Recently, I came across PostSharp, an excellent open source framework for .net. Among the great set of toys which it brings along is LAOS, an Aspect Oriented Programming solution which works very neatly.
Aspect oriented programming is quite a tricky subject to explain in short, so I won’t. While it’s an oversimplification, for the purposes of this post, we will just say that it is a technique where you intercept the control flow as it passes from one block to another, and attach non-default behaviour at these points.
PostSharp provides a mechanism which lets you define attributes to specify this behaviour; you can then decorate an assembly, class, method or field with these attributes. The specific methods you overload will then be triggered accordingly.
As an example, we’re going to add a logger to a method. Yes, it’s pretty boring – 99.99% of the AOP discussions on the web use this scenario as an example. However, I just want to demonstrate how to use these attributes, so no need to rock the boat yet. (If you’re feeling adventurous, visit Manageability for a couple of ideas on what else you can do with AOP).
Let’s say that we want to write a log message every time that an exception is thrown from a method. Since we’re going to use this attribute to handle something that has to do with a method, we’re going to use the
OnMethodBoundaryAspect attribute as a base. Among others, this attribute exposes an OnException method, which is just what we need:
Like all interception methods exposed by OnMethodBoundaryAspect, OnException accepts a MethodExecutionEventArgs parameter. This gives us access to a lot of useful fields, among others, the name of the method being executed before interception (and all its reflected details), any parameters that were passed to the method, and the full details of the exception. This is especially handy when trying to re-create an exception.
Note the value being set in eventArgs.FlowBehavior. FlowBehavior.Continue tells the application to digest the exception and go on. Not, I must say, the smartest thing to do in all cases; in fact, I’d be dead set against finishing such an intercept this way in most cases. It’s been set that way for the purposes of the example application, where I want and expect an exception to be thrown half-way.
One other possible value of FlowBehaviour which I would like to mention briefly is the Return value. This makes the intercept digest the exception in the same way as Continue, but also allows us to set the MethodExecutionEventArgs.ReturnValue property. In this way, we can return a default value after an exception.
To actually make our new attribute DO something, we have to apply it to a method. We have a number of ways we can do this. If we want, we can specifically apply the attribute to a method (the parameter in the attribute is a constructor parameter for the attribute – refer to the source code attached):
We can also apply an attribute to all methods in a class:
Even more neatly, we can specify an expression that identifies a selection of methods within a class. For example,
The named property tells the attribute that it should apply to all methods and fields in the decorated class where the name starts with “MyLogged”.
Yet another option is to specify the attribute at an assembly level… but if you plan to do that, you really should move on to the actual PostSharp documentation 😛
The source code extends some more methods from the OnMethodBoundaryAspect attribute. You will need to install PostSharp to build the code.