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在本文中,我們討論OOP中的熱點(diǎn)之一:抽象類。抽象類在各個編程語言中概念是一致的,但是C#稍微有些不一樣。本文中我們會通過代碼來實(shí)現(xiàn)抽象類,并一一進(jìn)行解析。
在微軟的MSDN中,對抽象類有如下的定義:
用abstract 關(guān)鍵字可定義抽象類,要求其子類必須實(shí)現(xiàn)抽象類的函數(shù)、屬性等。抽象類不可被實(shí)例化。抽象類提供了統(tǒng)一的定義,用于其不同子類直接共享數(shù)據(jù)、函數(shù)。 抽象類也可定義抽象函數(shù)。
在Visual Studio中添加Console程序,并命名為“InheritanceAndPolymorphism
”,添加ClassA.cs,添加抽象類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(); } } }
編譯報(bào)錯:
Compile time error: Cannot create an instance of the abstract class or interface 'InheritanceAndPolymorphism.ClassA'
結(jié)論:無法用new關(guān)鍵字來實(shí)例化一個抽象類。
給抽象類ClassA添加一些非抽象函數(shù)的代碼:
/// <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(); } }
編譯,依然報(bào)錯。 抽象類無論是否有抽象、非抽象函數(shù),均無法通過new關(guān)鍵字來實(shí)例化。
我們把抽象類作為基類,添加ClassB—使之繼承自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(); } }
編譯的結(jié)果:不再報(bào)錯。
結(jié)論:一個類可以繼承自abstract 修飾的抽象類,且可被new關(guān)鍵字初始化。
在ClassA中聲明YYY函數(shù)--無函數(shù)體。
/// <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(); } }
編譯,結(jié)果報(bào)錯:
Compile time error: 'InheritanceAndPolymorphism.ClassA.YYY()' must declare a body because it is not marked abstract, extern, or partial
結(jié)論是需要對YYY添加函數(shù)體,或者添加abstract的修飾符。
在ClassA的YYY前,添加abstract修飾符。
/// <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(); } }
編譯結(jié)果,報(bào)錯:
Compiler error: 'InheritanceAndPolymorphism.ClassB' does not implement inherited abstract member 'InheritanceAndPolymorphism.ClassA.YYY()'
結(jié)論:我們在abstract 類中聲明了一個
abstract 的函數(shù),但是并未在其子類ClassB中實(shí)現(xiàn)其內(nèi)容;當(dāng)使用new關(guān)鍵字初始化ClassB的時候則會報(bào)錯----無法使用new關(guān)鍵字初始化一個
abstract類。
在子類中添加YYY的實(shí)現(xiàn)。
/// <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(); } }
編譯結(jié)果,報(bào)錯:
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()'.
結(jié)論:要使得子類繼承基類的YYY函數(shù),需要用到override關(guān)鍵字,然后才可以用new關(guān)鍵字實(shí)例化ClassB。
我們再看看這些代碼:
/// <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(); } }
編譯,結(jié)果報(bào)錯:
Compiler error: 'InheritanceAndPolymorphism.ClassA.YYY()' is abstract but it is contained in non-abstract class 'InheritanceAndPolymorphism.ClassA'
結(jié)果分析:聲明abstract的函數(shù),必須同時聲明類為
abstract。
abstract 的函數(shù)不能同時添加static或virtual關(guān)鍵字。
/// <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(); } }
編譯,結(jié)果報(bào)錯:
Compile time error : Cannot call an abstract base member: 'InheritanceAndPolymorphism.ClassA.YYY()'
結(jié)果分析:ClassB中無法使用base調(diào)用基類的abstract函數(shù)--因?yàn)槠洳淮嬖凇?/p>
最后一個問題,可否在抽象類中添加sealed關(guān)鍵字,結(jié)果是不可以。
抽象類不能添加sealed、static類修飾符的。
通過下面幾點(diǎn),歸納一下本文的結(jié)論。
無法使用new來實(shí)例化abstract 抽象類
abstract 抽象類可以有子類,其子類實(shí)現(xiàn)抽象方法后,可被new實(shí)例化對象
如聲明了
abstract 的函數(shù),則必須聲明
abstract 的類
當(dāng)override抽象基類,無法修改基類函數(shù)的簽名
abstract函數(shù),無法同時添加static、virtual關(guān)鍵字
abstract 類無法被聲明為sealed、static類
原文鏈接:Diving in OOP (Day 4): Polymorphism and Inheritance (All About Abstract Classes in C#)
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