Mark’s Testblog

I’ve been faffing around with Castle.DynamicProxy2 a bit lately and it’s a pretty interesting bit of kit.  Castle Dynamic Proxy (CDP) allows you to dynamically generate proxies at runtime to weave aspects of behaviour into existing types.  Aspect oriented programming is typically employed for implementing crosscutting concerns such as logging, performance measuring, raising INotifyPropertyChanged and various other types of repetitive and/or orthogonal concerns.  I’m a newbie to this stuff so I won’t say much more on AOP.

While I really like CDP, I’ve found that the documentation and tutorials (the best of which is Krzysztof Koźmic‘s excellent tutorial series) aren’t particularly explicit on how CDP achieves its effects, and sometimes these details are important.

There are two main ways of creating proxies that most developers will encounter.

CreateClassProxy

This is nearly always the first demonstrated method in tutorials.  ProxyGenerator.CreateClassProxy dynamically subclasses the target class, so if you have a class named Pogo and you call ProxyGenerator.CreateClassProxy, what you’ll get back is an instance of a subclass of Pogo (i.e. the new type is-a Pogo) that weaves in the interception behaviour via overriding methods.  This is why it is a stipulation that methods / properties must be virtual when they’re intercepted.

With class based interceptors, you cannot intercept non virtual methods because unlike Java, C# does not make methods virtual by default.  If you try to intercept a non-virtual method, nothing will happen, though mechanisms do exist to allow you to identify these situations and warn the developer (the most common example of this is that NHibernate will die on its arse if you try to use lazy loading with a non-virtual member).

CreateInterfaceProxyWithTarget

The second method is ProxyGenerator.CreateInterfaceProxyWithTarget, and it is the primary reason for writing this blog post!  CreateInterfaceProxyWithTarget does not dynamically subclass target types, it simply creates a dynamically generated class, implements the same target interface and then passes through to it.  I.e. it’s an implementation of the decorator pattern.  This has two effects, one of which is very important!

1. You don’t need to mark your methods/properties as virtual
2. Since it is a proxy working via decoration rather than subclassing, for the effects of the interceptor to be applied, all calls must be made on the proxy instance.  Think about it.

The most salient point is #2.  I’ll elaborate.

A worked example: Rocky Balboa

Say you have an interface called IBoxer like this:

… and you implement it like this:

If you then turn to aspect oriented programming and decide to gather statistics on punches thrown for the duration of a boxing round, it’s a reasonable assumption that you can simply proxy the IBoxer interface and intercept only the StraightLeft/StraightRight punch calls, tally them up and report the metrics (ignore whether this is a good idea to be doing this, it’s a contrived example).  On the face of it this isn’t a horrible idea.  However, it won’t work as expected.

The key here is that OneTwo() calls through to StraightLeft() and StraightRight().  Once the proxy has delegated to the decorated type it loses the ability to intercept the calls.  We can follow the call graph easily enough.  We have a reference to the proxy via the IBoxer interface.   We call “OneTwo()” on it and when the invocation proceeds, it delegates to the decorated Rocky instance.  The Rocky instance then calls StraightLeft(), StraightRight().  Both of these calls will immediately go straight to the ‘real’ implementation, bypassing the proxy.

Just as with the normal decorator pattern, the decorator (the IBoxer dynamic proxy in this case) loses its influence when the decorated object calls methods declared in its own public interface.  In this particular situation we could write an interceptor that knows to add two punches when OneTwo() is called on the proxy, but compare this approach to one using class based proxy.  If we were using a class proxy we could rest safe in the knowledge that all calls to StraightLeft() and StraightRight() will always be intercepted, as the extra stuff we’ve bolted on resides in a method override.

The results vary depending on the type of proxy we generate and the way the types are written. In hindsight it’s pretty obvious, but it still caught me out.

It’s a fairly common occurrence — somebody wants to use NUnit’s data driven testing, but they want to vary either the action under test, or the expectation.  I.e. they’re not parametrising simple data, they’re parametrising the actions.

You cannot encode these things via normal data-driven testing (short of doing really nasty things like passing string names of methods to be invoked or using enums and a dictionary of methods) and even if you use a hackish workaround, it’s unlikely to be flexible or terse.

Test readability is paramount, so if you have some tests written in an unfamiliar style, it is very important to express the intent clearly, too.

NUnit’s data-driven testing

NUnit uses a few mechanisms to parametrise tests.  Firstly, for simple test cases, it offers the [TestCase] attribute which takes a params object[] array in its constructor.  Each argument passed to the TestCaseAttribute’s constructor is stored, ready for retrieval by the framework.  NUnit does the heavy lifting for us and casts/converts each argument to the test method’s parameter types.  Here’s an example where three ints are passed, then correctly mapped to a test method:

The main limitation here is that we can only store intrinsic types.  Strings, ints, shorts, bools etc.  We can’t new up classes or structs because .NET doesn’t allow it.  How the devil do we do something more complicated?

Passing more complicated types

It would appear we’re screwed, but fortunately, we can use the [TestCaseSource] attribute.  There are numerous options for yielding the data, and one of them is to define an IEnumerable<TestCaseData> as a public method of your test class (it works if it’s private, but since it’s accessed via reflection it’s a good idea to keep it public so that ReSharper or other tools do not flag it as unused).  You can then fill up and yield individual TestCaseData instances in the same fashion as before.  Once again, NUnit does the mapping and the heavy lifting for us.

If you do not require any of the fancy SetDescription, ExpectedException etc. stuff associated with the TestCaseData type, you can skip one piece of ceremony by simply yielding your own arbitrary type instead (i.e. change the IEnumerable<TestCaseData> to IEnumerable<MyType> and then simply yield return new MyType()).

Passing a delegate as a parameter (simple)

The simplest case is that you want to vary which methods are called.  For example, if you have multiple types implementing the same interface or multiple static methods, encoding which method to call is very simple.

Here’s an example from Stackoverflow that I answered recently where the author wanted to call one of three different static methods, each with the same signature and asserts.  The solution was to examine the method signature of the call and then use the appropriate Func<> type (Funcs and Actions are convenience delegates provided by the .NET framework).  It was then easy to parametrise the test by passing in a delegates targeting the appropriate methods.

More advanced applications

Beyond calling simple, stateless methods via delegates or passing non-intrinsic types, you can do a lot of creative and cool stuff.  For example, you could new up an instance of a type T in the test body and pass in an Action<T> to call.  The test body would create an instance of type T, then apply the action to it.  You can even go as far as expressing Act/Assert pairs via a combination of Actions and mocking frameworks.  E.g. you could say “when I call method X on the controller, I expect method Y on the model to be called”, and so forth.

The caveat is that as you do use more and more ‘creative’ types of data-driven testing, it gets less and less readable for other programmers.  Always keep checking what you’re doing and determine whether there is a better way to implement the type of testing you’re doing.  It’s easy to get carried away when applying new techniques, but it’s often the case that a more verbose but familiar pattern is a better choice.