Variables and Constants

' ── Explicit type declaration ──────────────────────────
Dim age      As Integer          ' default value: 0
Dim price    As Decimal          ' default value: 0D
Dim name     As String           ' default value: Nothing
Dim isActive As Boolean          ' default value: False

' ── Declaration with initialisation ───────────────────
Dim score     As Integer = 100
Dim taxRate   As Decimal = 0.06D
Dim greeting  As String  = "Hello, World!"
Dim today     As Date    = DateTime.Today

' ── Type inference (compiler deduces type) ─────────────
Dim count    = 42                  ' Integer
Dim pi       = 3.14159            ' Double
Dim city     = "Kuala Lumpur"     ' String

' ── Multiple declarations on one line ──────────────────
Dim x As Integer, y As Integer, z As Integer
Dim firstName As String, lastName As String

' ── Re-assigning values ─────────────────────────────────
score = 200
name  = "Alice"
age   = age + 1        ' increment
age  += 1              ' shorthand compound assignment

' ── Module-level (global across all forms) ──────────────
' In a separate Module file:
Module AppGlobals
    Public AppName  As String = "My App 2026"
    Public Version  As String = "1.0.0"
End Module

' ── Form-level (shared between all subs in this form) ───
Public Class Form1

    Private _clickCount  As Integer = 0      ' form-level: persists between clicks
    Private _userName    As String  = ""     ' form-level: accessible by all handlers
    Private _totalSales  As Decimal = 0D    ' form-level: accumulates across events

    ' ── Local scope — lives only inside this Sub ────────────
    Private Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click
        Dim amount As Decimal               ' local: only exists in this Sub
        If Decimal.TryParse(txtAmount.Text, amount) Then
            _totalSales += amount           ' update form-level variable
            _clickCount += 1
            lblTotal.Text = $"Total: {_totalSales:C}  ({_clickCount} sales)"
        End If
    End Sub

    Private Sub btnReset_Click(sender As Object, e As EventArgs) Handles btnReset.Click
        _totalSales = 0D                  ' both handlers can access form-level vars
        _clickCount  = 0
        lblTotal.Text = "Total: RM 0.00  (0 sales)"
    End Sub

End Class

Private Sub btnClick_Click(sender As Object, e As EventArgs) Handles btnClick.Click
    ' Static — retains value between button clicks
    Static clickCount As Integer = 0
    clickCount += 1
    lblCount.Text = $"Button clicked {clickCount} time(s)"
End Sub

' Compare: regular Dim resets to 0 every click
Private Sub btnClickNormal_Click(sender As Object, e As EventArgs) Handles btnClickNormal.Click
    Dim clickCount As Integer = 0       ' always resets — always shows "1"
    clickCount += 1
    lblNormal.Text = $"Count: {clickCount}"   ' always "Count: 1"
End Sub

' ── Local constants (inside a Sub/Function) ────────────
Private Sub CalcCircle_Click(sender As Object, e As EventArgs) Handles btnCalc.Click
    Const Pi          As Double  = 3.14159265358979
    Const MaxDiameter  As Integer = 1000

    Dim radius As Double
    If Double.TryParse(txtRadius.Text, radius) Then
        Dim area          = Pi * radius ^ 2
        Dim circumference = 2 * Pi * radius
        lblArea.Text = $"Area: {area:F4}  |  Circumference: {circumference:F4}"
    End If
End Sub

' ── Form-level constants (shared by all procedures) ────
Public Class Form1

    Private Const MaxStudents  As Integer = 40
    Private Const PassMark     As Integer = 50
    Private Const GSTRate      As Decimal = 0.06D
    Private Const AppTitle     As String  = "Student Portal 2026"

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Me.Text = AppTitle
        lblMax.Text = $"Max students: {MaxStudents}"
    End Sub

    Private Sub btnCheck_Click(sender As Object, e As EventArgs) Handles btnCheck.Click
        Dim mark As Integer
        If Integer.TryParse(txtMark.Text, mark) Then
            Dim result = If(mark >= PassMark, "PASS ✔", "FAIL ✘")
            lblResult.Text = $"{mark}/100 — {result} (Pass mark: {PassMark})"
        End If
    End Sub

End Class

' AppConstants.vb — a dedicated Module for all app-wide constants
Module AppConstants

    ' Application identity
    Public Const AppName       As String  = "School Management 2026"
    Public Const Version       As String  = "3.1.0"
    Public Const CompanyName   As String  = "EduSoft Sdn Bhd"

    ' Business rules — change here to affect the whole app
    Public Const MaxStudents   As Integer = 40
    Public Const PassMark      As Integer = 50
    Public Const DistinctionMark As Integer = 75
    Public Const GSTRate       As Decimal = 0.06D
    Public Const MaxLoginAttempts As Integer = 3

    ' Mathematical constants
    Public Const Pi            As Double  = 3.14159265358979
    Public Const GoldenRatio   As Double  = 1.61803398874989

End Module

' Usage in any form — reference directly by name
' Me.Text = AppConstants.AppName
' lblPass.Text = $"Pass mark: {AppConstants.PassMark}"

Public Class Form1

    ' ReadOnly — set in the constructor or field initialiser, then fixed
    Private ReadOnly _appStartTime As Date    = DateTime.Now
    Private ReadOnly _machineId    As String = Environment.MachineName
    Private ReadOnly _maxItems     As Integer

    Public Sub New()    ' Constructor — ReadOnly can be set here too
        InitializeComponent()
        _maxItems = GetMaxItemsFromConfig()
    End Sub

    Private Function GetMaxItemsFromConfig() As Integer
        Return 100   ' could read from file, database, registry...
    End Function

End Class

' Define an Enum — usually at module/class level
Enum DayOfWeek
    Monday    = 1
    Tuesday   = 2
    Wednesday = 3
    Thursday  = 4
    Friday    = 5
    Saturday  = 6
    Sunday    = 7
End Enum

Enum StudentGrade
    Fail        = 0    ' 0–49
    Pass        = 1    ' 50–64
    Credit      = 2    ' 65–74
    Distinction = 3    ' 75–84
    HighDist    = 4    ' 85–100
End Enum

' Using enums — much clearer than using raw integers
Private Sub btnGrade_Click(sender As Object, e As EventArgs) Handles btnGrade.Click
    Dim mark As Integer
    If Not Integer.TryParse(txtMark.Text, mark) Then Return

    Dim grade As StudentGrade
    Select Case mark
        Case 0 To 49  : grade = StudentGrade.Fail
        Case 50 To 64 : grade = StudentGrade.Pass
        Case 65 To 74 : grade = StudentGrade.Credit
        Case 75 To 84 : grade = StudentGrade.Distinction
        Case Else      : grade = StudentGrade.HighDist
    End Select

    lblGrade.Text = $"Mark: {mark} → Grade: {grade}"  ' displays "Grade: Distinction"
End Sub