Avoid Unsealed Attributes

Reports custom Attribute classes declared without the sealed modifier. The CLR’s attribute-reflection paths are significantly faster when the attribute type is sealed because they can skip the inheritance walk.

The .NET design guidelines explicitly recommend sealing attribute classes. Every attribute in the base class library is sealed (SerializableAttribute, ObsoleteAttribute, FlagsAttribute, …​). Leaving an attribute open for inheritance pays a reflection cost on every lookup and almost never reflects an intentional extensibility point.

Add the sealed modifier to the class header. If the attribute is genuinely intended as a base for further attributes, mark it abstract instead and document the extensibility point.