Abstract Classes in C#.NET

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Abstract Classes
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Listing 617: Specifying Which Base Constructor to Invoke
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public class PdaItem { public PdaItem(string name) { Name = name; } // }
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public class Contact : PdaItem { public Contact(string name) : base(name) { Name = name; } public string Name { get; set; } // }
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By identifying the base constructor in the code, you let the runtime know which base constructor to invoke before invoking the derived class constructor
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Abstract Classes
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Many of the inheritance examples so far have defined a class called PdaItem that defines the methods and properties common to Contact, Appointment, and so on, which are type objects that derive from PdaItem PdaItem is not intended to be instantiated itself, however A PdaItem instance has no meaning by itself; it has meaning only when it is used as a base class to share default method implementations across the set of data types that derive from it These characteristics are indicative of the need for PdaItem to be an abstract class rather than a concrete class Abstract classes are designed for derivation only It is not possible to instantiate an abstract class, except in the context of instantiating a class that derives from it Classes that are not abstract and can instead be instantiated directly are concrete classes
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6: Inheritance
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BEGINNER TOPIC Abstract Classes Abstract classes represent abstract entities Their abstract members define what an object derived from an abstract entity should contain, but they don t include the implementation Often, much of the functionality within an abstract class is unimplemented, and before a class can successfully derive from an abstract class, it needs to provide the implementation for the abstract methods in its abstract base class
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To define an abstract class, C# requires the abstract modifier to the class definition, as shown in Listing 618
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Listing 618: Defining an Abstract Class
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// Define an abstract class public abstract class PdaItem { public PdaItem(string name) { Name = name; } public virtual string Name { get; set; } }
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public class Program { public static void Main() { PdaItem item; // ERROR: Cannot create an instance of the abstract class item = new PdaItem("Inigo Montoya"); } }
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Although abstract classes cannot be instantiated, this restriction is a minor characteristic of an abstract class Their primary significance is achieved when abstract classes include abstract members An abstract member is a method or property that has no implementation Its purpose is to force all derived classes to provide the implementation Consider Listing 619
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Abstract Classes
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Listing 619: Defining Abstract Members
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// Define an abstract class public abstract class PdaItem { public PdaItem(string name) { Name = name; } public virtual string Name { get; set; } public abstract string GetSummary(); }
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public class Contact : PdaItem { public override string Name { get { return FirstName + " " + LastName; } set { string[] names = valueSplit(' '); // Error handling not shown FirstName = names[0]; LastName = names[1]; } } public string FirstName { get; set; } public string LastName { get; set; } public string Address { get; set; } public override string GetSummary() { return stringFormat( "FirstName: {0} + "LastName: {1} + "Address: {2}", FirstName, LastName, Address); } // }
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6: Inheritance
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public class Appointment : PdaItem { public Appointment(string name) : base(name) { Name = name; } public DateTime StartDateTime { get; set; } public DateTime EndDateTime { get; set; } public string Location { get; set; } // public override string GetSummary() { return stringFormat( "Subject: {0}" + EnvironmentNewLine + "Start: {1}" + EnvironmentNewLine + "End: {2}" + EnvironmentNewLine + "Location: {3}", Name, StartDateTime, EndDateTime, Location); } }
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Listing 619 defines the GetSummary() member as abstract, and therefore, it doesn t include any implementation Then, the code overrides it within Contact and provides the implementation Because abstract members are supposed to be overridden, such members are automatically virtual and cannot be declared so explicitly In addition, abstract members cannot be private because derived classes would not be able to see them