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  • c#教程之详解C#面相对象编程中的继承特性

  • 2018-09-12 23:10 来源:未知

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。

2016128171832088.jpeg (449×302)

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。

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// 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
2
1 - Fix Bugs
2 - Change the Design of the Base Class

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