Foundations of Programming - Karl Seguin

Chapter 9 - Back to Basics: Proxy This and Proxy That
Interception
The reason we're exploring a more technical side of inheritance is because two of the tools we've l ooked
at so far, RhinoMocks and NHibernate, make extensive use of proxies - even though you might not have
noticed. RhinoMocks uses proxies to support its core record/playback functionality. NHibernate relies on
proxies for its optional lazy-loading capabilities. We'll only look at NHibernate, since it's easier to
understand what's going on behind the covers, but the same high level pattern applies to RhinoMocks.
(A side note about NHibernate. It's considered a frictionless or transparent O/R mapper because it
doesn't require you to modify your domain classes in order to work. However, if you want to enable lazy
loading, all members must be virtual. This is still considered frictionless/transparent since you aren't
adding NHibernate specific elements to your classes - such as inheriting from an NHibernate base class
or sprinkling NHibernate attributes everywhere.)
Using NHibernate there are two distinct opportunities to leverage lazy loading. The first, and most
obvious, is when loading child collections. For example, you may not want to load all of a Model's
Upgrades until they are actually needed. Here's what your mapping file might look like:
...
By setting the lazy attribute to true on our bag element, we are telling NHibernate to lazily load the
Upgrades collection. NHibernate can easily do this since it returns its own collection types (which all
implement standard interfaces, such as IList, so to you, it's transparent).
The second, and far more interesting, usage of lazy loading is for individual domain objects. The general
idea is that sometimes you'll want whole objects to be lazily initialized. Why? Well, say that a sale has
just been made. Sales are associated with both a sales person and a car model:
Sale sale = new Sale();
sale.SalesPerson = session.Get(1);
sale.Model = session.Get(2);
sale.Price = 25000;
session.Save(sale);
Foundations of Programming Copyright © Karl Seguin www.codebetter.com
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