GrazingMammal myGrazingMammal = new GrazingMammal(...); // illegal in .NET framework

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GrazingMammal myGrazingMammal = new GrazingMammal(...); // illegal
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Abstract Methods
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An abstract class can contain abstract methods. An abstract method is similar in principle to a virtual method (you met virtual methods in 12) except that it does not contain a method body. A derived class must override this method. The following example de nes the DigestGrass method in the GrazingMammal class as an abstract method; grazing mammals might use the same code for chewing grass, but they must provide their own implementation of the DigestGrass method. An abstract method is useful if it does not make sense to provide
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Part II
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Understanding the C# Language
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a default implementation in the abstract class and you want to ensure that an inheriting class provides its own implementation of that method.
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abstract class GrazingMammal : Mammal, IGrazable { abstract void DigestGrass(); ... }
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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 dif cult business. With practice and experience, you can develop the skills to craft a exible, easy-to-use hierarchy of interfaces, abstract classes, and classes, but it takes effort and you also need a solid understanding of the problem you are modeling. 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. 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:
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sealed class Horse : GrazingMammal, ILandBound { ... }
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If any class attempts to use Horse as a base class, a compile-time error will be generated. Note that a sealed class cannot declare any virtual methods and that an abstract class cannot be sealed. Note A structure is implicitly sealed. You can never derive from a structure.
Sealed Methods
You can also use the sealed keyword to declare that an individual method in an unsealed class is sealed. This means that a derived class cannot then override the sealed method. You can seal only 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 rst implementation of a method. An override method is another implementation of a method. A sealed method is the last implementation of a method.
13
Creating Interfaces and De ning Abstract Classes
Implementing an Extensible Framework
In the following exercise, you will familiarize yourself with a hierarchy of interfaces and classes that together implement a simple framework for reading a C# source le and classifying its contents into tokens (identi ers, keywords, operators, and so on). This framework performs some of the tasks that a typical compiler might perform. The framework provides a mechanism for visiting each token in turn, to perform speci c tasks. For example, you could create: A displaying visitor class that displays the source le in a rich text box. A printing visitor class that converts tabs to spaces and aligns braces correctly. A spelling visitor class that checks the spelling of each identi er. A guideline visitor class that checks that public identi ers start with a capital letter and that interfaces start with the capital letter I. A complexity visitor class that monitors the depth of the brace nesting in the code. A counting visitor class that counts the number of lines in each method, the number of members in each class, and the number of lines in each source le. Note This framework implements the Visitor pattern, rst documented by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides in Design Patterns: Elements of Reusable Object-Oriented Software (Addison Wesley Longman, 1995).
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