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    <p>You've only mentioned basic CRUD actions. Doing these directly does mean you have to be aware of transactions, flushing and other things that a repository can wrap up, but I guess the value of repositories becomes more apparent when you think about complex retrieval queries.</p> <p>Imagine then that you do decide to use the NHibernate session directly in your application layer.</p> <p>You will need to do the equivalent of WHERE clauses and ORDER BYs etc, using either HQL or NHibernate criteria. This means your code has to reference NHibernate, and contains ideas specific to NHibernate. This makes your application hard to test and harder for others unfamiliar with NH to follow. A call to repository.GetCompletedOrders is much more descriptive and reusable than one that includes something like "where IsComplete = true and IsDeleted = false..." etc.</p> <p>You could use Linq to NHibernate instead, but now you have the situation where you can easily forget that you're working on an IQueryable. You could end up chaining Linq expressions which generate enormous queries when they execute, without realising it (I speak from experience)! Mike Hadlow sparked a conversation on essentially this topic in his post <a href="http://mikehadlow.blogspot.com/2009/01/should-my-repository-expose-iqueryable.html" rel="nofollow">Should my repository expose IQueryable</a>.</p> <p>N.b. If you don't like having lots of methods on custom repositories for different queries (like GetCompletedOrders), you can use specification parameters (like Get(specification)), which allow you to specify filters, orderings etc. without using data access language.</p> <p>Going back to the list of benefits of repository that you gave:</p> <ul> <li>Persistence</li> <li>OO View of the data</li> <li>Data Access Logic Abstraction</li> <li>Query Access Logic</li> </ul> <p>You can see that points 3 and 4 are not provided for by using the persistence framework classes directly, especially in real world retrieval scenarios.</p>
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