INTRODUCTION TO VB.NET PART 2

Before we study basic building blocks of the VB.Net programming language, let us look a bare minimum VB.Net program structure so that we can take it as a reference in upcoming chapters.

VB.Net Hello World Example

A VB.Net program basically consists of the following parts:

• Namespace declaration
• A class or module
• One or more procedures
• Variables
• The Main procedure
• Statements & Expressions
• Comments

Let us look at a simple code that would print the words "Hello World":

Imports System
Module Module1
   'This program will display Hello World 
   Sub Main()
      Console.WriteLine("Hello World")
      Console.ReadKey()
   End Sub
End Module

When the above code is compiled and executed, it produces the following result:

Hello, World!

Let us look various parts of the above program:

• The first line of the program Imports System is used to include the System namespace in the program.
• The next line has a Module declaration, the module Module1. VB.Net is completely object oriented, so every program must contain a module of a class that contains the data and procedures that your program uses.
• Classes or Modules generally would contain more than one procedure. Procedures contain the executable code, or in other words, they define the behavior of the class. A procedure could be any of the following:
  
  • Function
  • Sub
  • Operator
  • Get
  • Set
  • AddHandler
  • RemoveHandler
  • RaiseEvent
• The next line( 'This program) will be ignored by the compiler and it has been put to add additional comments in the program.
• The next line defines the Main procedure, which is the entry point for all VB.Net programs. The Main procedure states what the module or class will do when executed.
• The Main procedure specifies its behavior with the statement
  Console.WriteLine("Hello World")
  WriteLine is a method of the Console class defined in the System namespace. This statement causes the message "Hello, World!" to be displayed on the screen.
• The last line Console.ReadKey() is for the VS.NET Users. This will prevent the screen from running and closing quickly when the program is launched from Visual Studio .NET.

Compile & Execute VB.Net Program:

If you are using Visual Studio.Net IDE, take the following steps:

• Start Visual Studio.
• On the menu bar, choose File, New, Project.
• Choose Visual Basic from templates
• Choose Console Application.
• Specify a name and location for your project using the Browse button, and then choose the OK button.
• The new project appears in Solution Explorer.
• Write code in the Code Editor.
• Click the Run button or the F5 key to run the project. A Command Prompt window appears that contains the line Hello World.

You can compile a VB.Net program by using the command line instead of the Visual Studio IDE:

• Open a text editor and add the above mentioned code.
• Save the file as helloworld.vb
• Open the command prompt tool and go to the directory where you saved the file.
• Type vbc helloworld.vb and press enter to compile your code.
• If there are no errors in your code the command prompt will take you to the next line and would generate helloworld.exe executable file.
• Next, type helloworld to execute your program.
• You will be able to see "Hello World" printed on the screen.
  
  
  VB.Net is an object-oriented programming language. In Object-Oriented Programming methodology, a program consists of various objects that interact with each other by means of actions. The actions that an object may take are called methods. Objects of the same kind are said to have the same type or, more often, are said to be in the same class.
  When we consider a VB.Net program, it can be defined as a collection of objects that communicate via invoking each other's methods. Let us now briefly look into what do class, object, methods and instance variables mean.
  
• Object - Objects have states and behaviors. Example: A dog has states - color, name, breed as well as behaviors - wagging, barking, eating, etc. An object is an instance of a class.
• Class - A class can be defined as a template/blueprint that describes the behaviors/states that objects of its type support.
• Methods - A method is basically a behavior. A class can contain many methods. It is in methods where the logics are written, data is manipulated and all the actions are executed.
• Instance Variables - Each object has its unique set of instance variables. An object's state is created by the values assigned to these instance variables.

A Rectangle Class in VB.Net

For example, let us consider a Rectangle object. It has attributes like length and width. Depending upon the design, it may need ways for accepting the values of these attributes, calculating area and displaying details.
Let us look at an implementation of a Rectangle class and discuss VB.Net basic syntax on the basis of our observations in it:

Imports System
Public Class Rectangle
    Private length As Double
    Private width As Double

    'Public methods
    Public Sub AcceptDetails()
        length = 4.5
        width = 3.5
    End Sub

    Public Function GetArea() As Double
        GetArea = length * width
    End Function
    Public Sub Display()
        Console.WriteLine("Length: {0}", length)
        Console.WriteLine("Width: {0}", width)
        Console.WriteLine("Area: {0}", GetArea())

    End Sub

    Shared Sub Main()
        Dim r As New Rectangle()
        r.Acceptdetails()
        r.Display()
        Console.ReadLine()
    End Sub
End Class

When the above code is compiled and executed, it produces the following result:

Length: 4.5
Width: 3.5
Area: 15.75

In previous chapter, we created a Visual Basic module that held the code. Sub Main indicates the entry point of VB.Net program. Here, we are using Class that contains both code and data. You use classes to create objects. For example, in the code, r is a Rectangle object.
An object is an instance of a class:

Dim r As New Rectangle()

A class may have members that can be accessible from outside class, if so specified. Data members are called fields and procedure members are called methods.
Shared methods or static methods can be invoked without creating an object of the class. Instance methods are invoked through an object of the class:

Shared Sub Main()
   Dim r As New Rectangle()
   r.Acceptdetails()
   r.Display()
   Console.ReadLine()
End Sub

Identifiers

An identifier is a name used to identify a class, variable, function, or any other user-defined item. The basic rules for naming classes in VB.Net are as follows:

• A name must begin with a letter that could be followed by a sequence of letters, digits (0 - 9) or underscore. The first character in an identifier cannot be a digit.
• It must not contain any embedded space or symbol like ? - +! @ # % ^ & * ( ) [ ] { } . ; : " ' / and \. However, an underscore ( _ ) can be used.
• It should not be a reserved keyword.

VB.Net Keywords

The following table lists the VB.Net reserved keywords:

AddHandler	AddressOf	Alias	And	AndAlso	As	Boolean
ByRef	Byte	ByVal	Call	Case	Catch	CBool
CByte	CChar	CDate	CDec	CDbl	Char	CInt
Class	CLng	CObj	Const	Continue	CSByte	CShort
CSng	CStr	CType	CUInt	CULng	CUShort	Date
Decimal	Declare	Default	Delegate	Dim	DirectCast	Do
Double	Each	Else	ElseIf	End	End If	Enum
Erase	Error	Event	Exit	False	Finally	For
Friend	Function	Get	GetType	GetXML Namespace	Global	GoTo
Handles	If	Implements	Imports	In	Inherits	Integer
Interface	Is	IsNot	Let	Lib	Like	Long
Loop	Me	Mod	Module	MustInherit	MustOverride	MyBase
MyClass	Namespace	Narrowing	New	Next	Not	Nothing
Not Inheritable	Not Overridable	Object	Of	On	Operator	Option
Optional	Or	OrElse	Overloads	Overridable	Overrides	ParamArray
Partial	Private	Property	Protected	Public	RaiseEvent	ReadOnly
ReDim	REM	Remove Handler	Resume	Return	SByte	Select
Set	Shadows	Shared	Short	Single	Static	Step
Stop	String	Structure	Sub	SyncLock	Then	Throw
To	True	Try	TryCast	TypeOf	UInteger	While
Widening	With	WithEvents	WriteOnly	Xor