New experimental collections - MultiDictionary

Immo Landwerth posted an interesting article here on the concept of a MultiDictionary. I know it's something I've implemented myself on many occasion and here is his contribution, available via NuGet.

An excerpt from the article at http://blogs.msdn.com/b/dotnet/archive/2014/06/20/would-you-like-a-multidictionary.aspx:

What’s your favorite data structure? Mine is the dictionary; I love the near constant time operations, the huge number of use cases, the cleanliness! Although the dictionary has a wide variety of uses, there are times when I want to add multiple values per key and Dictionary just doesn’t quite cut it. In those situations the solution is simple: just build aDictionary>!

The issue with the dictionary of list is nearly every call to the Dictionary has to be wrapped in logic to check the current state of the dictionary before adding/removing/indexing etc. I’m never satisfied with the idea of surrounding my dictionary calls with a series of if statements, so I end up coding an entirely new data structure to wrap my dictionary of lists. I’ve had to do this more times than I’m proud of, which is why I’m pleased to code up that data structure just one last time.

MultiDictionary myDictionary = new MultiDictionary();
myDictionary.Add("key", 1);
myDictionary.Add("key", 2);
myDictionary.Add("key", 3);
//myDictionary["key"] now contains the values 1, 2, and 3