The IInterceptor interface provides callbacks from the session to the application allowing the application to inspect and/or manipulate properties of a persistent object before it is saved, updated, deleted or loaded. One possible use for this is to track auditing information. For example, the following IInterceptor automatically sets the createTimestamp when an IAuditable is created and updates the lastUpdateTimestamp property when an IAuditable is updated.

You may either implement IInterceptor directly or (better) extend EmptyInterceptor.

using System;
	
using NHibernate;
using NHibernate.Type;

public class AuditInterceptor : EmptyInterceptor {

    private int updates;
    private int creates;
    private int loads;

    public override void OnDelete(object entity,
                                  object id,
                                  object[] state,
                                  string[] propertyNames,
                                  IType[] types)
    {
        // do nothing
    }

    public override bool OnFlushDirty(object entity, 
                                      object id, 
                                      object[] currentState,
                                      object[] previousState,
                                      string[] propertyNames,
                                      IType[] types)
    {
        if ( entity is IAuditable ) {
            updates++;
            for ( int i=0; i < propertyNames.Length; i++ ) {
                if ( "lastUpdateTimestamp".Equals( propertyNames[i] ) ) {
                    currentState[i] = new DateTime();
                    return true;
                }
            }
        }
        return false;
    }

    public override bool OnLoad(object entity, 
                                object id, 
                                object[] state,
                                string[] propertyNames,
                                IType[] types)
    {
        if ( entity is IAuditable ) {
            loads++;
        }
        return false;
    }

    public override bool OnSave(object entity, 
                                object id, 
                                object[] state,
                                string[] propertyNames,
                                IType[] types)
    {
        if ( entity is IAuditable ) {
            creates++;
            for ( int i=0; i<propertyNames.Length; i++ ) {
                if ( "createTimestamp".Equals( propertyNames[i] ) ) {
                    state[i] = new DateTime();
                    return true;
                }
            }
        }
        return false;
    }

    public override void AfterTransactionCompletion(ITransaction tx)
    {
        if ( tx.WasCommitted ) {
            System.Console.WriteLine(
                "Creations: " + creates +
                ", Updates: " + updates +
                ", Loads: " + loads);
        }
        updates=0;
        creates=0;
        loads=0;
    }

}

Interceptors come in two flavors: ISession-scoped and ISessionFactory-scoped.

An ISession-scoped interceptor is specified when a session is opened using one of the overloaded ISessionFactory.OpenSession() methods accepting an IInterceptor.

ISession session = sf.OpenSession( new AuditInterceptor() );

An ISessionFactory-scoped interceptor is registered with the Configuration object prior to building the ISessionFactory. In this case, the supplied interceptor will be applied to all sessions opened from that ISessionFactory; this is true unless a session is opened explicitly specifying the interceptor to use. ISessionFactory-scoped interceptors must be thread safe, taking care to not store session-specific state since multiple sessions will use this interceptor (potentially) concurrently.

new Configuration().SetInterceptor( new AuditInterceptor() );

If you have to react to particular events in your persistence layer, you may also use the NHibernate2 event architecture. The event system can be used in addition or as a replacement for interceptors.

Essentially all of the methods of the ISession interface correlate to an event. You have a LoadEvent, a FlushEvent, etc (consult the XML configuration-file XSD or the NHibernate.Event namespace for the full list of defined event types). When a request is made of one of these methods, the ISession generates an appropriate event and passes it to the configured event listeners for that type. Out-of-the-box, these listeners implement the same processing in which those methods always resulted. However, you are free to implement a customization of one of the listener interfaces (i.e., the LoadEvent is processed by the registered implementation of the ILoadEventListener interface), in which case their implementation would be responsible for processing any Load() requests made of the ISession.

The listeners should be considered effectively singletons; meaning, they are shared between requests, and thus should not save any state as instance variables.

A custom listener should implement the appropriate interface for the event it wants to process and/or extend one of the convenience base classes (or even the default event listeners used by NHibernate out-of-the-box as their methods are declared virtual for this purpose). Custom listeners can either be registered programmatically through the Configuration object, or specified in the NHibernate configuration XML. Here's an example of a custom load event listener:

public class MyLoadListener : ILoadEventListener 
{
    // this is the single method defined by the LoadEventListener interface
    public void OnLoad(LoadEvent theEvent, LoadType loadType)
    {
        if ( !MySecurity.IsAuthorized( theEvent.EntityClassName, theEvent.EntityId ) ) {
            throw new MySecurityException("Unauthorized access");
        }
    }
}

You also need a configuration entry telling NHibernate to use the listener in addition to the default listener:

<hibernate-configuration>
    <session-factory>
        ...
        <event type="load">
            <listener class="MyLoadListener"/>
            <listener class="NHibernate.Event.Default.DefaultLoadEventListener"/>
        </event>
    </session-factory>
</hibernate-configuration>

Instead, you may register it programmatically:

Configuration cfg = new Configuration();
ILoadEventListener[] stack =
    new ILoadEventListener[] { new MyLoadListener(), new DefaultLoadEventListener() };
cfg.EventListeners.LoadEventListeners = stack;

Listeners registered declaratively cannot share instances. If the same class name is used in multiple <listener/> elements, each reference will result in a separate instance of that class. If you need the capability to share listener instances between listener types you must use the programmatic registration approach.

Why implement an interface and define the specific type during configuration? Well, a listener implementation could implement multiple event listener interfaces. Having the type additionally defined during registration makes it easier to turn custom listeners on or off during configuration.