| ClassAttr ::= … |
|---|
| abstract |
| sealed |
Attributes that specify special semantics are abstract and sealed. These attributes can be used together.
abstract specifies that this type shall not be instantiated. If a type contains abstract methods, that type shall be declared as an abstract type.
sealed specifies that a type shall not have derived classes. All value types shall be sealed.
[Rationale: Virtual methods of sealed types are effectively instance methods, since they cannot be overridden. Framework authors should use sealed classes sparingly since they do not provide a convenient building block for user extensibility. Sealed classes can be necessary when the implementation of a set of virtual methods for a single class (typically multiple interfaces) becomes interdependent or depends critically on implementation details not visible to potential derived classes.
A type that is both abstract and sealed should have only static members, and serves as what some languages call a "namespace" or "static class". end rationale]