深入淺出OOP（四）: 多態和繼承(抽象類) - powertoolsteam－IT工程師數位筆記本

文章出處

在本文中，我們討論OOP中的熱點之一：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現抽象類，并一一進行解析。

Abstract Classes

在微軟的MSDN中，對抽象類有如下的定義：

用abstract 關鍵字可定義抽象類，要求其子類必須實現抽象類的函數、屬性等。抽象類不可被實例化。抽象類提供了統一的定義，用于其不同子類直接共享數據、函數。抽象類也可定義抽象函數。

Abstract Classes實戰

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();
        }
    }
}

編譯報錯：

Compile time error: Cannot create an instance of the abstract class or interface 'InheritanceAndPolymorphism.ClassA'

結論：無法用new關鍵字來實例化一個抽象類。

Abstract Class的非抽象函數

給抽象類ClassA添加一些非抽象函數的代碼：

    /// <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，依然報錯。抽象類無論是否有抽象、非抽象函數，均無法通過new關鍵字來實例化。

Abstract Class作為基類

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();
        }
    }

編譯的結果：不再報錯。

結論：一個類可以繼承自abstract 修飾的抽象類，且可被new關鍵字初始化。

Abstract Class的非抽象函數聲明

在ClassA中聲明YYY函數--無函數體。

    /// <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，結果報錯：

Compile time error: 'InheritanceAndPolymorphism.ClassA.YYY()' must declare a body because it is not marked abstract, extern, or partial

結論是需要對YYY添加函數體，或者添加abstract的修飾符。

Abstract Class的抽象函數聲明

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();
        }
    }

編譯結果，報錯：

Compiler error: 'InheritanceAndPolymorphism.ClassB' does not implement inherited abstract member 'InheritanceAndPolymorphism.ClassA.YYY()'

結論：我們在abstract 類中聲明了一個abstract 的函數，但是并未在其子類ClassB中實現其內容；當使用new關鍵字初始化ClassB的時候則會報錯----無法使用new關鍵字初始化一個abstract類。

子類繼承實現抽象函數

在子類中添加YYY的實現。

/// <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();
        }
    }

編譯結果，報錯：

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()'.

結論：要使得子類繼承基類的YYY函數，需要用到override關鍵字，然后才可以用new關鍵字實例化ClassB。

非抽象類的抽象函數

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，結果報錯：

Compiler error: 'InheritanceAndPolymorphism.ClassA.YYY()' is abstract but it is contained in non-abstract class 'InheritanceAndPolymorphism.ClassA'

結果分析：聲明abstract的函數，必須同時聲明類為abstract。

abstract 的函數不能同時添加static或virtual關鍵字。

抽象基類函數

/// <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();
        }
    }