NHibernate: how to handle entity-based validation using session-per-request pattern, without controllers knowing about ISession?

Option 1 without a doubt. It's not counter intuitive, it's how NH works. Objects retrieved using NH are persistent and changes will be saved when the session is flushed.

Calling Evict makes the object transient which is exactly the behavior you want.

How about a validation service with an IsValid (or something similar) method which validates the object passed to it, if it fails it could publish a ValidationFailed event. Then when your request finishes instead of calling the session's flush you could publish a RequestEnd event. You could then have a handler that listens for both RequestEnd events and ValidationFailed events - if there is a ValidationFailed event then don't flush the session but if not then flush it.

Having said that I just do Option 2!

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