Sealed Classes

Sealed Sử dụng các lớp kế thừa không phải luôn luôn dễ dàng và đòi hỏi phải suy nghĩ. Nếu bạn tạo một giao diện hoặc lớp một trừu tượng, bạn đang cố ý viết cái gì đó sẽ được thừa hưởng từ trong tương lai. Điều rắc rối là dự đoán tương lai là một doanh nghiệp khó khăn.
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Sealed Classes Sealed ClassesUsing inheritance is not always easy and requires forethought. If you create an interfaceor an abstract class, you are knowingly writing something that will be inherited from inthe 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, unlessyou consciously design a class with the intention of using it as a base class, its extremelyunlikely that it will function very well as a base class. Fortunately, C# allows you to usethe sealed keyword to prevent a class from being used as a base class if you decide that itshould 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 begenerated. A sealed class cannot declare any virtual methods. The sole purpose of thevirtual keyword is to declare that this is the first implementation of a method that youintend to override in a derived class, but a sealed class cannot be derived from.NOTEA struct is implicitly sealed. You can never derive from a struct.Sealed MethodsYou can also use the sealed keyword to declare that an individual method is sealed. Thismeans that a derived class cannot then override the sealed method. You can only seal anoverride method (you declare the method as sealed override).You can think of theinterface, 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.