Can .NET 4 ISet<> HashSet<> replace NHibernate Iesi.Collections ISet , HashSet ? I am using Castle proxy, and NHibernate 3.0 .
No, not as of this reply.
The NHibernate engine uses the Iesi.Collections.ISet interface on internal collection classes which are used as wrappers around collections in your classes which NHibernate persists. There is no direct conversion to System.Collections.Generic.ISet.
Update: NHibernate 4 now uses HashSet from the BCL internally, and HashedSet has been removed from the Iesi.Collections dependency. The approach to use System.Collections.Generic.ISet is now available.