详解C#面相对象编程中的继承特性

继承（加上封装和多态性）是面向对象的编程的三个主要特性（也称为“支柱”）之一。 继承用于创建可重用、扩展和修改在其他类中定义的行为的新类。其成员被继承的类称为“基类”，继承这些成员的类称为“派生类”。派生类只能有一个直接基类。但是，继承是可传递的。如果 classb 派生出 classc，classa 派生出 classb，则 classc 会继承 classb 和 classa 中声明的成员。

注意
结构不支持继承，但可以实现接口。

从概念上来说，派生类是基类的特例。 例如，如果您有一个基类 animal，则可以有一个名为 mammal 的派生类和一个名为 reptile 的派生类。 mammal 是一个 animal，reptile 也是一个 animal，但每个派生类均表示基类的不同专用化。
定义一个类从其他类派生时，派生类隐式获得基类的除构造函数和析构函数以外的所有成员。因此，派生类可以重用基类中的代码而无需重新实现这些代码。可以在派生类中添加更多成员。派生类以这种方式扩展基类的功能。
下图演示一个 workitem 类，该类表示某业务流程中的一个工作项。和所有的类一样，该类派生自 system.object 并继承其所有方法。 workitem 添加了自己的五个成员。其中包括一个构造函数，因为构造函数不能继承。类changerequest 继承自 workitem 并表示特定种类的工作项。 changerequest 在它从 workitem 和 object 继承的成员中另外添加了两个成员。它必须添加其自己的构造函数，还添加 originalitemid。利用属性 originalitemid，可将 changerequest 实例与更改请求将应用到的原始 workitem 相关联。

类继承
下面的示例演示如何以 c# 表示上图所示的类关系。该示例还演示 workitem 如何重写虚方法 object.tostring，以及 changerequest 类如何继承该方法的 workitem 实现。

// workitem implicitly inherits from the object class.
public class workitem
{
  // static field currentid stores the job id of the last workitem that
  // has been created.
  private static int currentid;

  //properties.
  protected int id { get; set; }
  protected string title { get; set; }
  protected string description { get; set; }
  protected timespan joblength { get; set; }

  // default constructor. if a derived class does not invoke a base-
  // class constructor explicitly, the default constructor is called
  // implicitly. 
  public workitem()
  {
    id = 0;
    title = "default title";
    description = "default description.";
    joblength = new timespan();
  }

  // instance constructor that has three parameters.
  public workitem(string title, string desc, timespan joblen)
  {
    this.id = getnextid();
    this.title = title;
    this.description = desc;
    this.joblength = joblen;
  }

  // static constructor to initialize the static member, currentid. this
  // constructor is called one time, automatically, before any instance
  // of workitem or changerequest is created, or currentid is referenced.
  static workitem()
  {
    currentid = 0;
  }


  protected int getnextid()
  {
    // currentid is a static field. it is incremented each time a new
    // instance of workitem is created.
    return ++currentid;
  }

  // method update enables you to update the title and job length of an
  // existing workitem object.
  public void update(string title, timespan joblen)
  {
    this.title = title;
    this.joblength = joblen;
  }

  // virtual method override of the tostring method that is inherited
  // from system.object.
  public override string tostring()
  {
    return string.format("{0} - {1}", this.id, this.title);
  }
}

// changerequest derives from workitem and adds a property (originalitemid) 
// and two constructors.
public class changerequest : workitem
{
  protected int originalitemid { get; set; }

  // constructors. because neither constructor calls a base-class 
  // constructor explicitly, the default constructor in the base class
  // is called implicitly. the base class must contain a default 
  // constructor.

  // default constructor for the derived class.
  public changerequest() { }

  // instance constructor that has four parameters.
  public changerequest(string title, string desc, timespan joblen,
             int originalid)
  {
    // the following properties and the getnexid method are inherited 
    // from workitem.
    this.id = getnextid();
    this.title = title;
    this.description = desc;
    this.joblength = joblen;

    // property originalitemid is a member of changerequest, but not 
    // of workitem.
    this.originalitemid = originalid;
  }
}

class program
{
  static void main()
  {
    // create an instance of workitem by using the constructor in the 
    // base class that takes three arguments.
    workitem item = new workitem("fix bugs",
                   "fix all bugs in my code branch",
                   new timespan(3, 4, 0, 0));

    // create an instance of changerequest by using the constructor in
    // the derived class that takes four arguments.
    changerequest change = new changerequest("change base class design",
                         "add members to the class",
                         new timespan(4, 0, 0),
                         1);

    // use the tostring method defined in workitem.
    console.writeline(item.tostring());

    // use the inherited update method to change the title of the 
    // changerequest object.
    change.update("change the design of the base class",
      new timespan(4, 0, 0));

    // changerequest inherits workitem's override of tostring.
    console.writeline(change.tostring());

    // keep the console open in debug mode.
    console.writeline("press any key to exit.");
    console.readkey();
  }
}

输出：

  1 - fix bugs
  2 - change the design of the base class