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Inheritance

09_inheritance.ppt

Overview of Topics

• Inheritance
• Virtual Methods used for Overriding
• Constructors & Inheritance
• Abstract Classes and Methods
• Interface Classes
• Delegates and Events

Inheritance

• Inheritance is the process by which a new

class is created from an existing class, but the

new class has additional member variables

and/or methods.

• This is a powerful feature of object-oriented

languages.

• This allows us to reuse code that has already

been developed and tested.

Inheritance Terminology

• Base Class
• Parent Class
• Super Class

Derived Class Child Class Sub Class

A derived class would be able to do everything the

base class can do, plus it would add some attributes

or operations to increase its functionality.

Data Inheritance

• In CS8 we created a class to process orders, clsOrder.
• In CS9 we want to create a new class, clsOrderPreferred, to handle orders for preferred customers, which would include a discount in the calculation.
• If we went through the design phase, we would see that the class would include many of the items and calculations already defined in clsOrder.
• The best thing to do would be to inherit clsOrder, and then add or redefine properties and methods that are needed for a preferred order (Discount Rate, calcExtendedPrice with discount, etc.).

clsOrderPreferred

//Add only the variables and methods needed for a preferred Order

public class clsOrderPreferred : clsOrder {

const decimal cdecDISCOUNT_RATE = 0.05M; //5%

public override void calcExtendedPrice( ) { cdecExtendedPrice = cintQuantity * (cdecPrice - cdecPrice * cdecDISCOUNT_RATE); }

}

Method Overriding

• Method Overriding occurs when a method in the derived class has the exact same name and parameters as a method in the base class – same signature.
• C# determines which method is executed based on the class the object was created from. - If the object is created using the base class, then the base method is called. - If the object is created using the derived class, then the overriding method is called.
• We override methods because we want a standard name for our methods across classes, such calcExtendedPrice instead of calcExtendedPrice1, calcExtendedPrice2, etc.

Virtual calcExtendedPrice is Overridable

• In clsOrder there is a calcExtendedPrice.
• If we wanted to create a clsOrderPreferred that would calculated the extended amount with a discount, we can redefine calcExtendedPrice in the new class by Overriding it.
• To allow the calcExtendedPrice method to be overridden, declare method as virtual n the base class, clsOrder:

public virtual void calcExtendedPrice ( ) { cdecExtendedPrice = cintQuantity * cdecPrice; }

Constructors & Inheritance

• Constructors are NOT inherited.
• Each derived class must have a constructor defined, and it should call the constructor from the base class.
• The base constructor would be responsible to initialize variables in the base class, so the derived class would only need its constructor to initialized variables defined in its class.
• Do not duplicate code.
• Pass the parameter values to the constructor in the base class so it can assign the parameter values to the class variables through the property methods.
• Use the existing code in the base class.

Call Base Constructors

public class clsOrderPreferred : clsOrder { //The descr, qty, price and all the Get and Put methods are inherited //Only add the new properties and/or methods

public clsOrderPreferred( ) : base( ) { //Call default constructor in base class }

public clsOrderPreferred (string descr, int qty, decimal price) : base (descr, qty, price) { //Call overloaded constructor in base class //to assign parameter values to class variables. //Do not duplicate the code to assign values. } }

Inheritance and Form class

• In all of the programs we have written so far we have

seen the following statement:

public partial class CS8Form : Form

• The form we create is based on a predefined Form

class that includes such things as:

• Properties: .Font, .AcceptButton, .CancelButton,
• Methods: .Close( ), .Activate( ), .Hide( ).
• We then add our own buttons, textboxes, etc.

Abstract Classes

• An abstract class can NOT be instantiated.
• It can only be used as a base class that other

classes can inherit.

• Use keyword abstract to declare class:

public abstract class Employee

• Abstract classes may contain method

implementations or abstract methods.

Interface Classes

• A interface class is similar to an abstract class.
• Use keyword interface to declare class and

usually named with a capital I prefix:

public interface IEmployee

decimal calcGross( ); //no implementation

• All methods in an interface are basically abstract

methods – no implementation included.

Delegates and Events

• Coding delegates and events is an advanced topic,

so the terms are just introduce here.

• A delegate is used to specify the signature of a

method that can handle an event.

• EventHandler is a predefined delegate in System:

Public delegate void EventHandler(object sender, EventArgs e);

• Next indicate the method that a delegate should call.

New System.EventHandler(this.clickButton_Click); This code found in designer file.

• The event sender controls the generation of events

and calls the defined event handler.