C#4.0图解教程 - 第7章 类和继承

7.5使用基类的引用#

派生类的实例由基类的实例加上派生类附加的成员组成，派生类引用指向整个类对象，包括基类部分。

MyDerivedClass derived = new MyDerivedClass();

MyBaseClass mybc = (MyBaseClass)derived;

 

 

using System;

namespace Examples

{

class MyBaseClass

{

public void Print()

        {

Console.WriteLine("This is the base class.");

        }

    }

class MyDerivedClass : MyBaseClass

{

new public void Print()

        {

Console.WriteLine("This is the derived class.");

        }

    }

class Program

{

static void Main()

        {

MyDerivedClass derived = new MyDerivedClass();

MyBaseClass mybc = (MyBaseClass)derived;

            derived.Print();

            mybc.Print();

        }

    }

}

 

This is the derived class.
This is the base class.
请按任意键继续. . .

 #

---

7.5.1 虚方法和覆写方法#

虚方法可以使基类的引用方式“升至”派生类内。

using System;

namespace Examples

{

class MyBaseClass

{

virtual public void Print()

        {

Console.WriteLine("This is the base class.");

        }

    }

class MyDerivedClass : MyBaseClass

{

override public void Print()

        {

Console.WriteLine("This is the derived class.");

        }

    }

class Program

{

static void Main()

        {

MyDerivedClass derived = new MyDerivedClass();

MyBaseClass mybc = (MyBaseClass)derived;

            derived.Print();

            mybc.Print();

        }

    }

}

 

This is the derived class.
This is the derived class.
请按任意键继续. . .

 

 

 #

---

7.5.2  覆写标记为override的方法#

当使用对象基类部分的引用调用一个覆写方法时，方法的调用被沿派生层次上溯执行，一直到标记为override的方法的最派生（most-derived）版本。

 

---

情况1：使用override声明Print()

using System;

namespace Examples

{

class MyBaseClass                                    // Base class

{

virtual public void Print()

        {

Console.WriteLine("This is the base class.");

        }

    }

class MyDerivedClass : MyBaseClass                   // Derived class

{

override public void Print()

        {

Console.WriteLine("This is the derived class.");

        }

    }

class SecondDerived : MyDerivedClass

{

override public void Print()              //这里使用override

{

Console.WriteLine("This is the second derived class.");

        }

    }

class Program

{

static void Main()

        {

SecondDerived derived = new SecondDerived(); // Use SecondDerived.

MyBaseClass mybc = (MyBaseClass)derived;     // Use MyBaseClass.

derived.Print();

            mybc.Print();

        }

    }

}

 

 

This is the second derived class.
This is the second derived class.
请按任意键继续. . .

 

 

---

情况2：使用new声明Print()

class MyDerivedClass : MyBaseClass

{

new public void Print()

        {

Console.WriteLine("This is the derived class.");

        }

    }

class Program

{

static void Main()

        {

MyDerivedClass derived = new MyDerivedClass(); 

MyBaseClass mybc = (MyBaseClass)derived;

            derived.Print();

            mybc.Print();

        }

    }

 

This is the second derived class.
This is the derived class.
请按任意键继续. . .

 

 #

---

7.8 成员访问修饰符#

internal  同一程序集访问

protected 派生类可访问

protected internal  听那个一程序集和派生类可访问

 

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7.10 抽象类 abstract#

7.11 密封类 sealed#

抽象方法必须是基类，它不能实例化。密封类恰好相反。

 

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7.12 静态类#

类必须为静态，类成员也必须为静态。