详解C#面相对象编程中的继承特性

// 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();  }}

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