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APPENDIX A
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INTRODUCTION TO C#
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Since C# does not provide global variables, static fields are often used to store global values. Finally, classes can be nested:
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class C1 { int i, j; string s; void m() { // ... } class c2 { // ... } }
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The class is at the heart of object-oriented programming in C#. We ll return to this topic when we consider inheritance. A.6.2 Programming with structs C# provides the ability to create a lightweight class using a struct. Structs are value types and are created on the stack at run time. (Refer to chapter 2 for a complete discussion of value and reference types.) This removes the overhead of using references and obviates the need for garbage collection. The following example uses a struct to create a type that stores the x and y coordinates of a point:
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struct Point { public int X, Y; public Point(int x, int y) { X = x; Y = y; } }
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Although a struct can contain methods, typically structs are used for types that contain just a few data members. Like classes, structs can also be nested. However, unlike a class, a struct may not inherit from another class or struct, nor may it serve as a base class for inheritance purposes.
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INHERITANCE
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C# allows us to design a class by using inheritance to embrace and extend an existing class. Unlike C++, which supports multiple inheritance, C# supports only single inheritance. However, like Java, C# also supports implementation inheritance using interfaces which provide some of the advantages of multiple inheritance. We ll explore interfaces later in this appendix. Many application domains contain hierarchies that are naturally modeled by inheritance. Object-oriented GUI libraries often use this technique. For example,
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INHERITANCE
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depending on the implementation, a check box may be a special type of button, and a button is a control, and a control is a type of component, and a component is an object. In such cases we might implement this as an inheritance hierarchy. The check box would extend the button class by adding a checked property, and so forth. A class that inherits from another is sometimes referred to as a derived class. A.7.1 Simple inheritance Let s return to our Person class. Listing A.3 provides a new implementation of the Person class, together with a class called Man which derives from Person.
Listing A.3 Simple inheritance
using System; class Person { protected string firstName, lastName; // constructor method... public Person(string firstName, string lastName) { this.firstName = firstName; this.lastName = lastName; } // method... public void Greet() { Console.WriteLine("Hello " + firstName + " " + lastName + "!"); } } // Man derives from Person class... class Man : Person { // create a Man by calling base Person constructor... public Man(string fName, string lName) : base(fName, lName) {} // replace base Greet method with a new implementation... public new void Greet() { Console.WriteLine("Hello Mr. " + lastName + "!"); } } class Test { public static void Main() { Person p = new Person("Joe", "Bloggs"); p.Greet(); // displays "Hello Joe Bloggs!" Man m = new Man("Joe", "Bloggs"); m.Greet(); // displays "Hello Mr. Bloggs!" } }
APPENDIX A
INTRODUCTION TO C#
Inheritance is used to model an is-a relationship. (A man is a person). In this example, we ve changed the accessibility of the firstName and lastName fields of the Person class to protected, thus making them accessible to any derived class. We specify that Man derives from Person, as follows:
// Man derives from Person class... class Man : Person { ... }
The only difference between the Man and Person classes is the greeting displayed. Therefore, we leverage the base Person class constructor to build an instance of Man, as follows:
// create a Man by calling base Person constructor... public Man(string fName, string lName) : base(fName, lName) {}
The base keyword is used to refer to the parent object. In this example, the body of the Man constructor is empty. Instead the Person constructor is called to construct the object. In the derived Man class, we reimplement the Greet method using the new keyword to make clear to the compiler that we are not inadvertently hiding the parent s Greet method. A.7.2 Using virtual methods One of the advantages of inheritance is the ability to use a base class reference to refer to an instance of a derived class. This allows us to write code without caring whether the reference is to a parent or derived class, as follows:
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