Why is the OnLoad method of the IInterceptor only called as a consequence of ISession.Load(object id), and not as a consequence of ISession.CreateCriteria(typeof(T))?

I want to change the state of each object loaded from database so that the object graph always points to objects that are already in memory, rather than introducing duplicates. The OnLoad method of the IInterceptor:

public bool OnLoad(object entity, object id, object[] state, string[] propertyNames, IType[] types)

provides the state of each loaded object as an object[]. Unfortunately, OnLoad is not called when objects are fetched via ISession.CreateCriteria(typeof(T)).

NHibernate version: 2.0

You should be able to provide references to objects already in memory by implementing IInterceptor.Instantiate. We did exactly that -- keep a cache of WeakReferences, periodically pruning dead references, to avoid endless hassles with NonUniqueObjectException when we try to reattach entities to new sessions and somehow something else has already put a different instance of the same type & id in the session.

Our initial testing with this approach came out clean, it eliminated the NonUniqueObjectException problem, but we never enabled this logic for production use. We were concerned that some field values might get leftover if we recycle instances this way -- if either NHibernate doesn't set every field on every load, and because we have some non-persisted fields in our entities that might not get cleared.

A better approach would be if the session's 1st level cache implementation could be replaced. This would would not only seem cleaner, but (I think) it would handle the case where a proxy is created. IInterceptor.Instantiate() doesn't get called until a proxy is initialized, so it can't prevent a different proxy instance from getting created.

I did some testing and I have the impression that IInterceptor.Instantiate is never called, although a massive amount of objects are fetched from database and instantiated. The IInterceptor documentation is very brief on explaining the exact semantics of its methods. In which situation should Instantiate method get called upon?

IInterceptor.Instantiate is probably not called because the entities returned by the fetches are uninitialized proxies.