关于C#面向对象三个特征:继承,封装,多态的说明(1)

下面重点说明面向对象的三个特征.继承,封装和多态.
C#是一种现代的面向对象的语言.
继承(inheritance):继承是一个面向对象的词语.说明,一个类(派生类)能分享,其它类(基类)的特征和行为.派

生类和基类是"is a"的关系.
base classes(基类):通常基类可以自己实例化,或被继承.派生类继承基类中的成员,被标记为protected或更大

的权限.语法: class (derive class name):(base class name)
例子:
//基类
public class Contact
{
  //默认私有的字段
  string name;
  string email;
  string address;
  //构造函数
  public Contact()
  {
    // statements ...
  }
  //属性
  public string Name
  {
    get
    {
      return name;
    }
    set
    {
      name = value;
    }
  }

  public string Email
  {
    get
    {
      return email;
    }
    set
    {
      email = value;
    }
  }

  public string Address
  {
    get
    {
      return address;
    }
    set
    {
      address = value;
    }
  }
}
//派生类
public class Customer : Contact
{
  //自己的私有字段
  string gender;
  decimal income;

  public Customer()
  {
    // statements ...
  }
}
在上面的例子中,Customer 是

Contact的子类,不但,继承了父类的成员,name,email,address;还有自己的成员,gender,income.

abstract classes(抽象类):抽象类是一种特殊的基类.除过普通的类成员,它们还有抽象的类成员.抽象类成员,

是不能被实例化的方法和属性.所有直接从抽象类派生的类,必须实现抽象的方法和属性.抽象类不能被实例化.
例子:
//抽象类
abstract public class Contact
{
  protected string name;

  public Contact()
  {
    // statements...
  }

  //抽象方法
  public abstract void generateReport();
  //抽象属性
  abstract public string Name
  {
    get;
    set;
  }}

public class Customer : Contact
{
  string gender;
  decimal income;
  int numberOfVisits;

  public Customer()
  {
    // statements
  }

  public override void generateReport()
  {
    // unique report
  }

  public override string Name
  {
    get
    {
      numberOfVisits++;
      return name;
    }
    set
    {
      name = value;
      numberOfVisits = 0;
    }
  }
}

public class SiteOwner : Contact
{
  int siteHits;
  string mySite;

  public SiteOwner()
  {
    // statements...
  }

  public override void generateReport()
  {
    // unique report
  }

  public override string Name
  {
    get
    {
      siteHits++;
      return name;
    }
    set
    {
      name = value;
      siteHits = 0;
    }
  }
}
上面的例子,定义了三个类.一个抽象类,两个派生类.实现了父类的方法和属性."override"修饰符,实现了抽象

类的方法.    
Calling Base Class Members(调用基类成员)
派生类能调用基类成员,如果,成员的修饰符是"protected"或更大权限.在适当的上下文条件下,好像调用自己的

成员一样.
例子:
abstract public class Contact
{
  private string address;
  private string city;
  private string state;
  private string zip;

  public string FullAddress()
  {
    string fullAddress =
      address + '\n' +
      city + ',' + state + ' ' + zip;

    return fullAddress;
  }
}

public class Customer : Contact
{
  public string GenerateReport()
  {
    string fullAddress = FullAddress();
    // do some other stuff...
    return fullAddress;
  }
}
上面的例子中,派生类调用基类的方法:FullAddress();
基类的构造函数,可以被派生类调用,用base().
例子:
abstract public class Contact
{
  private string address;

  public Contact(string address)
  {
    this.address = address;
  }
}

public class Customer : Contact
{
  public Customer(string address) : base(address)
  {
  }
}
例子中,派生类没有address成员,可以调用基类的构造函数.
Hiding Base Class Members(隐藏基类成员)
派生类可以和基类有同样名字的成员.这时,就会隐藏基类的成员.
例子:
abstract public class Contact
{
  private string address;
  private string city;
  private string state;
  private string zip;

  public string FullAddress()
  {
    string fullAddress =
      address + '\n' +
      city + ',' + state + ' ' + zip;

    return fullAddress;
  }
}

public class SiteOwner : Contact
{
  public string FullAddress()
  {
    string fullAddress;

    // create an address...
    return fullAddress;
  }
}
在例子中,派生类和基类有同样的成员,FullAddress(),当调用时,基类的方法会被隐藏.

尽管基类的成员被隐藏,仍然可以访问基类的成员,通过,base关键字,调用基类的引用.
例子:
abstract public class Contact
{
  private string address;
  private string city;
  private string state;
  private string zip;

  public string FullAddress()
  {
    string fullAddress =
      address + '\n' +
      city + ',' + state + ' ' + zip;

    return fullAddress;
  }
}

public class SiteOwner : Contact
{
  public string FullAddress()
  {
    string fullAddress = base.FullAddress();

    // do some other stuff...
    return fullAddress;
  }
}
在例子中,派生类调用基类的成员,用base引用.
visioning(版本)
例子:
using System;
public class WebSite
{
  public string SiteName;
  public string URL;
  public string Description;

  public WebSite()
  {
  }

  public WebSite( string strSiteName, string strURL, string strDescription )
  {
    SiteName  = strSiteName;
    URL     = strURL;
    Description = strDescription;
  }

  public override string ToString()
  {
    return SiteName + ", " +
        URL   + ", " +
        Description;
  }
}


public class Contact
{
  public string address;
  public string city;
  public string state;
  public string zip;

  public string FullAddress()
  {
    string fullAddress =
      address + '\n' +
      city + ',' + state + ' ' + zip;

    return fullAddress;
  }
}

public class SiteOwner : Contact
{
  int   siteHits;
  string name;
  WebSite mySite;

  public SiteOwner()
  {
    mySite = new WebSite();
    siteHits = 0;
  }

  public SiteOwner(string aName, WebSite aSite)
  {
    mySite = new WebSite(aSite.SiteName,
               aSite.URL,
               aSite.Description);

    Name = aName;
  }

  new public string FullAddress()
  {
    string fullAddress = mySite.ToString();

    return fullAddress;
  }

  public string Name
  {
    get
    {
      siteHits++;
      return name;
    }
    set
    {
      name = value;
      siteHits = 0;
    }
  }
}

public class Test
{
  public static void Main()
  {
    WebSite mySite = new WebSite("Le Financier",
                   "http://www.LeFinancier.com",
                   "Fancy Financial Site");

    SiteOwner anOwner = new SiteOwner("John Doe", mySite);
    string address;

    anOwner.address = "123 Lane Lane";
    anOwner.city  = "Some Town";
    anOwner.state  = "HI";
    anOwner.zip   = "45678";

    address = anOwner.FullAddress(); // Different Results
    Console.WriteLine("Address: \n{0}\n", address);

  }
}
例子中,派生类用new修饰符,说明,和基类有同样名字的成员.
sealed classed(密封类)
密封类是不能被继承的类.为了避免从一个类中继承,就要生成密封类.
例子:
//密封类
public sealed class CustomerStats
{
  string gender;
  decimal income;
  int numberOfVisits;

  public CustomerStats()
  {
  }
}

public class CustomerInfo : CustomerStats // error
{
}

public class Customer
{
  CustomerStats myStats; // okay
}
例子中,密封类不能被继承.