gistfile1.cs

public abstract class PreInsertListenerBase<TEntity> : IPreInsertEventListener 
	where TEntity : class
{
	public virtual bool OnPreInsert(PreInsertEvent @event)
	{
		var entity = @event.Entity as TEntity;
		return entity != null && OnPreInsert(entity, @event);
	}

	protected abstract bool OnPreInsert(TEntity entity, PreInsertEvent @event);

	protected static void UpdateState(PreInsertEvent @event, 
					Expression<Func<TEntity,object>> propertyExpression)
	{
		var entity = (TEntity)@event.Entity ;
		ChangeState(@event, propertyExpression, propertyExpression.Compile()(entity));
	}

	protected static void ChangeState(PreInsertEvent @event, 
					Expression<Func<TEntity,object>> propertyExpression, 
					object value)
	{
		var propertyName = TypeUtils.DecodeMemberAccessExpression(propertyExpression).Name;
		var index = Array.IndexOf(@event.Persister.PropertyNames, propertyName);
		if (index == -1) return;
		@event.State[index] = value;
	}
}



//Example usage


public class PreInsertFoo : PreInsertListenerBase<Foo>
{
	
	protected override bool OnPreInsert(Foo entity, PreInsertEvent @event)
	{
		//update the live object in memory
		entity.SomeData = entity.SomeData.Replace("%js", "'" + @event.Id + "'");
			
		//update the state to persist
		UpdateState(@event, j => j.SomeData);
		
		return false;
	}
}

//another example

public class PreInsertOfAuditable : PreInsertListenerBase<IAuditable>
{
	
	protected override bool OnPreInsert(IAuditable entity, PreInsertEvent @event)
	{
		//update the live object in memory
		entity.InsertDate = DateTime.Today();
			
		//update the state to persist
		UpdateState(@event, j => j.InsertDate);
		
		return false;
	}
}