using System;

public class Program

{

    public 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));

        // Wait... What is a TimeSpan? https://msdn.microsoft.com/en-us/library/bk8a3558(v=vs.110).aspx

        // 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());

    }

}

​

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;

    }

}

​

​