Inheritance
Introduction
Inheritance, together with encapsulation and polymorphism, is one of the three primary characteristics of object-oriented programming. Inheritance enables you to create new classes that reuse, extend, and modify the behavior defined in other classes. The class whose members are inherited is called the base class, and the class that inherits those members is called the derived class. A derived class can have only one direct base class. However, inheritance is transitive. If ClassC is derived from ClassB, and ClassB is derived from ClassA, ClassC inherits the members declared in ClassB and ClassA.
When you define a class to derive from another class, the derived class implicitly gains all the members of the base class, except for its constructors and finalizers. The derived class reuses the code in the base class without having to reimplement it. You can add more members in the derived class. The derived class extends the functionality of the base class.
This example shows how WorkItem overrides the virtual method Object.ToString, and how the ChangeRequest class inherits the WorkItem implementation of the method. The first block defines the classes:
// 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;
// currentID is a static field. It is incremented each time a new
// instance of WorkItem is created.
protected int GetNextID() => ++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() =>
$"{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;
}
}
This next block shows how to use the base and derived classes:
// 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());
/* Output:
1 - Fix Bugs
2 - Change the Design of the Base Class
*/