January 2008 – The Simple Part

January 29, 2008

Validating attribute usage with PostSharp Aspects

This post expands on my previous article on PostSharp. If you haven’t already, read it here.

Most of January has flown by with just one post, effectively shooting down my resolution to blog more often. In my defence, I have to state that I’ve just moved on to newer pastures, with all the re-settling in that involves.

Apart from that, I also spent a few hours twiddling with the Jasema sources, so I do have something to post about at least. 🙂

The .net attribute architecture supports an Attribute Usage definition, which is itself an attribute that can be applied to attributes. AttributeUsageAttribute determines whether said attribute can be applied to fields, methods, classes, assemblies, or any combination thereof. This allows you to identify, at compilation time, whether the attributes you have specified are used correctly, which is great.

There are situations, however, where you might want to enforce some more detailed constraints on an attribute; for example, you might want to make sure that it is only used by classes that implement a given interface, or check what parameters have been defined for it.

It is possible to implement this as a runtime check within the attribute itself, however this is not always a very elegant solution. If you check the constraints every time the attribute is invoked in some way, you’re incurring a performance hit, however slight. If one considers that PostSharp aspects may be called every time data changes in a given field, or every time a method (or even any method in an assembly is called), this may add up to quite a bit, depending on what sort of checking you’re doing.

By making this sort of check at run time, you’re also depriving yourself of compile time checking. Even if you have unit tests in place to make sure you catch incorrect behaviour. it can still take time to run the suite; wouldn’t it be a lot nicer if the compiler could be instructed to raise an error when an attribute is given the wrong parameters, or is used with the wrong sort of class?

While PostSharp aspects don’t allow you to give such instructions to the compiler, they do allow you to do something that’s so similar that the difference, to someone like me, is purely academic (so, flame me). These aspects define an OnCompileValidate method, which can be overridden in your custom aspects. Continue reading “Validating attribute usage with PostSharp Aspects”

January 3, 2008

AOP using PostSharp LAOS

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. Continue reading “AOP using PostSharp LAOS”

January 3, 2008

C# 3.0 – Linq queries and Implicit Types

Finally played a bit with Linq today. About time, a lot of people will say, and rightly so. Just dipping my toes right now: wrote a short console application to list all current processes running more than 5 threads (no real use – just playing). Continue reading “C# 3.0 – Linq queries and Implicit Types”

January 2, 2008

Writing a Custom Appender for log4net

On most of my projects, I use log4net (a habit I carried over from Java really) to maintain traces and logs. I find it preferable to using the trace mechanism, and it certainly beats Console.Write as a technique. One of the best things about it is that you can configure appenders to write to pretty much anything. The appenders that come bundled with log4net cover most of the stuff you’d actually want to log to – the event logs, console, a database, file system, and so on. However, sometimes you may need something a little more specific.

In my case, I needed to have one of my projects log issues directly into the issue tracking system we use here. Said tracking system exposes a web service to do just that, so I set about writing an appender that would consume this service. I’m not going to go into the details of how to connect to this specific system, but hopefully this short post will illustrate how incredibly easy it is to write a custom appender. Continue reading “Writing a Custom Appender for log4net”