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SUMMARY
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This chapter explained C# constructs for classes and object orientation in C# This included a discussion of fields, and a discussion of how to access them on a class instance This chapter also discussed the key concept of whether to store data on a per-instance basis or across all instances of a type Static data is associated with the class and instance data is stored on each object In addition, the chapter explored encapsulation in the context of access modifiers for methods and data The C# construct of properties was introduced, and you saw how to use it to encapsulate private fields The next chapter focuses on how to associate classes with each other via inheritance, and the benefits derived from this object-oriented construct
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Inheritance
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how one class can reference other classes via fields and properties This chapter discusses how to use the inheritance relationship between classes to build class hierarchies
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HE PRECEDING CHAPTER DISCUSSED
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Casting protected Single Inheritance Sealed Classes
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is Operator
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Derivation
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Inheritance
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Overriding
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virtual new sealed
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SystemObject
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Abstract Classes
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BEGINNER TOPIC Inheritance Definitions The preceding chapter provided an overview of inheritance Here s a review of the defined terms
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Derive/inherit: Specialize a base class to include additional members
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or customization of the base class members
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6: Inheritance Derived/sub/child type: The specialized type that inherits the members
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of the more general type
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Base/super/parent type: The general type whose members a derived
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type inherits Inheritance forms an is a relationship The derived type is always implicitly also of the base type Just as a hard drive is a storage device, any other type derived from the storage device type is a type of storage device
Derivation
It is common to want to extend a given type to add features, such as behavior and data The purpose of inheritance is to do exactly that Given a Person class, you create an Employee class that additionally contains EmployeeId and Department properties The reverse approach may also occur Given, for example, a Contact class within a Personal Digital Assistant (PDA), you decide you also can add calendaring support Toward this effort, you create an Appointment class However, instead of redefining the methods and properties that are common to both classes, you refactor the Contact class Specifically, you move the common methods and properties on Contact into a base class called PdaItem from which both Contact and Appointment derive, as shown in Figure 61 The common items in this case are Created, LastUpdated, Name, ObjectKey, and the like Through derivation, the methods defined on the base class, PdaItem, are accessible from all subclasses of PdaItem When defining a derived class, follow the class identifier with a colon and then the base class, as Listing 61 demonstrates
Listing 61: Deriving One Class from Another
public class PdaItem { public string Name { get; set; } public DateTime LastUpdated { get; set; } }
Derivation
// Define the Contact class as inheriting the PdaItem class public class Contact : PdaItem { public string Address { get; set; } public string Phone { get; set; } }
Listing 62 shows how to access the properties defined in Contact
Listing 62: Using Inherited Methods
public class Program { public static void Main() { Contact contact = new Contact(); contactName = "Inigo Montoya"; // } }
Figure 61: Refactoring into a Base Class
6: Inheritance
Even though Contact does not directly have a property called Name, all instances of Contact can still access the Name property from PdaItem and use it as though it was part of Contact Furthermore, any additional classes that derive from Contact will also inherit the members of PdaItem, or any class from which PdaItem was derived The inheritance chain has no practical limit and each derived class will have all the exposed members of its base class inheritance chain combined (see Listing 63)
Listing 63: Classes Deriving from Each Other to Form an Inheritance Chain
public class PdaItem : object { // }
public class Appointment : PdaItem { // }
public class Contact : PdaItem { // }
public class Customer : Contact { // }
In other words, although Customer doesn t derive from PdaItem directly, it still inherits the members of PdaItem In Listing 63, PdaItem is shown explicitly to derive from object Although C# allows such syntax, it is unnecessary because all classes that don t have some other derivation will derive from object, regardless of whether it is specified
Casting between Base and Derived Types As Listing 64 shows, because derivation forms an is a relationship, a derived type can always be directly assigned to a base type