n interface is defined as a syntactical contract that all the classes inheriting the interface should follow. The interface defines the 'what' part of the syntactical contract and the deriving classes define the 'how' part of the syntactical contract.
Interfaces define properties, methods, and events, which are the members of the interface. Interfaces contain only the declaration of the members. It is the responsibility of the deriving class to define the members. It often helps in providing a standard structure that the deriving classes would follow
// 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();
}
}
/* Output:
1 - Fix Bugs
2 - Change the Design of the Base Class
*/
