Custom Attributes

Chapter 13

Custom Attributes

Every system worth its name needs extensibility. The languages that describe an extensible system and their compilers need extensibility as well; otherwise, they are describing not the system but rather its glorious past.

A system and the associated languages can be extended in three ways. The first way is to tinker with the system itself, changing its inner structure and changing the languages accordingly. This approach is good as long as the system has a negligible number of users, because each new version of the system (and hence the languages) is basically different from the previous version. This approach is characteristic of the early stages of the life cycle of a complex system.

The second way is to leave the system and the languages as they are and build a parallel system (and parallel languages and their compilers) providing additional functionality. A classic example of this approach was the introduction of the remote procedure call (RPC) standard and the interface description language (IDL) in parallel with existing C runtime and C/C++ compilers.

The third way is to build into the system (and the languages) some formal means of extensibility and then employ these means when needed. This approach allows the system developers to sneak in new features and subsystems without changing the basic characteristics of the system. The only challenge is to devise a means of extensibility that is both efficient and universal—efficient because we need productivity, and universal because we don’t know what we’ll need tomorrow or a year from now. These requirements are contradictory, and usually universality wins out. If efficiency wins at universality’s expense, sooner or later the designers run out of options and must switch to the second way.

The Microsoft .NET platform, including the common language runtime, the .NET Framework, and the compilers, has a universal extensibility mechanism built in. This mechanism is known as custom attributes.