A Discovery in Linq: Lookup

I've been using Linq to Objects for quite some time - so it was quite a surprise to me when I discovered something new recently - the LookUp class.

LookUp is much like the more familiar Dictionary class, but with a few differences that make it easier to use with Linq.

• Doesn't require unique keys - there can be multiple instances with the same key. When you lookup a key, the result is a sequence of values;
• If you index with a key that isn't there, you get a sequence with zero elements, not an exception.
• Immutability - an instance of LookUp is created fully formed, with all instances, by the Enumerable.ToLookup() factory extension method.

It seems to me that LookUp addresses the major deficiencies of the Dictionary class, giving us a better tool to use. I know I'll be making frequent use of LookUp in my own code.