Friday, February 03, 2006

 

Introduction To OOP Concepts - Object Oriented Concepts In VB.NET

Contents

Introduction

VB.NET is completely Object Oriented. This article uncovers some basic Object Oriented Programming Features of Visual Basic.NET. The whole article is divided to 10 lessons. The source code for these lessons is provided with the article.

This tutorial is designed with the following objectives.

  1. To provide a sound knowledge about Object Oriented Programming in VB.NET
  2. To educate how Object Oriented techniques are used in VB.NET
  3. To explain the following concepts simply and easily.
    • Creating And Using Classes And Objects In VB.NET
    • Encapsulation, Abstraction, Inheritance And Polymorphism
    • Overloading And Overriding
    • Constructors And Destructors
    • Static Functions

Go through this tutorial and you will start making sense of almost any .NET code. Also, Java/CPP programmers can use this to understand OOPs in VB.NET easily.

Using the code

The source code for each lesson is available as a .vb source code file. You need Microsoft.NET framework SDK installed in your system to compile and execute the excercises in this article. You can download it from the Microsoft website. The VB.NET compiler (vbc.exe) normally resides in your FrameworkSDK\bin folder.

To manually compile a source code file, you may use the command prompt to type

 vbc filename.vb /out:"filename.exe"  /r:"System.Windows.Forms.dll","System.dll"

 

Lesson 1: Namespaces, Classes, Objects And Modules

1) A Namespace

In VB.NET, classes and other data structures for a specific purpose are grouped together to form a namespace.You can use the classes in a namespace, by simply importing the namespace.The 'imports' keyword is used to import a namespace to your project..NET framework provides a rich set of built in classes, groupedto various namespaces.In this lesson, we are using the system namespace.Import the System namespace (already available in .NET)
Imports System

2) A Class

Probably, you are already familiar with classes and objects. Simply speaking, a class is a definition of a real life object. For example, Human is a class for representing allhuman beings. Dog is a class to represent all Dogs.Classes can contain functions too.Animals is a namespace
Namespace Animals

Dog is a class in the namespace Animals
Class Dog

Bark is a function in this class
 Function Bark()
  Console.Writeline ("Dog is barking")
 End Function
End Class
End Namespace

3) An Object

An object is an instance of a class. For example,Jimmy is an object of type Dog. We will createan object in the next section. Read on.

4) Modules

You can use modules to write common functions. A module is agroup of functions. Unlike functions in classes, public functionsin modules can be called directly from some where else.VB provides Functions and Subroutines. Functions and Subroutines are almost same, but subroutines can't returna value.
Public Module modMain

Execution will start from the Main() subroutine
Sub Main()
  
    'Call our function. See below
    OurFunction() 
End sub

OurFunction: Our own little function to use the class Dog
Function OurFunction() 
    'Here is how we declare a variable Jimmy of type Dog.
    'We use Animals.Dog because, the class Dog is in the
    'namespace Animals (see above).
 
    Dim Jimmy as Animals.Dog
 
    'Create an object. Unlike in VB 6, it is not required to use
    'the 'set' keyword.
     
    Jimmy = new Animals.Dog()
 
    'Another way to create an object is
    'Dim Jimmy as new Dog
 
    'Call Jimmy's Main Function  
    Jimmy.Bark()
 
End Function
End module

 

Lesson 2: Access Types

The major access types are Public, Private, Friend And Protected.A class may contain functions, variables etc, either publicor private or protected or friend. If they are public,they can be accessed by creating objects of the class.Private and Protected members can be accessed only by functionsinside the class. Protected members are much like private members,but they have some special use while inheriting the class. Wewill see this later, in Inheritance (Lesson 5).Friend members can be accessed only from elements in thesame project, and not by elements outer the current project.Let us expand our dog class.

Import the System namespace (already available in .NET)

Imports System

Animals is a namespace
Namespace Animals

Dog is a class in the namespace Animals
Public Class Dog
 'A public variable 
 Public AgeOfDog as Integer

Bark is a function in this class. It is public
 Public Function Bark()
  Console.Writeline ("Dog is barking")
 End Function
 

Walk is a function in this class. It is private
 Private Function Walk()
  Console.Writeline ("Dog is walking")
 End Function
 
End Class
End Namespace

Our Module
Public Module modMain

Execution will start from the Main() subroutine
Sub Main()
  
    'Call our function. See below
    OurFunction() 
End sub
 'OurFunction: Called from Main()
 
 Function OurFunction()
  
  Dim Jimmy as Animals.Dog
  Jimmy=new Animals.Dog()
  
  
  'This will work, because Bark & Ageofdog are public
  Jimmy.Bark
  Jimmy.AgeOfDog=10
  
  'Calling the Walk function will not work here, because
  'Walk() is outside the class Dog  
  'So this is wrong. Uncomment this and try to compile, it will
  'cause an error.
  
  'Jimmy.Walk
  
 End Function  
 
End Module

Additional Notes:

Encapsulation:

Putting all the data and related functions, in the class is calledas Encapsulation.

Data Hiding or Abstraction:

Normally, in a class, variables used to hold data (like the age ofa dog) is declared as private. Functions or property routines are usedto access these variables. Protecting the data of an object fromouter functions is called as Abstraction or Data Hiding. This prevents accidental modification of data by functions outside the class.

 

Lesson 3: Shared Functions

The shared members in a class (both functions and variables)can be used with out creating objects of the class as shown.The Shared modifier indicates the method does not operate on aspecific instance of a type and may be invoked directly froma type rather than through a particular instance of a type.

Import the System namespace (already available in .NET)

Imports System

Animals is a namespace
Namespace Animals

Dog is a class in the namespace Animals
Class Dog

Bark is a now a public, shared function in this class
 Public Shared Function Bark()
  Console.Writeline ("Dog is barking")
 End Function
 
 
Walk is a public function in this class. It is not shared
 Public Function Walk()
  Console.Writeline ("Dog is walking")
 End Function
 
End Class
End Namespace

Our Module
Public Module modMain

Execution will start from the Main() subroutine
Sub Main()
  
  
  'We can call the Bark() function directly,
  'with out creating an object of type Dog -
  'because it is shared.
   
  Animals.Dog.Bark()
  
  
  'We can call the Walk() function only
  'after creating an object, because
  'it is not shared.
  
  Dim Jimmy as Animals.Dog
  Jimmy=new Animals.Dog()
  Jimmy.Walk()
  
  'Now Guess? The WriteLine() function we used so far
  'is a shared function in class Console :)
  
  'Also, we can write the Main() function itself as a shared
  'function in a class. i.e Shared Sub Main(). Try
  'moving Main() from this module to the above class
  
  
End sub
End Module

Lesson 4: Over Loading

Overloading is a simple technique, to enable a single function name to accept parameters of different type.

Let us see a simple Adder class

Import the System namespace (already available in .NET)
Imports System
Class Adder

Here, we have two Add() functions.This one adds two integers.Convert.ToString is equivalent to good old Cstr
 Overloads Public Sub Add(A as Integer, B as Integer)
  Console.Writeline ("Adding Integers: " + Convert.ToString(a + b))
 End Sub
 

This one adds two strings
 Overloads Public Sub Add(A as String, B as String)
  Console.Writeline ("Adding Strings: " + a + b)
 End Sub
 
 'And both have the same name. This is possible because, we used the
 'Overloads keyword, to overload them.
 
 
 'Here, we have the Main Function with in this class. When you write.
 'your main function inside the class, it should be a shared function.
 
 Shared Sub Main()
  
  Dim AdderObj as Adder
  
  'Create the object
  AdderObj=new Adder
  
  'This will invoke first function
  AdderObj.Add(10,20)
  'This will invoke second function
  AdderObj.Add("hello"," how are you")
  
  
 End Sub
 
 
End Class

Lesson 5: Inheritance

Inheritance is the property in which, a derived classaquires the attributes of its base class. In simple terms,you can create or 'inherit' your own class (derived class),using an existing class (base class). You can use the Inherits keyword for this. 

Let us see a simple example.

Import the System namespace (already available in .NET)
Imports System

Our simple base class

Class Human
 'This is something that all humans do
 Public Sub Walk()
  Console.Writeline ("Walking")
 End Sub
  
End Class

Now, let us derive a class from human

A programmer IS_A Human
Class Programmer
 Inherits Human
 
 'We already have the above Walk() function
 
 'This is something that all programmers do ;)
 Public Sub StealCode()
  Console.Writeline ("Stealing code")
 End Sub

End Class
Just a main class
Class MainClass
 'Our main function
 Shared Sub Main()
  Dim Tom as Programmer
  Tom=new Programmer
  
  'This call is okie because programmer got this function
  'from its base class
  Tom.Walk()
  
  'This is also correct because Tom is a programmer
  Tom.StealCode()
 
 End Sub
End Class

Additional Notes:

MustInherit: The MustInherit keyword specifies that a class cannotbe instantiated and can be used only as a base class.i.e, if you declare our Human class as 'MustInherit Class Human' ,then you can't create object of type Human with out inheriting it.

NotInheritable: The NotInheritable keyword specifies that a class cannotbe inherited. I.e, if you specify 'NotInheritable Class Human', noderived classes can be made from Human class

Lesson 6: Overriding

By default, a derived class inherits methods from its base class. If an inherited property or method needs to behave differently inthe derived class it can be overridden; that is, you can define anew implementation of the method in the derived class.The 'Overridable' keyword is used to mark a function as overridable. The keyword 'Overrides' is used to mark that a functionis overriding some base class function.Let us see an example

Import the System namespace (already available in .NET)

Imports System

Our simple base class

Class Human
 'Speak() is declared Overridable
 Overridable Public Sub Speak()
  Console.Writeline ("Speaking")
 End Sub
  
End Class

Now, let us derive a class from human

An Indian IS_A Human
Class Indian
 Inherits Human
 
 'Let us make Indian speak Hindi, the National Language
 'in India
 
 'Speak() is overriding Speak() in its base class (Human)
 
 Overrides Public Sub Speak()
  Console.Writeline ("Speaking Hindi")
 'Important: As you expect, any call to Speak() inside this class
 'will invoke the Speak() in this class. If you need to
 'call Speak() in base class, you can use MyBase keyword.
 
 'Like this
 'Mybase.Speak()
 End Sub
 
 
 
 
End Class

 

Just a class to put our Main()

Class MainClass
 'Our main function
 Shared Sub Main()
  
  'Tom is a generic Human
  Dim Tom as Human
  Tom=new Human
  
  'Tony is a human and an Indian
  Dim Tony as Indian
  Tony=new Indian
  
  
  'This call will invoke the Speak() function
  'in class Human
  Tom.Speak()
  
  'This call will invoke the Speak() function
  'in class Indian
  Tony.Speak()
  
 End Sub
End Class

Lesson 7: Polymorphism

Polymorphism is the property in which a single object can take more than one forms. For example, if you have a base class named human, an object of human type can be used to hold an object of any of its derved type. When you call a function in your object, the system will automatically determine the type of the object to call the appropriate function. For example, let us assume that you have a function named speak() in your base class. You derived a child class from your base class and overloaded the function speak(). Then, you created a child class object and assigned it to a base class variable. Now, if you call speak() function using the base class variable, the speak() function defined in your child class will work. On the contrary, if you are assigning an object of the base class to the base class variable, the speak() function in base class will work. This is acheived through run time type identification of objects.  See the example. 

 Import the System namespace (already available in .NET)

Imports System

This example is exactly the same one we saw in previous lesson.The only difference is in the Shared Sub Main() in class MainClass. So scroll down.Let us see an example

Our simple base class

Class Human
 'Speak() is declared Overridable
 Overridable Public Sub Speak()
  Console.Writeline ("Speaking")
 End Sub
  
End Class

Now, let us derive a class from human

An Indian IS_A Human
Class Indian
 Inherits Human
 
 'Let us make Indian speak Hindi, the National Language
 'in India
 
 'Speak() is overriding Speak() in its base class (Human)
 
 Overrides Public Sub Speak()
  Console.Writeline ("Speaking Hindi")
 'Important: As you expect, any call to Speak() inside this class
 'will invoke the Speak() in this class. If you need to
 'call Speak() in base class, you can use MyBase keyword.
 
 'Like this
 'Mybase.Speak()
 End Sub
 
 
 
 
End Class

Carefully examine the code in Main()

Class MainClass
 'Our main function
 Shared Sub Main()
  
  'Let us define Tom as a human (base class)
  Dim Tom as Human
  
  'Now, I am assiging an Indian (derived class)
  Tom=new Indian
  
  'The above assignment is legal, because
  'Indian IS_A human.
  
  'Now, let me call Speak as
  Tom.Speak()
  
  'Which Speak() will work? The Speak() in Indian, or the
  'Speak() in human?
  
  'The question arises because, Tom is declared as a Human,
  'but an object of type Indian is assigned to Tom.
  
  'The Answer is, the Speak() in Indian will work. This is because,
  'most object oriented languages like Vb.net can automatically
  'detect the type of the object assigned to a base class variable.
  
  'This is called Polymorphism
  
  
 End Sub
End Class

 

Lesson 8: Constructors & Destructors

Import the System namespace (already available in .NET)
Imports System

A Constructor is a special function, which is calledautomatically when a class is created. In VB.NET, you should useNew() to create constructors as in the below examples.Constructors can be overloaded (see Lesson 4), but unlikein functions, the Overloads keyword is not required.A Destructor is a special function, which is calledautomatically when a class is Destroyed. In VB.NET, you should useFinalize() routine to create Destructors.They are similiar to Class_Initialize and Class_Terminatein VB 6.0

Dog is a class

Class Dog
 'The age variable
 Private Age as integer
 

The Default Constructor.
 Public Sub New()
  Console.Writeline ("Dog is Created With Age Zero")
  Age=0
 End Sub

The Parameterized Constructor
 Public Sub New(val as Integer)
  Console.Writeline ("Dog is Created With Age " + Convert.ToString(val))
  Age=val
 End Sub

This is the destructor.
 Overrides Protected Sub Finalize()
  Console.Writeline ("Dog is Destroyed")
 End Sub
 
 
 'The Main Function
 Shared Sub Main()
  Dim Jimmy, Jacky as Dog
  
  'Create the objects
  
  'This will call the default constructor
  Jimmy=new Dog
  
  'This will call the parameterized constructor
  Jacky=new Dog(10)
  
 End Sub
 
 'The Destruction will be done automatically, when
 'the program ends. This is done by the Garbage
 'Collector.
 
End Class

 

Lesson 9: Property Routines

You can use both properties and fields to store information in an object. Whereas fields are simply public variables, properties use property proceduresto control how values are set or returned.You can use the Get/Set keywords for getting/setting properties.See the following example.

Import the System namespace (already available in .NET)

Imports System

Dog is a class
Public Class Dog
 'A private variable to hold the value
 Private mAgeOfDog as Integer

This is our property routine
Public Property Age() As Integer

 'Called when someone tries to retreive the value
Get
 Console.Writeline ("Getting Property")
 Return mAgeOfdog
End Get

Set(ByVal Value As Integer)
 'Called when someone tries to assign a value  
 Console.Writeline ("Setting Property")
 mAgeOfDog=Value
End Set
End Property
 
End Class

Another Class

Class MainClass
 'Our main function. Execution starts here.
 Shared Sub Main()
 
 'Let us create an object.
 Dim Jimmy as Dog
 Jimmy=new Dog  
 
 'We can't access mAgeofDog directly, so we should
 'use Age() property routine.
 
 'Set it. The Age Set routine will work
 Jimmy.Age=30
 
 'Get it back. The Age GEt routine will work
 Dim curAge=Jimmy.Age()
  
 End Sub
End Class

Lesson 10: A Simple Program

Let us analyze a simple program. First, let us import required namespaces
Imports System
Imports System.ComponentModel
Imports System.Windows.Forms
Imports System.Drawing

 'We are inheriting a class named SimpleForm, from the
 'class System.Windows.Forms.Form
 '
 'i.e, Windows is a namespace in system, Forms is a
 'namespace in Windows, and Form is a class in Forms.
 
Public Class SimpleForm
Inherits System.Windows.Forms.Form
  'Our constructor
Public Sub New()
 'This will invoke the constructor of the base
 'class
MyBase.New()

Set the text property of this class. We inheritedthis property from the base class.
Me.Text = "Hello, How Are You?"
End Sub
End Class

Public Class MainClass
 Shared Sub Main()
  'Create an object from our SimpleForm class
  Dim sf as SimpleForm
  sf=new SimpleForm
  
  'Pass this object to the Run() function to start
   System.Windows.Forms.Application.Run(sf)
 End Sub
End Class

 

History

Nov 13,2004 - Prepared this article for publishing


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