Inheritance in C

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Inheritance in C#

C# supports a single inheritance model. Thus a class may derive from a single base class, and not from more than one. (In fact, as we saw in the previous chapter, every class in C# ultimately derives from the root class System.Object . In C# we may use the alias object for this root class.) This single inheritance model is simple and avoids the complexities and ambiguities associated with multiple inheritance in C++. Although a C# class can inherit only from a single base class , it may inherit from several interfaces , a topic we will discuss in the next chapter.

In this section we discuss inheritance in connection with a further elaboration of our hotel reservation case study. In the following section we will cover additional features of inheritance in C#, illustrated by an employee class hierarchy.

Inheritance Fundamentals

With inheritance, you factor the abstractions in your object model, and put the more reusable abstractions in a high-level base class. You can add or change features in more specialized derived classes, which "inherit" the standard behavior from the base class. Inheritance facilitates code reuse and extensibility. A derived class can also provide a more appropriate interface to existing members of the base class.

Consider Reservable as a base class, with derived classes such as Hotel . All reservables share some characteristics, such as an id, a capacity, and a cost. Different kinds of reservables differ in other respects. For example, a hotel has a City and a HotelName .

C# Inheritance Syntax

You implement inheritance in C# by specifying the derived class in the class statement with a colon followed by the base class. The file HotelBroker.cs in the CaseStudy folder illustrates deriving a new class Hotel from the class Reservable .

// HotelBroker.cs 

namespace OI.NetCs.Acme
^[1]
{ 
   using System;

public class Hotel : Reservable

{ 
      public string City; 
      public string HotelName; 

      public Hotel(string city, string name, 
                   int number, decimal cost) 
         : base(number, cost) 
      { 
         City = city; 
         HotelName = name; 
      } 
      public int HotelId 
      { 
         get 
         { 
            return unitid; 
         } 
      } 
      public int NumberRooms 
      { 
         get 
         { 
            return capacity; 
         } 
      } 
      public decimal Rate 
      { 
         get 
         { 
            return cost; 
         } 
      } 
   }

^[1] We discuss creating a namespace with the namespace directive later in the chapter.

The class Hotel automatically has all the members of Reservable , and in addition has the fields City and HotelName .

Changing the Interface to Existing Members

The base class Reservable has members unitid , capacity , and cost , which are designed for internal use and are not intended to be exposed as such to the outside world. In the Hotel class we provide public properties HotelId , NumberRooms , and Rate to give clients read-only access to these fields. When we implement a property in this way, we can choose a name that is meaningful, such as NumberRooms , in place of a more abstract name, such as capacity , used in the base class.

Invoking Base Class Constructors

If your derived class has a constructor with parameters, you may wish to pass some of these parameters along to a base class constructor. In C# you can conveniently invoke a base class constructor by using a colon, followed by the base keyword and a parameter list.

public Hotel(string city, string name, 
             int number, decimal cost)

: base(number, cost)

{ 
   City = city; 
   HotelName = name; 
}

Note that the syntax allows you to explicitly invoke a constructor only of an immediate base class. There is no notation that allows you to directly invoke a constructor higher up the inheritance hierarchy.

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Stiefel M., Oberg R. J.

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