.

public class Lookup : Collection, ILookup
{
  public Lookup(Func keyForItem)
    : base((IList)new Collection(keyForItem))
  {
  }

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

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

  class Collection : KeyedCollection
  {
    Func KeyForItem { get; }      
    public Collection(Func keyForItem) => KeyForItem = keyForItem;
    protected override TKey GetKeyForItem(Grouping item) => item.Key;

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

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