I was confused when looked while debugging
two equal objects (they have equal fields and id)
but references to them was not equal!!!

Is it a bug or a feature?
Why Doesn't NHibernate guarantee that obj1 == obj2 if obj1.id == obj2.id.
(both are persisted).

Best regards,
Sergey

Perhpas the Equals and GetHashCode overrides aren't correctly created for the objects so NHibernate though they were different objects?

BOb

This is a fundemental issue you need to understand in .NET. The only way .NET can compare two objects is by reference, unless one or both of the objects implement an appropriate interface like IEquatable<T> (.NET 2.0).

In .NET 1.1, you can override Object.Equals() to give the desired behavior.

You might want to look in to IComparable too.

But be careful when you implement Equals(). What if two objects have the same ID but different data. Are they really equal? The answer is, "It depends."

NHibernate actually guarantees this uniqueness, but only for objects belonging to the same session.

Right, but you can load an object from the DB, putting it in the session, and then end up getting an object with the same ID but with different de-serialized from somewhere else (not in the session, right?).

Not a huge, frequent problem, but still something you have to keep in mind.

Also, my "be careful when overriding Equals()" statement was more geared toward .NET in general. Equals() is used in lots of places (like various Contains() and Hashtable lookups etc.) in the .NET Framework, so if you override Equals() you can get some tricky behavior in a few cases.

IIRC, whenever you override Equals(), it's also very strongly suggested that you override GetHashCode() and vice versa.

Yes, two equal objects must have the same HashCode and vice versa...

Not vice versa of course! Two objects with equal hash codes are not required to be equal.



Yes, of course that's possible, and you can just create a new object and set it to the same ID, the possibilities are endless :) That's why uniqueness is guaranteed only for objects attached to the same session.

what happens when I try to work with object that was loaded from different session and that session is already closed.

can i retrieve lazy attributes of that object?

of a query?

I mean th following:

can a pass as a param of a query object from OTHER session?

Long threads in the hibernate pages about equals and hashcode can be found here. As far as I can tell it translates directly to NHibernate and C#.