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4. How to Choose an AOP Framework?

4.1 DynamicProxy or IL Level Weaving

Generally, the AOP frameworks mentioned in last chapter can be divided into two categories: DynamicProxy and IL Level Weaving.

4.1.1 DynamicProxy

An AOP framework based on DynamicProxy, like Aspect#, dynamically (at runtime) constructs and builds sub classes of classes wanted to be intercepted with advices according to the configurations. Because it depends on inheritance, there come some implicit constraints: classes wanted to be intercepted must be un-sealed (inheritable); methods or properties of classes wanted to be intercepted must be virtual (overridable). In fact, an AOP framework of this kind is an extension of the DynamicProxy Pattern, including a set of configuration language and helper and factory classes for creating proxy objects or doing advising and introducing. For more about DynamicProxy Pattern, see Castle Project/DynamicProxy.

4.1.2 IL Level Weaving

Different from DynamicProxy, IL Level Weaving has less limitation. In essence, an AOP framework of this kind directly accesses/modifies the compiled IL code format assemblies, so in theory, any kinds of modifications and controlling to existing assemblies (unobfuscated) are possible. So through it, it becomes easy to do any AOP operations to existing assemblies, such as advising and introducing. But mostly this kind of processing can be done only at static time or build time. (Another so called load-time weaving is only the composition use of  System.Assembly.Load and static-time or build-time processing.) And of course, specific IL Level Weaving framework must have its specific additional limitations.

4.2 Different Configuration Methods

Each AOP framework must have its specific configuration language. Although the format of a configuration language may mot affect much of the performance and implementing essence of the framework, it is the user interface of the framework and it represents how easy it is for users to use the AOP framework. There are four most common configuration methods:

• Custom script like configuration language, e.g. Aspect#.

• Xml based configuration, e.g. AspectDNG.
• Custom Attribute Classes based Configuration, e.g. PapierLoom.Net (Dynamic Part of Loom.Net)
• AspectJ like specific programming language extension with extended compiler, e.g. Eos.

4.3 Suggestion

How to choose the most suitable AOP framework for your project depends on the facts of your project.

If your project is a completely new one from design, which means your project doesn't depend on lots of existing codes, then all mentioned frameworks are suitable. So you should just choose the most familiar and the most stable one. And I suggest Aspect# and AspectDNG.

If your new project based on lots of existing code that expensive to change or based on existing assemblies without source code, AOP frameworks based on IL level weaving become the only choice. And I suggest AspectDNG.

Briefly, If you are not sure which one to choose, choose an IL level weaving based framework will not be too worse. And I suggest AspectDNG.