Variables, Constants and Scope

Option Strict On

Module Program
    Const MaxLoanDays As Integer = 21
    Const Greeting As String = "Welcome to the shelf"
    Const LongLoanDays As Integer = MaxLoanDays * 2

    Sub Main()
        ' One Dim, several declarators: every name gets the type that follows As.
        Dim shelfCount, bookCount As Integer
        Dim title As String = Nothing, copies As Integer = 3

        ' Type inference (Option Infer On): the type comes from the initializer.
        Dim inferredInt = 42
        Dim inferredDouble = 4.5
        Dim inferredString = "Dune"
        Dim inferredDate = #1/15/2026#
        Dim nothingOnly = Nothing

        ' Default values: numbers 0, Boolean False, String/Object Nothing.
        Console.WriteLine("shelfCount = " & shelfCount & ", bookCount = " & bookCount)
        Console.WriteLine("title is Nothing: " & (title Is Nothing))
        Console.WriteLine("copies = " & copies)

        Console.WriteLine(shelfCount.GetType().Name & " " & bookCount.GetType().Name)
        Console.WriteLine(inferredInt.GetType().Name & " " & inferredDouble.GetType().Name & " " &
                          inferredString.GetType().Name & " " & inferredDate.GetType().Name)
        nothingOnly = "an Object variable accepts anything"      ' proves the inferred type is Object
        Console.WriteLine(nothingOnly)

        Console.WriteLine(Greeting & ": " & MaxLoanDays & " days, long loans " & LongLoanDays)
    End Sub
End Module

shelfCount = 0, bookCount = 0
title is Nothing: True
copies = 3
Int32 Int32
Int32 Double String DateTime
an Object variable accepts anything
Welcome to the shelf: 21 days, long loans 42

Option Strict On

Module Program
    Sub Main()
        Dim total As Integer = 1
        Dim Total As Integer = 2
        Console.WriteLine(total)
    End Sub
End Module

Program.vb(6,13): error BC30288: Local variable 'Total' is already declared in the current block.

Option Strict On

Module Program
    Sub Main()
        Dim list = {1, 2, 3}                                  ' Integer()
        Dim item = New With {.Title = "Dune", .Pages = 412}   ' anonymous type
        Dim ratio = 7 / 2                                     ' Double
        Dim big = 3000000000                                  ' Long: does not fit Integer
        Dim letter = "A"c                                     ' Char
        Dim price = 9.99D                                     ' Decimal

        Console.WriteLine(TypeName(list) & ", " & TypeName(ratio) & ", " & TypeName(big) & ", " &
                          TypeName(letter) & ", " & TypeName(price))
        Console.WriteLine(item.Title & " " & item.Pages)
        For Each entry In list
            Console.Write(TypeName(entry) & " ")
        Next
        Console.WriteLine()
    End Sub
End Module

Integer(), Double, Long, Char, Decimal
Dune 412
Integer Integer Integer

Option Strict On
Option Infer Off

Module Program
    Sub Main()
        Dim x = 5
        Console.WriteLine(x)
    End Sub
End Module

Program.vb(6,13): error BC30209: Option Strict On requires all variable declarations to have an 'As' clause.

Option Strict On

Public Class LoanPolicy
    Public Const DefaultDays As Integer = 21
    Public Shared ReadOnly DefaultPeriod As TimeSpan = TimeSpan.FromDays(DefaultDays)
    Public ReadOnly Created As Date
    Private ReadOnly _limit As Integer

    Public Sub New(limit As Integer)
        _limit = limit                     ' ReadOnly: assignable here ...
        Created = New Date(2026, 10, 19)
    End Sub

    Public ReadOnly Property Limit As Integer
        Get
            Return _limit
        End Get
    End Property

    Public Sub Change()
        ' _limit = 5                       ' ... but not here: BC30064
    End Sub
End Class

Module Program
    Sub Main()
        Dim policy As New LoanPolicy(5)
        Console.WriteLine("DefaultDays   = " & LoanPolicy.DefaultDays)
        Console.WriteLine("DefaultPeriod = " & LoanPolicy.DefaultPeriod.ToString())
        Console.WriteLine("Limit         = " & policy.Limit)
        Console.WriteLine("Created       = " & policy.Created.ToString("yyyy-MM-dd"))
    End Sub
End Module

DefaultDays   = 21
DefaultPeriod = 21.00:00:00
Limit         = 5
Created       = 2026-10-19

Option Strict On

Public Class Policy
    Public ReadOnly Limit As Integer = 5

    Public Sub Change()
        Limit = 6
    End Sub
End Class

Module Program
    Sub Main()
        Console.WriteLine(New Policy().Limit)
    End Sub
End Module

Program.vb(7,9): error BC30064: 'ReadOnly' variable cannot be the target of an assignment.

Option Strict On

Public Class Counter
    Private count As Integer = 100          ' field: lives as long as the object

    Public Sub Show()
        Dim count As Integer = 1            ' local shadows the field
        Console.WriteLine("local count      = " & count)
        Console.WriteLine("Me.count (field) = " & Me.count)
        If count > 0 Then
            Dim inner As Integer = count + 1
            Console.WriteLine("block-level inner = " & inner)
        End If
        ' Console.WriteLine(inner)         ' BC30451: 'inner' is not declared (out of scope)
    End Sub
End Class

Module Program
    Private moduleLevel As Integer = 7      ' module scope

    Sub Main()
        Dim c As New Counter()
        c.Show()
        Console.WriteLine("moduleLevel = " & moduleLevel)

        ' A For control variable is scoped to the loop.
        For i As Integer = 1 To 2
            Console.WriteLine("i = " & i)
        Next
        For i As Integer = 5 To 6           ' a second loop may reuse the name
            Console.WriteLine("second loop i = " & i)
        Next
    End Sub
End Module

local count      = 1
Me.count (field) = 100
block-level inner = 2
moduleLevel = 7
i = 1
i = 2
second loop i = 5
second loop i = 6

Option Strict On

Module Program
    Sub Main()
        Dim n As Integer = 1
        If n > 0 Then
            Dim n As Integer = 2
            Console.WriteLine(n)
        End If
    End Sub
End Module

Program.vb(7,17): error BC30616: Variable 'n' hides a variable in an enclosing block.

Option Strict On

Module Program
    Sub Main()
        For i As Integer = 1 To 2
            Console.WriteLine("i = " & i)
        Next
        Dim i As Integer = 99
        Console.WriteLine(i)
    End Sub
End Module

Program.vb(5,13): error BC30616: Variable 'i' hides a variable in an enclosing block.

Option Strict On

Module Program
    Sub Main()
        Console.WriteLine(later)
        Dim later As Integer = 5
        Dim number = 5
        number = "five"
    End Sub
End Module

Program.vb(5,27): error BC32000: Local variable 'later' cannot be referred to before it is declared.
Program.vb(8,18): error BC30512: Option Strict On disallows implicit conversions from 'String' to 'Integer'.

Option Strict On

Module Program
    Sub Main()
        Dim readings As Integer() = {3, 4, 5}

        Console.WriteLine("Without an initializer the total survives every iteration:")
        For Each reading In readings
            Dim total As Integer          ' not reset on each pass
            total += reading
            Console.WriteLine("  total = " & total)
        Next

        Console.WriteLine("With an initializer the variable is reset on each pass:")
        For Each reading In readings
            Dim total As Integer = 0
            total += reading
            Console.WriteLine("  total = " & total)
        Next

        Console.WriteLine("The same trap with a While loop:")
        Dim i As Integer = 0
        While i < 3
            Dim copies As Integer
            copies += 1
            Console.WriteLine("  copies = " & copies)
            i += 1
        End While
    End Sub
End Module

Without an initializer the total survives every iteration:
  total = 3
  total = 7
  total = 12
With an initializer the variable is reset on each pass:
  total = 3
  total = 4
  total = 5
The same trap with a While loop:
  copies = 1
  copies = 2
  copies = 3

Option Strict On

Module Program
    Private Function NextTicket() As Integer
        Static issued As Integer = 1000     ' initialized once, survives calls
        issued += 1
        Return issued
    End Function

    Private Sub Visit()
        Static visits As Integer
        Dim local As Integer                ' ordinary local: reset each call
        visits += 1
        local += 1
        Console.WriteLine("visits = " & visits & ", local = " & local)
    End Sub

    Sub Main()
        Console.WriteLine(NextTicket() & " " & NextTicket() & " " & NextTicket())
        Visit()
        Visit()
        Visit()
    End Sub
End Module

1001 1002 1003
visits = 1, local = 1
visits = 2, local = 1
visits = 3, local = 1

Option Strict On

Public Structure Tally
    Public Sub Increment()
        Static calls As Integer
        calls += 1
    End Sub
End Structure

Module Program
    Sub Main()
        Dim a, b As Integer = 5
        Console.WriteLine(a & b)
    End Sub
End Module

Program.vb(5,9): error BC31400: Local variables within methods of structures cannot be declared 'Static'.
Program.vb(12,13): error BC30671: Explicit initialization is not permitted with multiple variables declared with a single type specifier.

Option Strict On

Imports System.Reflection

Friend Class Container
    Dim implicitField As Integer
    Const ImplicitConst As Integer = 1
    Sub ImplicitSub()
    End Sub
    Property ImplicitProperty As Integer
    Event ImplicitEvent()
End Class

Friend Structure Pair
    Dim implicitField As Integer
    Sub ImplicitSub()
    End Sub
End Structure

Public Interface IImplicit
    Sub Member()
End Interface

Class UndeclaredLevelClass
End Class

Enum UndeclaredLevelEnum
    A
End Enum

Module Program
    Private Function Access(member As MemberInfo) As String
        Dim f = TryCast(member, FieldInfo)
        If f IsNot Nothing Then Return If(f.IsPrivate, "Private", If(f.IsPublic, "Public", If(f.IsAssembly, "Friend", "other")))
        Dim m = TryCast(member, MethodBase)
        If m IsNot Nothing Then Return If(m.IsPrivate, "Private", If(m.IsPublic, "Public", If(m.IsAssembly, "Friend", "other")))
        Return "n/a"
    End Function

    Sub Main()
        Const flags As BindingFlags = BindingFlags.Instance Or BindingFlags.Static Or BindingFlags.Public Or BindingFlags.NonPublic Or BindingFlags.DeclaredOnly
        For Each t In {GetType(Container), GetType(Pair)}
            Console.WriteLine(t.Name & " (" & If(t.IsNotPublic, "Friend", "Public") & ")")
            For Each member In t.GetMembers(flags).OrderBy(Function(x) x.Name)
                If member.MemberType = MemberTypes.Field OrElse member.MemberType = MemberTypes.Method Then
                    If member.Name.StartsWith("_") OrElse member.Name.StartsWith("get_") OrElse member.Name.StartsWith("set_") OrElse member.Name = ".ctor" Then Continue For
                    Console.WriteLine("  " & member.MemberType.ToString().PadRight(7) & member.Name.PadRight(22) & Access(member))
                End If
            Next
        Next
        Console.WriteLine("UndeclaredLevelClass is " & If(GetType(UndeclaredLevelClass).IsNotPublic, "Friend", "Public"))
        Console.WriteLine("UndeclaredLevelEnum  is " & If(GetType(UndeclaredLevelEnum).IsNotPublic, "Friend", "Public"))
        Dim p = GetType(Container).GetProperty("ImplicitProperty")
        Console.WriteLine("ImplicitProperty getter: " & If(p.GetMethod.IsPublic, "Public", "not public"))
        Console.WriteLine("ImplicitEvent add: " & If(GetType(Container).GetEvent("ImplicitEvent").AddMethod.IsPublic, "Public", "not public"))
    End Sub
End Module

Container (Friend)
  Method add_ImplicitEvent     Public
  Field  ImplicitConst         Private
  Field  ImplicitEventEvent    Private
  Field  implicitField         Private
  Method ImplicitSub           Public
  Method remove_ImplicitEvent  Public
Pair (Friend)
  Field  implicitField         Public
  Method ImplicitSub           Public
UndeclaredLevelClass is Friend
UndeclaredLevelEnum  is Friend
ImplicitProperty getter: Public
ImplicitEvent add: Public

Option Strict On

Imports System.Reflection

Public Class Outer
    Class NestedClass
    End Class
    Structure NestedStructure
    End Structure
    Enum NestedEnum
        A
    End Enum
    Sub New()
    End Sub
    Shared Sub New()
    End Sub
    Dim OuterField As Integer
End Class

Structure TopStructure
End Structure

Module TopModule
    Dim moduleField As Integer
    Sub ModuleSub()
    End Sub
End Module

Interface ITop
End Interface

Delegate Sub TopDelegate()

Module Program
    Private Function Vis(t As Type) As String
        If t.IsNested Then Return If(t.IsNestedPublic, "Public", If(t.IsNestedAssembly, "Friend", If(t.IsNestedPrivate, "Private", "other")))
        Return If(t.IsPublic, "Public", "Friend")
    End Function

    Sub Main()
        For Each t In {GetType(Outer), GetType(TopStructure), GetType(TopModule), GetType(ITop), GetType(TopDelegate),
                       GetType(Outer.NestedClass), GetType(Outer.NestedStructure), GetType(Outer.NestedEnum)}
            Console.WriteLine(t.Name.PadRight(18) & Vis(t))
        Next
        Const all As BindingFlags = BindingFlags.Instance Or BindingFlags.Static Or BindingFlags.Public Or BindingFlags.NonPublic Or BindingFlags.DeclaredOnly
        For Each c In GetType(Outer).GetConstructors(all)
            Console.WriteLine("constructor (" & If(c.IsStatic, "Shared", "instance") & ") " & If(c.IsPublic, "Public", If(c.IsPrivate, "Private", "other")))
        Next
        Console.WriteLine("Outer.OuterField: " & If(GetType(Outer).GetField("OuterField", all).IsPrivate, "Private", "not private"))
        Console.WriteLine("TopModule.moduleField: " & If(GetType(TopModule).GetField("moduleField", all).IsPrivate, "Private", "not private"))
        Console.WriteLine("TopModule.ModuleSub: " & If(GetType(TopModule).GetMethod("ModuleSub", all).IsPublic, "Public", "not public"))
    End Sub
End Module

Outer             Public
TopStructure      Friend
TopModule         Friend
ITop              Friend
TopDelegate       Friend
NestedClass       Public
NestedStructure   Public
NestedEnum        Public
constructor (instance) Public
constructor (Shared) Private
Outer.OuterField: Private
TopModule.moduleField: Private
TopModule.ModuleSub: Public

Option Strict On

Namespace Global.Shelf.Demo
    Public Class [Class]
        Public Property [Select] As String = "reserved word as a name"
    End Class
End Namespace

Module Program
    Sub Main()
        Dim item As New Shelf.Demo.[Class]()
        Console.WriteLine(item.[Select])

        ' Global.System.String reaches the real System namespace even if a local name hides it.
        Dim s As Global.System.String = "hello"
        Console.WriteLine(s.ToUpperInvariant())
    End Sub
End Module

reserved word as a name
HELLO

Option Strict On

Module Program
    Sub Main()
        Dim o As Object = "text"
        Console.WriteLine(o.Length)
    End Sub
End Module

Program.vb(6,27): error BC30574: Option Strict On disallows late binding.