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Just as you can prevent inheritance using the sealed modifier on a class, virtual members may be sealed, too (see Listing 615) This prevents a subclass from overriding a base class member that was originally declared as virtual higher in the inheritance chain The situation arises when a subclass B overrides a base class A's member and then needs to prevent any further overriding below subclass B
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Listing 615 Sealing Members
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class A { public virtual void Method() { } } class B : A { public override sealed void Method() { } } class C : B { // ERROR: Cannot override sealed members //public override void Method() // { // } }
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In this example, the use of the sealed modifier on class B's Method() declaration prevents C's overriding of Method()
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In choosing to redeclare a member, developers often want to invoke the member on the base class (see Listing 616)
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Listing 616 Accessing a Base Member
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public class Address { public string StreetAddress; public string City; public string State; public string Zip; public override string ToString() { return stringFormat("{0}" + EnvironmentNewLine + "{1}, {2} {3}", StreetAddress, City, State, Zip); } } public class InternationalAddress : Address { public string Country; public override string ToString() { return baseToString()+ EnvironmentNewLine + Country; } }
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In Listing 616, InternationalAddress inherits from Address and implements ToString() To call the base class's implementation you use the base keyword The syntax is virtually identical to this, including support for using base as part of the constructor (discussed shortly) Parenthetically, in the AddressToString() implementation, you are required to override as well, because ToString() is also a member of object Any members that are decorated with override are automatically designated as virtual, so additional child classes may further specialize the implementation
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When instantiating a derived class, the runtime first invokes the base class's constructor so that the base class initialization is not circumvented However, if there is no accessible (nonprivate) default constructor on the base class, then it is not clear how to construct the base class and the C# compiler reports an error To avoid the error caused by no accessible default constructor, programmers need to designate explicitly, in the derived class constructor header, which base constructor to run (see Listing 617)
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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; } // } ___________________________________________________________________________ ___________________________________________________________________________ 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 classto 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 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
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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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