T3 - Object Oriented Programming in C#

8/7/2019 T3 - Object Oriented Programming in C#

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Objectives

Essentials of Object-OrientedProgramming

Classes and Objects Using Encapsulation

C# and Object Orientation

Defining Object-Oriented Systems

The Object Hier archy



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Objectives

Use Constructors In C#

Use Destructors In C#

Explain the working of Garbage Collector

Discuss Method Overloading

Discuss Operator Overloading

Use Inheritance in C#

Discuss Overriding



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Objectives

Discuss Polymorphism

Use Virtual Functions

Discuss relationship between polymorphism and

inheritance Discuss Abstract Base classes

Discuss the concept of Interfaces

Use Interfaces Discuss Properties

Discuss Indexers



Object ± OrientedProgramming Overview



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1. Object ± Oriented Progr amming

History

Object-oriented progr amming has roots that can betr aced to the 1960s:

As har dware and software became increasinglycomplex, quality was often compromised.

Researchers studied ways to maintain software qualityand developed object-oriented progr amming in part toaddress common problems by strongly emphasizing discrete, reusable units of progr amming logic.

The methodology focuses on data r ather than processes, with progr ams composed of self-sufficientmodules (objects) each containing all the information needed to manipulate its own data structure.



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1. Object ± Oriented Progr amming

History

The Object-Oriented methodology focuses on datarather than processes, with progr ams composed of self-sufficient modules (objects) each containing all theinformation needed to manipulate its own data

structure. This is in contr ast to the existing modular

progr amming which had been dominant for manyyears that focused on the function of a module

An object-oriented progr am may thus be viewed as a collection of cooper ating objects, as opposed to theconventional model, in which a progr am is seen as a list of tasks (subroutines) to perform



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2. OOP Fundamental Concepts

Fundamental Concepts in OOP are: Class

Object

Instance

Method

Message passing

Inheritance

Abstr action

Encapsulation Polymor physm

De-coupling



I. Classes and Objects



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1. What is a class?

A class define the abstr act char acteristics of a thing (object), including the thing'schar acteristics (its attributes, fields or

properties) and the thing's behaviors (thethings it can do, or methods, operations or features).

For example, the class Dog would consist of

tr aits shared by all dogs, such as: Breed and fur color (char acteristics), and

The ability to bark and sit (behaviors)



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1. What is a class?

In C#.NET, classes are declared using

the keywor d class, as shown in the

following example:



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An example: A class with its attributes,

fields, properties, methods,«



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2. Members of a class

The members of a class type are divided into thefollowing categories: Constants

Fields

Methods Properties

Events

Indexers

Oper ators

Constructors Destructors

Types



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2.1. Compare Properties and Public

Fields

Properties can be recognizeby IDEs (such as Visual Studio.NET) but public fields

Properties are the hybrid of fields and methods

A property is like a clock-knob: when you tur n it, the

hour hand and the minutehand and the second hand are automatically ad justed



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2.2. Constructor

Whenever a class or struct is created, itsconstructor is called. A class or struct mayhave multiple constructors that take differentar guments.

Constructors enable the progr ammer to setdef ault values, limit instantiation, and writecode that is f lexible and easy to read.

If you do not provide a constructor for your object, C# will create one by def ault that

instantiates the object and sets member variables to the def ault values

Static classes and structs can also haveconstructors



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2.3. Destructor

Destructors are used to destruct instances of classes. Destructors cannot be defined in structs. They are only used

with classes.

A class can only have one destructor .

Destructors cannot be inherited or over loaded.

Destructors cannot be called. They are invoked automatically.

A destructor does not take modifiers or have par ameters.

The destructor implicitly calls Finalize on the base class of the object.



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2.4. Indexers

Indexers allow you to index a class or a

struct instance in the same way as an

arr ay

An indexer is declared like a property

except that the name of the member is

this followed by a par ameter list written

between the delimiters [ and ]



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Indexer example

BEFORE



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Indexer example



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3. Access Modifiers

Access modifiers are keywor ds used to specify the declared accessibility of a member or a type. This section introduces the four access modifiers: Public: The public keywor d is an access modifier for types and type

members. Public access is the most permissive access level. There areno restrictions on accessing public members

Protected: The protected keywor d is a member access modifier . Aprotected member is accessible from within the class in which it isdeclared, and from within any class derived from the class that declared this member .

Internal: Inter nal members are accessible only within files in the sameassembly. A common use of inter nal access is in component-based development because it enables a group of components to cooper ate in a private manner without being exposed to the rest of the application code.

Private: The private keywor d is a member access modifier . Privateaccess is the least permissive access level. Private members areaccessible only within the body of the class or the struct in which they aredeclared. Nested types in the same body can also access those privatemembers.



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3. What is the meaning of ³partial´

keywor d?

In .NET fr amework 2.0 or higher, Microsoft introducesa very new modifier named ³partial´.

Before, the definition a a class must be placed in asingle source file.

But now, with ³partial´ modifier, It is possible to splitthe definition of a class or a struct, or an interf ace over

two or more source files. Each source file contains a section of the class definition, and all parts arecombined when the application is compiled



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³partial´ class definition example



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3. What is the meaning of ³partial´

keywor d?

Reasons to use ³partial´ modifier:

When working on lar ge projects, spreading a class

over separ ate files allows multiple progr ammers to

work on it simultaneously. When working with automatically gener ated source,

code can be added to the class without having to

recreate the source file. Visual Studio uses this

approach when creating Windows Forms, Web

Service wr apper code, and so on. You can create

code that uses these classes without having to edit

the file created by Visual Studio



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4. What is an Object?

An Object is a patter n of a class.

Exp: The class of Dog defines all possible dogs by listing thechar acteristics and behaviors they can have; the object Lassie is one particular

dog, with particular versions of thechar acteristics. A Dog has fur; Lassiehas brown-and-white fur .



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4. What is an Object?

In OOP, each object is capable of receiving messages, processing data,and sending messages to other objects

and can be viewed as an independent'machine' with a distinct role or responsibility.

The actions (or "oper ators") on theseobjects are closely associated with theobject.



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5. What is an ³Instance´

The instance is the actual object created

at runtime (and placed in RAM).

The set of values of the fields and

properties of a particular object is called

its state



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6. Object Data (or Object State)

Object data describes information for

individual objects



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7. ³static´ keywor d

The static modifier can be used with

classes, fields, methods, properties,

oper ators, events, and constructors

But ³static´ keywor d cannot be used with

indexers, destructors, or types other than

classes.



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7. ³static´ keywor d example



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7. ³static´ keywor d

The static modifier on a class means that the classcannot be instantiated, and that all of its members arestatic.

A static member has one version regar dless of how many instances of its enclosing type are created.

A constant is implicitly a static member .

A static member cannot be referenced through an instance. Instead, it is referenced through the typename



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8. Nested Classes

A class defined within a class or struct is

called a nested class



II. Encapsulation



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1. What is encapsulation?

Encapsulation is the hiding of the inter nal mechanisms and data structures of a softwarecomponent behind a defined interf ace, in such a waythat users of the component (other pieces of software)only need to know what the component does, and cannot make themselves dependent on the details of how it does it.

For example, the Dog class has a bark() method. Thecode for the bark() method defines exactly how a bark

happens (e.g., by inhale() and then exhale(), at a particular pitch and volume). Timmy, Lassie's friend,however, does not need to know exactly how shebarks.



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2. Why must encapsulate?

The pur pose of encapsulation is to achievepotential for change: the inter nal mechanismsof the component can be improved withoutimpact on other components, or the

component can be replaced with a differentone that supports the same public interf ace.

Encapsulation also protects the integrity of the

component, by preventing users from setting the inter nal data of the component into an invalid or inconsistent state.



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2. Why must encapsulate?

Encapsulation reduces system

complexity and thus increases

robustness, by limiting the

inter dependencies between software

components



III. Inheritance



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1. What is the inheritance?

Inheritance is a way to form new classes (instances of which are called objects) using classes that have already

been defined. The inheritance concept was invented in

1967

The new classes, known as derivedclasses, take over (or inherit) attributesand behavior of the pre-existing classes



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Inheritance - Examples



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2. Single and Multiple inheritances

Single inheritance: deriving from one

base class

Multiple inheritance: deriving from two or more base classes



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3. What are Over-loads?

Over loading is the technical term for

declaring two or more methods in the

same scope with the same name

By specifying different number of

par ameters

By specifying different types of par ameters



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4. Oper ator Over-loading

Like C++, C# allows you to over load oper atorsfor use on your own classes. This makes itpossible for a user-defined data type to look as

natur al and be as logical to use as a fundamental data type

To over load an oper ator, you write a function

that has the name oper ator followed by thesymbol for the oper ator to be over loaded



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5. Hiding Base Class Members

Sometimes derived class members have the samename as a corresponding base class member . In thiscase, the derived member is said to be "hiding" thebase class member

When hiding occurs, the derived member is masking the functionality of the base class member . Users of the derived class won't be able to see the hidden member; they'll see only the derived class member



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7. ³base´ keywor d

The base keywor d is used to access membersof the base class from within a derived class: Call a method on the base class that has been

overridden by another method.

Specify which base-class constructor should becalled when creating instances of the derived class.

A base class access is permitted only in a constructor, an instance method, or an

instance property accessor . It is an error to use the base keywor d fromwithin a static method.



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7. ³base´ keywor d example



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8. ³sealed´ keywor d

When applied to a class, the sealed modifier

prevents other classes from inheriting from it

You can also use the sealed modifier on a

method or property that overrides a virtual

method or property in a base class. This

enables you to allow classes to derive from

your class and prevent them from overriding

specific virtual methods or properties.



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8. ³sealed´ keywor d example

In the following example, no class can

inherit from class Musician.



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8. ³sealed´ keywor d example

In the following example, Z inherits from Y but

Z cannot override the virtual function F that is

declared in X and sealed in Y.



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8. ³sealed´ keywor d

Notes: When you define new methods or properties in a

class, you can prevent deriving classes fromoverriding them by not declaring them as virtual.

It is an error to use the abstr act modifier with a sealed class, because an abstr act class must beinherited by a class that provides an implementation of the abstr act methods or properties.

When applied to a method or property, the sealed modifier must always be used with override.

Because structs are implicitly sealed, they cannotbe inherited.



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8. ³new´ keywor d

In C#, the new keywor d can be used as

an oper ator or as a modifier .

new oper ator Used to create objects on

the heap and invoke constructors.

new modifier Used to hide an inherited

member from a base class member .



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8. ³new´ keywor d examples

In the following example, ³new´ keywor d

is used as an oper ator



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8. ³new´ keywor d examples

In the following example, ³new´ keywor d

is used as a modifier:



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9. Why use inheritances?

Enables a series of classes to be related together accor ding to their common parts:

common members only specified once, in base class

can be accessed in objects of all classesderived from the base class

Increases efficiency of production and f lexibility





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1. What is the polymor phism?

Polymor phism is derived from two Greek wor ds. Poly (meaning many) and morph (meaning forms). Polymor phism means manyforms

Understanding Polymor phism is crucial to any OO language

professional , be it a Java , C++ or C# progr ammer

If a Dog is commanded to speak(), it may emit a bark, while if a Pig is asked to speak(), it may respond with an oink. Both inheritspeak() from Animal, but their subclass methods override themethods of the superclass, known as overriding polymor phism

We implement polymor phism by using virtual and overridekeywor ds



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Polymor physm example

V mt hình thc, i su vào ca hàm Speakphi có kiu là Animal,và khi hàm này cgi, thì phng thc

Speak ca Animal ó sc gi và s không in

ra ni dung gì

Nhng thc t, khi truyn i s

u vào cho hàm Speak, lúc là Pig,lúc là Dog, thì hàm Speak li lúc thìgi hàm Speak ca Pig, lúc thì li

gi hàm Speak ca Dog => Chính vìlý do này, hàm Speak c gi là

³a hình´



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3. ³override´ and ³virtual´ keywor ds

³override´ and ³virtual´ keywor ds are used toimplement polymor phism in C#.NET

The override modifier is required to extend or

modify the abstr act or virtual implementation of an inherited method, property, indexer, or event

The virtual keywor d is used to modify a method, property, indexer, or event declar ation and allow for it to be overridden in aderived class



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4. Why polymor phism?

Why include polymor phism in a

progr amming language?

Because it significantly reduces

progr amming effort. In particular,

polymor phism allows you to write one

method for a variety of types, r ather than to write one method per type



V. Abstractions:Abstract Base Classes &

Interfaces



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1. What is the abstr action?

Abstr action (from the Latin abs, meaning away

fr om and t raher e, meaning to draw ) is the

process of taking away or removing

char acteristics from something in or der toreduce it to a set of essential char acteristics

In object-oriented progr amming, abstr action is

one of three central principles (along with

encapsulation and inheritance)



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1. What is the abstr action?

Through the process of abstr action, a

progr ammer hides all but the relevant

data about an object in or der to reduce

complexity and increase efficiency



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2. Abstr act base classes

Abstr act base classes are classes that cannotbe instantiated, and are frequently either partially implemented, or not at all implemented.

In C#, the abstract modifier is used to declarean abstr act class



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3. Interf aces

An interf ace is a collection of method definitions (without implementations) and constant values.

An interf ace is like a pure abstr act baseclass

Notes:

A class that implements a particular interf ace must implement the memberslisted by that interf ace

4 Wh t k thi



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4. Why must make everything

abstr act?

The importance of abstr action is derived from

its ability to hide irrelevant details and from the

use of names to reference objects.

Abstr action is essential in the construction of progr ams. It places the emphasis on what an

object is or does r ather than how it is

represented or how it works. Thus, it is the

primary means of managing complexity inlarge programs

5 Diff b t Ab t t



5. Differences between Abstr act

Classes Vs. Interf aces

The biggest difference between an

abstr act class and an interf ace is: A

class may implement an unlimited

number of interfaces, but may inheritfrom only one abstract

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T3 - Object Oriented Programming in C#