A.11. Chapter 11: Inheritance and Polymorphism A.11.1. Quiz -
Solution to Question 111 .
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This relationship is reciprocal: if you have three types with similar functionality, you can refactor that similarity out of the three types into a generalized type. At the same time, each of the three types are now specialized forms of the more generalized type. Inheritance is also implicitly hierarchical: you can imagine a tree with the most generalized type at the top and each level of specialization descending from levels above. A generalized type may have many specializations, but each specialized type may have only one generalization. -
Solution to Question 112 .
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Through inheritance. -
Solution to Question 113 .
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class <identifier> : <base class> -
Solution to Question 114 .
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Create a virtual method in the base class, and then override it in the derived class. -
Solution to Question 115 .
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The more usual meaning is to allocate memory on the heap. The special meaning in inheritance is that you are not overriding a base method; you are creating a new method that intentionally hides and replaces the base class method. -
Solution to Question 116 .
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After the parameter list, but before the opening brace , put a colon followed by the keyword base and two parentheses. Pass the parameters for the base class constructor within the parentheses. -
Solution to Question 117 .
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A member of class A is visible to class B if it is marked public , or if it is marked protected and class B derives directly or indirectly from A. If the member is marked private in A, it is always invisible to class B. -
Solution to Question 118 .
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An abstract method has no implementation in the base class, but must be overridden and implemented in any derived class that does not itself want to be abstract. Any class with an abstract method (even if inherited) is itself abstract and may not be instantiated . -
Solution to Question 119 .
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A sealed class is one that the compiler will not let you derive from. Classes are marked sealed when the designer of the class does not want anyone to create a derived version. -
Solution to Question 1110 .
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Object (which is the ultimate base class (root) of all types in C#). -
Solution to Question 1111 .
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Object . -
Solution to Question 1112 .
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If you use a value type where an Object is expected, the value type will be automatically and invisibly wrapped in an Object and will take on reference characteristics. -
Solution to Question 1113 .
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When you return a boxed type back to its original type, you must unbox it and explicitly cast it. A.11.2. Exercises -
Solution to Exercise 11-1 .
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Create a base class, Telephone , and derive a class Electronic-Phone from it. In Telephone , create a protected string member phonetype , and a public method Ring( ) that outputs a text message such as: "Ringing the <phonetype>." In ElectronicPhone , the constructor should set the phonetype to "Digital." In the Run( ) method, call Ring( ) on the ElectronicPhone to test the inheritance. using System; public class Tester { public class Telephone { protected string phonetype; public void Ring( ) { Console.WriteLine( "Ringing the {0} phone...", phonetype ); } } public class ElectronicPhone : Telephone { public ElectronicPhone( ) { this.phonetype = "Digital"; // access protected member } } public void Run( ) { ElectronicPhone phone = new ElectronicPhone( ); phone.Ring( ); // accessing the base method } static void Main( ) { Tester t = new Tester( ); t.Run( ); } }
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Solution to Exercise 11-2 .
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Extend Exercise 11-1 to illustrate a polymorphic method. Have the derived class override the Ring( ) method to display a different message. using System; public class Tester { public class Telephone { protected string phonetype; public virtual void Ring( ) // now virtual { Console.WriteLine( "Ringing the {0} phone. Ring Ring.", phonetype ); } } public class ElectronicPhone : Telephone { public ElectronicPhone( ) { this.phonetype = "Digital"; // access protected member } public override void Ring( ) // override { Console.WriteLine( "Ringing the {0} phone. Beep Beep.", phonetype ); } } public void Run( ) { // assign derived instance to base reference Telephone phone = new ElectronicPhone( ); phone.Ring( ); // accessing the polymorphic method } static void Main( ) { Tester t = new Tester( ); t.Run( ); } }
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Solution to Exercise 11-3 .
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Change the Telephone class to abstract, and make Ring( ) an abstract method. Derive two new classes from Telephone : DigitalPhone and Talking-Phone . Each derived class should set the phonetype , and override the Ring( ) method. using System; public class Tester { public abstract class Telephone { protected string phonetype; public abstract void Ring( ); // now abstract } public class DigitalPhone : Telephone { public DigitalPhone( ) { this.phonetype = "Digital"; // access protected member } public override void Ring( ) // implement { Console.WriteLine( "Ringing the {0} phone. Beep Beep.", phonetype ); } } public class TalkingPhone : Telephone { public TalkingPhone( ) { this.phonetype = "Talking"; // access protected member } public override void Ring( ) // implement { Console.WriteLine( "Ringing the {0} phone. You have a call.", phonetype ); } } public void Run( ) { // assign derived instance to base reference Telephone phone1 = new DigitalPhone( ); Telephone phone2 = new TalkingPhone( ); phone1.Ring( ); // accessing the polymorphic method phone2.Ring( ); // accessing the polymorphic method } static void Main( ) { Tester t = new Tester( ); t.Run( ); } }
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