Quick and Dirty Multiple Value Dictionary Using Extension Methods

Though it's not a collection I tend to reach for often, there have been times when I really need a multiple value dictionary (that is, a dictionary that contains more than one value per key). In the past, I've usually reached for the excellent PowerCollections library to fill the gap. However, that requires bringing in another library and it can be a little heavy-weight for just this one collection class. There are also a ton of other implementations out there. But perhaps there's a better way to fill this need, one that doesn't require a lot of extra code. These two extension methods do most of the work of a multiple value dictionary, but don't require any extra classes or libraries:

public static void AddMulti<TKey, TCollection, TValue>(
    this IDictionary<TKey, TCollection> dictionary, TKey key, TValue value)
    where TCollection : ICollection<TValue>, new()
{
    TCollection collection;
    if (!dictionary.TryGetValue(key, out collection))
    {
        collection = new TCollection();
        dictionary.Add(key, collection);
    }
    collection.Add(value);
}

public static bool ContainsMultiValue<TKey, TCollection, TValue>(
    this IDictionary<TKey, TCollection> dictionary, TKey key, TValue value)
    where TCollection : ICollection<TValue>, new()
{
    TCollection collection;
    return dictionary.TryGetValue(key, out collection) && collection.Contains(value);
}

The extensions apply to any IDictionary<TKey, TCollection> where TCollection implements ICollection<TValue>. The idea is that you bring your own dictionary with an arbitrary collection for values and the extension manages adding a new collection when the key doesn't exist and adding a value to the collection for a given key. The nice this is that by varying the type of collection you can get different behavior. Use a List<TValue> when you want duplicate values to be allowed and a HashSet<TValue> when you don't. Here's an example:

Dictionary<string, List<int>> duplicateValuesDictionary
  = new Dictionary<string, List<int>>();
duplicateValuesDictionary.AddMulti("foo", 1);
duplicateValuesDictionary.AddMulti("foo", 1);
duplicateValuesDictionary.AddMulti("foo", 2);
duplicateValuesDictionary.ContainsMultiValue(1);  // True
duplicateValuesDictionary["foo"].Contains(1);     // True
duplicateValuesDictionary.ContainsMultiValue(3);  // False

Dictionary<string, HashSet<int>> uniqueValuesDictionary
  = new Dictionary<string, HashSet<int>>();
uniqueValuesDictionary.AddMulti("foo", 1);
uniqueValuesDictionary.AddMulti("foo", 1);  // This is throw an exception

Obviously this approach won't work in all cases. For example, you don't have a lot of control over the comparer or construction logic that the inner ICollection<TValue> uses since it gets constructed using the general new() statement. But in general, this can be a good lightweight way to add multiple value dictionary support without the overhead of an additional library.