Custom Attributes

The previous two lessons covered what attributes are and walked through the built-in catalog. The real power shows up when you define your own. A custom attribute lets you tag a class, property, method, or parameter with declarative metadata that some other piece of code (validation, serialization, routing, authorization, test discovery) reads at runtime and acts on. This lesson covers how to define a custom attribute, what data it can carry, where it can be applied, and how to read it back with just enough reflection to make the pattern useful.