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

Microsoft just added an official prelease version of exactly what you're looking for (called a MultiDictionary) available through NuGet here: https://www.nuget.org/packages/Microsoft.Experimental.Collections/

Info on usage and more details can be found through the official MSDN blog post here: http://blogs.msdn.com/b/dotnet/archive/2014/06/20/would-you-like-a-multidictionary.aspx

I'm the developer for this package, so let me know either here or on MSDN if you have any questions about performance or anything.

Hope that helps.

Answer 2

Alternative to custom type can be a generic extension that adds key and value when not found:

public static V getValue<K, V>(this IDictionary<K, V> d, K key) where V : new() {
    V v; if (!d.TryGetValue(key, out v)) { v = new V(); d.Add(key, v); } return v; } 

Sample use:

var d = new Dictionary<int, LinkedList<int>>();
d.getValue(1).AddLast(2);

Answer 3

You can easily make one from a dictionary of lists:

public class MultiValueDictionary<Key, Value> : Dictionary<Key, List<Value>> {

  public void Add(Key key, Value value) {
    List<Value> values;
    if (!this.TryGetValue(key, out values)) {
      values = new List<Value>();
      this.Add(key, values);
    }
    values.Add(value);
  }

}

Answer 4

You can always use a Tuple for your second generic parameter:

var dict = new Dictionary<string,Tuple<string,int,object>>();
dict.Add("key", new Tuple<string,int,object>("string1", 4, new Object()));

Or even , a generic List as a second generic parameter:

var dict = new Dictionary<string,List<myType>>();

That will allow you to bind multiple values to a single key.

For ease of use, you can create an extension method that will check for existence of a key and addition to the list.

Answer 5

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

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

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

Answer 6

You could use MultiDictionary class from PowerCollections.

It returns ICollection{TValue} for the key asked.