在本文中,我们讨论OOP中的热点之一:抽象类。抽象类在各个编程语言中概念是一致的,但是C#稍微有些不一样。本文中我们会通过代码来实现抽象类,并一一进行解析。
深入理解OOP(一):多态和继承(初期绑定和编译时多态)
深入理解OOP(二):多态和继承(继承)
深入理解OOP(三):多态和继承(动态绑定和运行时多态)
深入理解OOP(四):多态和继承(C#中的抽象类)
深入理解OOP(五):C#中的访问修饰符(Public/PRivate/Protected/Internal/Sealed/Constants/Static and Readonly Fields)
深入理解OOP(六):枚举(实用方法)
深入理解OOP(七):属性(实用方法)
深入理解OOP(八):索引器(实用方法)
深入理解OOP(九):事件(深入理解)
在微软的MSDN中,对抽象类有如下的定义:
用abstract 关键字可定义抽象类,要求其子类必须实现抽象类的函数、属性等。抽象类不可被实例化。抽象类提供了统一的定义,用于其不同子类直接共享数据、函数。 抽象类也可定义抽象函数。
在Visual Studio中添加Console程序,并命名为“InheritanceAndPolymorphism
”,添加ClassA.cs,添加抽象类ClassA。
using System;namespace InheritanceAndPolymorphism{ public abstract class ClassA { } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassA classA = new ClassA(); Console.ReadKey(); } }}
编译报错:
Compile time error: Cannot create an instance of the abstract class or interface 'InheritanceAndPolymorphism.ClassA'
结论:无法用new关键字来实例化一个抽象类。
给抽象类ClassA添加一些非抽象函数的代码:
/// <summary> /// Abstract class ClassA /// </summary> public abstract class ClassA { public int a; public void XXX() { } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassA classA = new ClassA(); Console.ReadKey(); } }
编译,依然报错。 抽象类无论是否有抽象、非抽象函数,均无法通过new关键字来实例化。
我们把抽象类作为基类,添加ClassB—使之继承自ClassA。
/// <summary> /// Abstract class ClassA /// </summary> public abstract class ClassA { public int a; public void XXX() { } } /// <summary> /// Derived class. /// Class derived from abstract class ClassA /// </summary> public class ClassB:ClassA { } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassB classB = new ClassB(); Console.ReadKey(); } }
编译的结果:不再报错。
结论:一个类可以继承自abstract 修饰的抽象类,且可被new关键字初始化。
在ClassA中声明YYY函数--无函数体。
/// <summary> /// Abstract class ClassA /// </summary> public abstract class ClassA { public int a; public void XXX() { } public void YYY(); } /// <summary> /// Derived class. /// Class derived from abstract class ClassA. /// </summary> public class ClassB:ClassA { } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassB classB = new ClassB(); Console.ReadKey(); } }
编译,结果报错:
Compile time error: 'InheritanceAndPolymorphism.ClassA.YYY()' must declare a body because it is not marked abstract, extern, or partial
结论是需要对YYY添加函数体,或者添加abstract的修饰符。
在ClassA的YYY前,添加abstract修饰符。
/// <summary> /// Abstract class ClassA /// </summary> public abstract class ClassA { public int a; public void XXX() { } abstract public void YYY(); } /// <summary> /// Derived class. /// Class derived from abstract class ClassA. /// </summary> public class ClassB:ClassA { } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassB classB = new ClassB(); Console.ReadKey(); } }
编译结果,报错:
Compiler error: 'InheritanceAndPolymorphism.ClassB' does not implement inherited abstract member 'InheritanceAndPolymorphism.ClassA.YYY()'
结论:我们在abstract 类中声明了一个
abstract 的函数,但是并未在其子类ClassB中实现其内容;当使用new关键字初始化ClassB的时候则会报错----无法使用new关键字初始化一个
abstract类。
在子类中添加YYY的实现。
/// <summary> /// Abstract class ClassA /// </summary> public abstract class ClassA { public int a; public void XXX() { } abstract public void YYY(); } /// <summary> /// Derived class. /// Class derived from abstract class ClassA. /// </summary> public class ClassB:ClassA { public void YYY() { } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassB classB = new ClassB(); Console.ReadKey(); } }
编译结果,报错:
Compile time error: 'InheritanceAndPolymorphism.ClassB' does not implement inherited abstract member 'InheritanceAndPolymorphism.ClassA.YYY()' Compile time warning: 'InheritanceAndPolymorphism.ClassB.YYY()' hides inherited member 'InheritanceAndPolymorphism.ClassA.YYY()'.
结论:要使得子类继承基类的YYY函数,需要用到override关键字,然后才可以用new关键字实例化ClassB。
我们再看看这些代码:
/// <summary> /// Abstract class ClassA /// </summary> public class ClassA { public int a; public void XXX() { } abstract public void YYY(); } /// <summary> /// Derived class. /// Class derived from abstract class ClassA. /// </summary> public class ClassB:ClassA { public override void YYY() { } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassB classB = new ClassB(); Console.ReadKey(); } }
编译,结果报错:
Compiler error: 'InheritanceAndPolymorphism.ClassA.YYY()' is abstract but it is contained in non-abstract class 'InheritanceAndPolymorphism.ClassA'
结果分析:声明abstract的函数,必须同时声明类为
abstract 的函数不能同时添加static或virtual关键字。abstract。
/// <summary> /// Abstract class ClassA /// </summary> public abstract class ClassA { public int a; public void XXX() { } abstract public void YYY(); } /// <summary> /// Derived class. /// Class derived from abstract class ClassA. /// </summary> public class ClassB:ClassA { public override void YYY() { base.YYY(); } } /// <summary> /// Program: used to execute the method. /// Contains Main method. /// </summary> public class Program { private static void Main(string[] args) { ClassB classB = new ClassB(); Console.ReadKey(); } }
编译,结果报错:
Compile time error : Cannot call an abstract base member: 'InheritanceAndPolymorphism.ClassA.YYY()'
结果分析:ClassB中无法使用base调用基类的abstract函数--因为其不存在。
最后一个问题,可否在抽象类中添加sealed关键字,结果是不可以。
抽象类不能添加sealed、static类修饰符的。
通过下面几点,归纳一下本文的结论。
abstract 抽象类
abstract 抽象类可以有子类,其子类实现抽象方法后,可被new实例化对象
如声明了abstract 的函数,则必须声明abstract 的类
当override抽象基类,无法修改基类函数的签名
abstract函数,无法同时添加static、virtual关键字
abstract 类无法被声明为sealed、static类
原文链接:Diving in OOP (Day 4): Polymorphism and Inheritance (All About Abstract Classes in C#)
译文链接:http://www.VEVb.com/powertoolsteam/p/Diving-in-OOP-Day-Polymorphism-and-Inheritance-All.html
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