Where Is System.Collections.Collections.Generic.ISet?

.NET Framework 3.5 is released! Among the new enhancements? System.Collections.Generic.HashedSet (Reference: BCL Team Blog). Finally, proper Set semantics in the .NET language. Those of you who are familiar with NHibernate and the Iesi collections should be plenty familiar with Iesi.Collections.HashedSet and the other set based collections. Does this mean that NHibernate could finally be implemented with just the framework's collection structures?

It turns out no! Microsoft has chosen to not add an interface which matches this collections set based operations. There is no matching ISet interface which the new HashedSet implements. While some people will say "yeah but it still uses IEnumerable", I think this is a huge shortcoming of the .NET 3.5 framework. There is enough semantically different regarding a set than the other collections that it warranted its own interface to control how someone would interact with it.

The only thought I could come up with regarding why this obvious interface was left out is that if it is just an interface you can not guarantee that the collection behaves like a set. For example with a set you can not add the same item twice. If a custom collection were to implement said ISet and decided to let the Add method add the same item multiple times, what would stop them?

I suppose a set isn't something you can just define by an interface. Maybe I should let my emotions cool down prior to jumping over Microsoft for leaving out this interface. I guess I was just imagining the possibility of using NHibernate without having to add Iesi.Collections to my project. Then again, what would make me think that this would change, or change in a relevant time frame?

All that being said, I still wish they included the interface! After all there is an IDictionary which could theoretically be implemented in such a way that the custom collection could add the same key multiple times. If the Dictionary semantics can't be guaranteed by the already existing interface, then I say it would have been ok for such an interface to exist for the set.

--John Chapman