What would i have to do to implement a Second Level Cache that handles instances instead of the object's internal state.

I believe this won't work in general because of concurrency issues, that's why it isn't done. Immutable objects could be cached this way, but it seems that nobody wanted to do it, and I don't know how much NH code would have to be changed to implement this.

Thank you for the reply and advice Sergey, but I really need to implement this kind of cache, because I need to handle the same instance.
What I still need to know is which classes should I extend, or which interfaces need to be implemented to get the desired result.

Customizing a project like NHibernate can cost you a lot in the long term...

If you need the cache for a specific case, can you create a "fake" cache that you feed and check manually?

Analysing the framework, I understand that is's possible to implement ICache
or to use the current HashtableCache that supports an object instance as a value.
But SessionImpl uses an internal class CacheEntry. here is where i loose hope because
this class limits itself to keep the internal's object state. (object[] state)
As far as i can see, whether I try to extend the framework or use the standard
ISession implementation, the last will allways transform the instance to it's internal
state.
I would like to know if my observations on this subject are correct.
Thank you.

Am my observations correct?