Procedures: Sub and Function

Option Strict On

Module Program
    Private Function FirstLetter(text As String) As Char
        If text.Length > 0 Then
            Return text(0)
        End If
        ' no Return on this path: the function returns the default value of Char
    End Function

    Private Function Find(titles As String(), wanted As String) As String
        For Each t In titles
            If t = wanted Then Return t
        Next
    End Function

    Private Function Count(values As Integer()) As Integer
        For Each v In values
            Count += v
        Next
        ' falls off the end: returns the running value of the implicit local
    End Function

    Sub Main()
        Console.WriteLine("[" & AscW(FirstLetter("")).ToString() & "]")
        Console.WriteLine(Find({"Dune", "Emma"}, "Ulysses") Is Nothing)
        Console.WriteLine(Count({1, 2, 3}))
    End Sub
End Module

/w/e2/Program.vb(15,5): warning BC42105: Function 'Find' doesn't return a value on all code paths. A null reference exception could occur at run time when the result is used.
/w/e2/Program.vb(22,5): warning BC42353: Function 'Count' doesn't return a value on all code paths. Are you missing a 'Return' statement?
/w/e2/Program.vb(9,5): warning BC42353: Function 'FirstLetter' doesn't return a value on all code paths. Are you missing a 'Return' statement?

[0]
True
6

Option Strict On

Module Program
    ' Main may be a Function that returns the process exit code.
    Function Main(args As String()) As Integer
        If args.Length = 0 Then
            Console.Error.WriteLine("usage: shelf <isbn>")
            Return 2
        End If
        Console.WriteLine("looking up " & args(0))
        Return 0
    End Function
End Module

usage: shelf <isbn>
looking up 9780441172719

Option Strict On

Public Class Loan
    Public Property Copies As Integer
    Public Property Title As String = ""
End Class

Public Structure Slot
    Public Copies As Integer
End Structure

Module Program
    Private Sub Fill(slot As Slot)
        slot.Copies = 99                  ' changes the copy
    End Sub

    Private Sub FillByRef(ByRef slot As Slot)
        slot.Copies = 99                  ' changes the caller's structure
    End Sub

    Private Sub Grow(items As Integer())
        items(0) = 100                    ' the array is shared
        ReDim items(5)                    ' ReDim creates a new array: only the local reference changes
    End Sub

    Private Sub AddByVal(n As Integer)            ' ByVal is the default
        n += 10
        Console.WriteLine("  inside AddByVal: " & n)
    End Sub

    Private Sub AddByRef(ByRef n As Integer)
        n += 10
        Console.WriteLine("  inside AddByRef: " & n)
    End Sub

    Private Sub RenameByVal(loan As Loan)
        loan.Title = "renamed"                    ' mutates the shared object
        loan = New Loan() With {.Title = "replaced"}   ' rebinds only the local copy of the reference
    End Sub

    Private Sub RenameByRef(ByRef loan As Loan)
        loan.Title = "renamed"
        loan = New Loan() With {.Title = "replaced"}   ' rebinds the caller's variable
    End Sub

    Private Sub Swap(Of T)(ByRef a As T, ByRef b As T)
        Dim temp As T = a
        a = b
        b = temp
    End Sub

    Private Sub AddBoth(ByRef a As Integer, ByRef b As Integer)
        a += 1
        b += 1
    End Sub

    Sub Main()
        Dim copies As Integer = 1
        AddByVal(copies)
        Console.WriteLine("after AddByVal: " & copies)
        AddByRef(copies)
        Console.WriteLine("after AddByRef: " & copies)

        Dim loan As New Loan() With {.Title = "original"}
        RenameByVal(loan)
        Console.WriteLine("after RenameByVal: " & loan.Title)
        RenameByRef(loan)
        Console.WriteLine("after RenameByRef: " & loan.Title)

        Dim a As String = "left", b As String = "right"
        Swap(a, b)
        Console.WriteLine(a & " " & b)

        ' Parentheses around an argument pass a copy, even for a ByRef parameter.
        Dim n As Integer = 5
        AddByRef((n))
        Console.WriteLine("after AddByRef((n)): " & n)

        ' Literals and expressions are passed through a temporary; nothing to write back to.
        AddByRef(n + 1)
        AddByRef(42)

        Dim slot As Slot
        Fill(slot)
        Console.WriteLine("structure after Fill: " & slot.Copies)
        FillByRef(slot)
        Console.WriteLine("structure after FillByRef: " & slot.Copies)

        Dim items As Integer() = {1, 2, 3}
        Grow(items)
        Console.WriteLine("array after Grow: " & items.Length & " items, first = " & items(0))

        ' Aliasing: the same variable passed twice.
        Dim x As Integer = 3
        AddBoth(x, x)
        Console.WriteLine("AddBoth(x, x): " & x)
    End Sub
End Module

  inside AddByVal: 11
after AddByVal: 1
  inside AddByRef: 11
after AddByRef: 11
after RenameByVal: renamed
after RenameByRef: replaced
right left
  inside AddByRef: 15
after AddByRef((n)): 5
  inside AddByRef: 16
  inside AddByRef: 52
structure after Fill: 0
structure after FillByRef: 99
array after Grow: 3 items, first = 100
AddBoth(x, x): 5

Option Strict On

Public Class Counter
    Private _value As Integer
    Public Property Value As Integer
        Get
            Console.WriteLine("  get Value")
            Return _value
        End Get
        Set(newValue As Integer)
            Console.WriteLine("  set Value = " & newValue)
            _value = newValue
        End Set
    End Property
    Public Items As Integer
End Class

Module Program
    Private Sub Bump(ByRef n As Integer)
        n += 1
        Console.WriteLine("  Bump saw " & n)
    End Sub

    ' Reads the property inside the callee while the caller's property still holds the old value.
    Private Sub BumpAndPeek(ByRef n As Integer, c As Counter)
        n += 100
        Console.WriteLine("  peek through the object: " & c.Items)
    End Sub

    Private Function TryParseCopies(text As String, ByRef copies As Integer) As Boolean
        Return Integer.TryParse(text, copies) AndAlso copies > 0
    End Function

    Sub Main()
        Dim c As New Counter()

        Console.WriteLine("property argument (copy-in, copy-out):")
        Bump(c.Value)

        Console.WriteLine("field argument (a real reference):")
        BumpAndPeek(c.Items, c)
        Console.WriteLine("  Items = " & c.Items)

        Console.WriteLine("array element argument:")
        Dim a As Integer() = {10, 20}
        Bump(a(1))
        Console.WriteLine("  a(1) = " & a(1))

        Console.WriteLine("consuming a C# out parameter:")
        Dim copies As Integer
        Console.WriteLine(TryParseCopies("12", copies) & " " & copies)
        Console.WriteLine(TryParseCopies("abc", copies) & " " & copies)
        Console.WriteLine(TryParseCopies("-4", copies) & " " & copies)
    End Sub
End Module

property argument (copy-in, copy-out):
  get Value
  Bump saw 1
  set Value = 1
field argument (a real reference):
  peek through the object: 100
  Items = 100
array element argument:
  Bump saw 21
  a(1) = 21
consuming a C# out parameter:
True 12
False 0
False -4

Option Strict On

Imports System.Globalization

Module Program
    Private Function Label(title As String, Optional copies As Integer = 1, Optional shelf As String = "A") As String
        Return title & " x" & copies.ToString(CultureInfo.InvariantCulture) & " on shelf " & shelf
    End Function

    Private Function Join(separator As String, ParamArray parts As String()) As String
        Return parts.Length.ToString(CultureInfo.InvariantCulture) & ": " & String.Join(separator, parts)
    End Function

    Private Function Total(ParamArray values As Integer()) As Integer
        Dim sum As Integer = 0
        For Each v In values
            sum += v
        Next
        Return sum
    End Function

    Private Function Describe(ParamArray items As Object()) As String
        If items Is Nothing Then Return "Nothing"
        Return items.Length.ToString(CultureInfo.InvariantCulture) & " object(s)"
    End Function

    Private Function Build(title As String, Optional year As Integer = 0, Optional isbn As String = Nothing, Optional tags As List(Of String) = Nothing) As String
        Return title & "/" & year & "/" & If(isbn, "no isbn") & "/" & If(tags Is Nothing, "no tags", tags.Count.ToString(CultureInfo.InvariantCulture))
    End Function

    Sub Main()
        Console.WriteLine(Label("Dune"))
        Console.WriteLine(Label("Dune", 3))
        Console.WriteLine(Label("Dune", shelf:="Z"))                    ' skip an optional parameter by name
        Console.WriteLine(Label(shelf:="B", copies:=2, title:="Emma"))  ' any order when every argument is named
        Console.WriteLine(Label(title:="Hamlet", 4))                    ' named first, positional after (VB 15.5)

        Console.WriteLine(Join(", "))
        Console.WriteLine(Join(", ", "a"))
        Console.WriteLine(Join(", ", "a", "b", "c"))
        Console.WriteLine(Join(", ", New String() {"x", "y"}))          ' an array is passed as is
        Console.WriteLine(Total() & " " & Total(1, 2, 3))

        Console.WriteLine(Describe())
        Console.WriteLine(Describe(1, "two", 3.0))
        Console.WriteLine(Describe(New Object() {1, 2}))
        Console.WriteLine(Describe(New String() {"a", "b", "c"}))       ' array covariance: still the array itself
        Console.WriteLine(Describe(CType(Nothing, Object())))

        Console.WriteLine(Build("Dune"))
        Console.WriteLine(Build("Dune", isbn:="9780441172719"))
        Console.WriteLine(Build("Dune", tags:=New List(Of String) From {"sf"}))
    End Sub
End Module

Dune x1 on shelf A
Dune x3 on shelf A
Dune x1 on shelf Z
Emma x2 on shelf B
Hamlet x4 on shelf A
0:
1: a
3: a, b, c
2: x, y
0 6
0 object(s)
3 object(s)
2 object(s)
3 object(s)
Nothing
Dune/0/no isbn/no tags
Dune/0/9780441172719/no tags
Dune/0/no isbn/1

Option Strict On

Module Program
    Private Function Show(value As Integer) As String
        Return "Integer"
    End Function
    Private Function Show(value As Long) As String
        Return "Long"
    End Function
    Private Function Show(value As Double) As String
        Return "Double"
    End Function
    Private Function Show(value As Object) As String
        Return "Object"
    End Function
    Private Function Show(value As String) As String
        Return "String"
    End Function

    Private Function Pick(a As Integer, b As Integer) As String
        Return "(Integer, Integer)"
    End Function
    Private Function Pick(a As Integer, ParamArray rest As Integer()) As String
        Return "(Integer, ParamArray)"
    End Function
    Private Function Pick(a As Double, Optional b As Double = 0) As String
        Return "(Double, Optional Double)"
    End Function

    Private Function Gen(Of T)(value As T) As String
        Return "Gen(Of " & GetType(T).Name & ")"
    End Function
    Private Function Gen(value As Integer) As String
        Return "Gen(Integer)"
    End Function

    Sub Main()
        Dim b As Byte = 1
        Dim s As Short = 2
        Dim sg As Single = 1.5F
        Dim d As Decimal = 1D
        Console.WriteLine("Byte    -> " & Show(b))
        Console.WriteLine("Short   -> " & Show(s))
        Console.WriteLine("Integer -> " & Show(3))
        Console.WriteLine("Long    -> " & Show(3L))
        Console.WriteLine("Single  -> " & Show(sg))
        Console.WriteLine("Decimal -> " & Show(d))
        Console.WriteLine("Char    -> " & Show("a"c))
        Console.WriteLine("String  -> " & Show("a"))

        Console.WriteLine(Pick(1, 2))
        Console.WriteLine(Pick(1))
        Console.WriteLine(Pick(1, 2, 3))
        Console.WriteLine(Pick(1.5, 2))
        Console.WriteLine(Pick(1.5))

        Console.WriteLine(Gen(5))
        Console.WriteLine(Gen("x"))
        Console.WriteLine(Gen(Of Integer)(5))
        Console.WriteLine(Gen(5L))
    End Sub
End Module

Byte    -> Integer
Short   -> Integer
Integer -> Integer
Long    -> Long
Single  -> Double
Decimal -> Double
Char    -> String
String  -> String
(Integer, Integer)
(Integer, ParamArray)
(Integer, ParamArray)
(Double, Optional Double)
(Double, Optional Double)
Gen(Integer)
Gen(Of String)
Gen(Of Int32)
Gen(Of Int64)

Option Strict On

Imports System.Runtime.CompilerServices

Public Class Printer
    Public Sub Print(value As Integer)
        Console.WriteLine("Printer.Print(Integer)")
    End Sub
End Class

Public Class FancyPrinter
    Inherits Printer

    ' Without Overloads the base overloads would be hidden by name (warning BC40003 about shadowing).
    Public Overloads Sub Print(value As String)
        Console.WriteLine("FancyPrinter.Print(String)")
    End Sub
End Class

Public Class ShadowPrinter
    Inherits Printer

    ' Shadows hides every base member with this name, whatever its signature.
    Public Shadows Sub Print(value As String)
        Console.WriteLine("ShadowPrinter.Print(String)")
    End Sub
End Class

Public Module Api
    <OverloadResolutionPriority(1)>
    Public Sub Log(Of T)(value As T)
        Console.WriteLine("Log(Of T)  [priority 1] T=" & GetType(T).Name)
    End Sub

    Public Sub Log(value As String)
        Console.WriteLine("Log(String)  [priority 0]")
    End Sub
End Module

Module Program
    Sub Main()
        Dim f As New FancyPrinter()
        f.Print(1)
        f.Print("x")

        Dim sp As New ShadowPrinter()
        sp.Print("x")
        DirectCast(sp, Printer).Print(1)
        ' sp.Print(1) would not compile: the Integer overload is hidden by Shadows

        Log("a")
        Log(5)
    End Sub
End Module

Printer.Print(Integer)
FancyPrinter.Print(String)
ShadowPrinter.Print(String)
Printer.Print(Integer)
Log(Of T)  [priority 1] T=String
Log(Of T)  [priority 1] T=Int32

Option Strict On

Module Program
    ' Generic procedure with a constraint
    Private Function Larger(Of T As IComparable(Of T))(a As T, b As T) As T
        Return If(a.CompareTo(b) >= 0, a, b)
    End Function

    ' VB has no nested (local) procedures; a lambda held in a local variable is the substitute.
    Private Function Summary(titles As IEnumerable(Of String)) As String
        Dim shorten = Function(t As String) If(t.Length > 6, t.Substring(0, 6) & "~", t)
        Return String.Join(" | ", titles.Select(shorten))
    End Function

    ' A recursive lambda must be declared before it is assigned.
    Private Function Fib(n As Integer) As Integer
        Dim f As Func(Of Integer, Integer) = Nothing
        f = Function(k As Integer) If(k < 2, k, f(k - 1) + f(k - 2))
        Return f(n)
    End Function

    ' Methods can return tuples
    Private Function MinMax(values As Integer()) As (Min As Integer, Max As Integer)
        Return (values.Min(), values.Max())
    End Function

    Sub Main()
        Console.WriteLine(Larger(3, 9) & " " & Larger("pear", "apple"))
        Console.WriteLine(Summary({"Dune", "Frankenstein", "Emma"}))
        Console.WriteLine(Fib(10))
        Dim r = MinMax({4, 9, 1})
        Console.WriteLine(r.Min & ".." & r.Max)
        Dim ByRefLambdaOk As Action(Of Integer) = Sub(n As Integer) Console.WriteLine("lambda " & n)
        ByRefLambdaOk(1)
    End Sub
End Module

9 pear
Dune | Franke~ | Emma
55
1..9
lambda 1