A while ago in the NHibernate mailing list we got a report that NHibernate is making use of a dictionary with an enum as the key, and that is causing a big performance issue.
The response was almost unanimous, I think, “what?! how can that be?!!?”. Several people went in to and tried to figure out what is going on there. The answer is totally non oblivious, Dictionary<K,V> force boxing for any value type that is used as the key.
That sound completely contradictory to what you would expect, after all, one of the major points in generics was the elimination of boxing, so what happened?
Well, the issue is that Dictionary<K,V> has to compare the keys, and for that, it must make some assumptions about the actual key. It is abstracted into EqualityComparer, and that is where the actual problem starts. EqualityComparer has some special cases for the common types (anything that is IEquatable<T>, which most of the usual suspects implements), to speed this up.
The problem is that the fall back is to an ObjectComparer, and that, of course, will box any value type.
And enum does not implements IEquatable<T>…
Omer has a good coverage on the subject, with really impressive results. Take a look at his results.
I am not going to steal his thunder, but I suggest going over and reading the code, it is very elegant.