AddHandler Visual Basic Example

VB.NET > Statements > AddHandler

AddHandler link an event with an event handler at run time.

Example:

1. Add event to an object

Public Class Form1

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim Obj As New Class1
        ' Associate an event handler with an event
        AddHandler Obj.myEvent, AddressOf EvHandler
        Obj.Class1Event()
    End Sub

    Sub EvHandler()

        ' Handle the event
        MsgBox("EvHandler")
    End Sub

    Public Class Class1
        Public Event myEvent()
        Public Sub Class1Event()
            RaiseEvent myEvent()
        End Sub
    End Class

End Class

2. Handle events for dynamic runtime control

Visual Basic Statements

VB.NET > Statements

AddHandler	Call	Class	Const
Continue	Declare	Delegate	Dim
Do...Loop	Else	End	End
Enum	Erase	Error	Event
Exit

RaiseEvent	ReDim	REM	RemoveHandler
Resume	Return	Select...Case	Set
Stop	Structure	Sub	SyncLock
Then	Throw	Try...Catch...Finally	Using
While...End While	With...End With	Yield

Form VB.NET

VB.NET > Form

Form

Is a representation of any window displayed in an application.

Type

• standard
• tool
• borderless
• floating

Example

Create dialog box in runtime

Dim form1 As New Form()

Dim btnYes As New Button()

Dim btnNo As New Button()
btnYes.Text = "Yes"
btnYes.Location = New Point(5, 5)
btnNo.Text = "No"
btnNo.Location = New Point(btnYes.Left, btnYes.Top + 30)

form1.Controls.Add(btnYes)

form1.Controls.Add(btnNo)

form1.Text = "Confirm Dialog Box"

form1.FormBorderStyle = FormBorderStyle.FixedDialog
form1.MaximizeBox = False
form1.MinimizeBox = False
form1.AcceptButton = btnYes
form1.CancelButton = btnNo
form1.StartPosition = FormStartPosition.CenterScreen

form1.ShowDialog()

Object-Oriented Programming VB.NET

VB.NET > Object-Oriented Programming VB.NET

Visual Basic .NET has been entirely rewritten to be fully object-oriented and everything in Visual Basic .NET can be treated as an object.

OOP language:

• Abstraction
• Encapsulation
• Polymorphism
• Inheritance

Examplea

• Abstract class employee 
• Class with events

System Namespace VB.NET

VB.NET > System namespace

System namespace in VB.NET contains fundamental classes and base classes.

String

•  Concat

Visual Basic run-time member functions

VB.NET  > Run-time member functions

AppActivate	Asc	AscW	Beep
CallByName	CBool	CByte	CChar
CDate	CDbl	CDec	ChDir
ChDrive	Choose	Chr	ChrW
CInt	CLng	CObj	Command
CreateObject	CShort	CSng	CStr
CType	CurDir

DateAdd	DateDiff	DatePart	DateSerial
DateValue	Day	DDB	DeleteSetting
Derived Math	Dir	Environ	EOF
ErrorToString	FileAttr	FileClose	FileCopy
FileDateTime	FileGet	FileGetObject	FileLen
FileOpen	FilePut	FilePutObject	FileWidth
Filter	Fix	Format	FormatCurrency
FormatDateTime	FormatNumber	FormatPercent	FreeFile
FV	GetAllSettings	GetAttr	GetChar
GetException	GetObject	GetSetting

What is Visual Basic .NET

Visual Basic .NET is the result of a significant rebuild of Visual Basic for the Microsoft .NET Framework.
Visual Basic .NET gives you access to system resources that in the past required the use of languages like C++. One of the most important additions is object inheritance.

• System 
• Run time functions
• Object-Oriented Programming
• Form
• Statements

Read Connection String From Web.Config VB.Net

VB.NET > Data > SqlConnection > ConnectionString

 

Dim con As SqlConnection

Dim conString As String = System.Configuration.ConfigurationManager.ConnectionStrings("ConnectionString").ToString
con = New SqlConnection(conString)
con.Open()

Asc Method Visual Basic

VB.NET > Functions > ASC

Asc Method returns an Integer value of character code corresponding to a character.

Example:

Dim IntCode As Integer
IntCode = Asc("A") '65
IntCode = Asc("a") '97
IntCode = Asc("@") '64
IntCode = Asc("?") '93

CType Function Visual Basic NET

VB.NET > Functions > CType

CType function returns the result of explicitly converting an expression to a specified data type.

Example:

Public Class Form1

  Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Dim mStr As String = "100"
    Dim mInt As Integer = CType(mStr, Integer) 'convert "100" string to mInt = 100 OK

    mStr = "aaa"
    mInt = CType(mStr, Integer) ' Error: Conversion from string "aaa" to type 'Integer' is not valid. NOK
  End Sub
End Class

Abstract classes, MustInherit, MustOverride , Overrides VB NET

VB.NET > OOP > Abstract Class

Abstract classes specify members that must be implemented in the inheriting classes. Any class can inherit only from one abstract class. You we cannot create objects of an abstract class.

MustInherit

Specifies that a class can be used only as a base class and that you cannot create an object directly from it.

MustOverride  

Specifies that a property or procedure is not implemented in this class and must be overridden in a derived class before it can be used.

Overrides

Specifies that a property or procedure overrides an identically named property or procedure inherited from a base class.




Example:

Public Class Form1

  Public MustInherit Class Employee

    Private mFirstName As String
    Private mLastName As String

    Public Sub New(ByVal FirstName, ByVal LastName)
      mFirstName = FirstName
      mLastName = LastName
    End Sub

    Public Property FirstName() As String
      Get
        Return mFirstName
      End Get
      Set(ByVal value As String)
        mFirstName = value
      End Set
    End Property

    Public Property LastName() As String

      Get
        Return mLastName
      End Get
      Set(ByVal value As String)
        mLastName = value
      End Set
    End Property

    Public MustOverride Function Income() As Double
  End Class

  Public Class Teacher

    Inherits Employee

    Private mSalary As Double

    Public Sub New(ByVal FirstName, ByVal Lastname, ByVal salary)
      MyBase.New(FirstName, Lastname)
      mSalary = salary
    End Sub

    Public Property Salary() As String

      Get
        Return mSalary
      End Get
      Set(ByVal value As String)
        mSalary = value
      End Set
    End Property

    Public Overrides Function Income() As Double

      Return Salary() * 1.2
    End Function
  End Class

  Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    Dim mTeacher As New Teacher("John", "Smith", 100)
    Dim sal = mTeacher.Income ' 120
  End Sub

End Class