Sealed Classes Using inheritance is not always easy and requires forethought. If you create an interface or an abstract class, you are knowingly writing something that will be inherited from in the future. | Sealed Classes Using inheritance is not always easy and requires forethought. If you create an interface or an abstract class you are knowingly writing something that will be inherited from in the future. The trouble is that predicting the future is a difficult business. It takes skill effort and knowledge of the problem you are trying to solve to craft a flexible easy-to-use hierarchy of interfaces abstract classes and classes. To put it another way unless you consciously design a class with the intention of using it as a base class it s extremely unlikely that it will function very well as a base class. Fortunately C allows you to use the sealed keyword to prevent a class from being used as a base class if you decide that it should not be. For example sealed class LiteralToken DefaultTokenImpl IToken . If any class attempts to use LiteralToken as a base class a compile-time error will be generated. A sealed class cannot declare any virtual methods. The sole purpose of the virtual keyword is to declare that this is the first implementation of a method that you intend to override in a derived class but a sealed class cannot be derived from. NOTE A struct is implicitly sealed. You can never derive from a struct. Sealed Methods You can also use the sealed keyword to declare that an individual method is sealed. This means that a derived class cannot then override the sealed method. You can only seal an override method you declare the method as sealed override .You can think of the interface virtual override and sealed keywords as follows An interface introduces the name of a method. A virtual method is the first implementation of a method. An override method is another implementation of a method. A sealed method is the last implementation of a method. Subscribing to an Event Like delegates events come ready-made with a operator. You subscribe to an event by using this operator. In the automated factory the software controlling each machine can arrange for the shutdown
