.VB File Format – Procedures and events

Subs, functions and parameters

Public Sub Greet(name As String, Optional greeting As String = "Hello")
    Console.WriteLine(greeting & ", " & name)
End Sub

Public Function Sum(ParamArray numbers() As Integer) As Integer
    Dim total = 0
    For Each n In numbers
        total += n
    Next
    Return total
End Function

Public Sub Swap(ByRef a As Integer, ByRef b As Integer)
    Dim t = a
    a = b
    b = t
End Sub

Public Function Max(a As Integer, b As Integer) As Integer
    Max = If(a > b, a, b)          ' assigning to the function's name sets the result
End Function

Public Function Max(a As Double, b As Double) As Double   ' an overload
    Return If(a > b, a, b)
End Function

Greet("Ada")
Greet(greeting:="Hi", name:="Bob")  ' named arguments, in any order
Dim s = Sum(1, 2, 3)

• Parameters are ByVal by default (a copy; ByVal may be left out, and a modifier-free parameter is by value) or ByRef (the caller’s variable). Optional parameters need a default and come last; ParamArray takes any number of trailing arguments as an array.
• Overloading: procedures of one name with different parameter lists; the compiler picks the best match. The Overloads, Overrides and Shadows modifiers are described under classes.
• Returning: Return value, or assign to the function’s own name and reach End Function or Exit Function.
• Declare imports a function of a native library: Declare Auto Function MessageBox Lib "user32.dll" Alias "MessageBoxW" (hWnd As IntPtr, text As String, caption As String, type As UInteger) As Integer. Ansi, Unicode or Auto choose the character set; Alias names the entry point. (<DllImport> does the same with an attribute.)
• Extension methods (version 9): a <Extension()> method (from System.Runtime.CompilerServices) in a module whose first parameter is the type being extended can be called as if it were a member of that type.

Async and iterator methods

Public Async Function FetchLengthAsync(delayMs As Integer) As Task(Of Integer)
    Await Task.Delay(delayMs)
    Return delayMs * 2
End Function

Public Iterator Function Squares(count As Integer) As IEnumerable(Of Integer)
    For i = 1 To count
        Yield i * i
    Next
End Function

An Async method returns a Task, a Task(Of T) (the Return then gives the T), or, for a Sub that fires and forgets, nothing; Await suspends it until the awaited task completes without blocking the thread. An Iterator function returns IEnumerable, IEnumerator or their generic forms, and each Yield produces the next element lazily. Lambdas can be Async or Iterator too. Main cannot be Async.

Properties

Private _value As Integer
Public Property Value As Integer
    Get
        Return _value
    End Get
    Set(newValue As Integer)
        _value = newValue
        RaiseEvent Changed(Me, newValue)
    End Set
End Property

Public ReadOnly Property Double1 As Integer
    Get
        Return _value * 2
    End Get
End Property

Public Property Label As String = "counter"    ' automatic property with an initialiser
Public Property Tag As Object

Default Public ReadOnly Property Item(i As Integer) As Integer
    Get
        Return _value + i
    End Get
End Property

A property has a Get and/or a Set accessor (ReadOnly and WriteOnly properties have one); an accessor may be less accessible than the property (Private Set). A property with no accessors written is an automatic property (version 10) with a hidden backing field, and may have an initialiser (= "counter") and be ReadOnly (version 14). A Default property has parameters and is used as obj(i).

Events

Public Event Changed(sender As Object, newValue As Integer)
Public Event Reset As EventHandler

Public Sub Clear()
    RaiseEvent Reset(Me, EventArgs.Empty)
End Sub

Public Custom Event Custom As EventHandler
    AddHandler(value As EventHandler)
    End AddHandler
    RemoveHandler(value As EventHandler)
    End RemoveHandler
    RaiseEvent(sender As Object, e As EventArgs)
    End RaiseEvent
End Event

' a subscriber
Private WithEvents source As New Counter

Private Sub OnChanged(sender As Object, newValue As Integer) Handles source.Changed
    Console.WriteLine(newValue)
End Sub

Public Sub Hook()
    AddHandler source.Reset, AddressOf OnReset
    RemoveHandler source.Reset, AddressOf OnReset
End Sub

• An Event is declared with a parameter list or with a delegate type (As EventHandler); RaiseEvent calls its subscribers.
• A Custom Event writes its own AddHandler, RemoveHandler and RaiseEvent blocks.
• Handles: a procedure with Handles variable.Event subscribes for as long as the WithEvents variable holds an object (Windows Forms designers use this for controls).
• AddHandler and RemoveHandler subscribe and unsubscribe at run time, with a method (AddressOf) or a lambda.

Operators

Public Shared Operator +(a As Counter, b As Counter) As Counter
    Dim c As New Counter
    c.Value = a.Value + b.Value
    Return c
End Operator

Public Shared Widening Operator CType(c As Counter) As Integer
    Return c.Value
End Operator

An operator is a Public Shared Operator member. Unary: + - Not IsTrue IsFalse; binary: + - * / \ Mod ^ & << >> = <> < > <= >= Like And Or Xor (comparison operators come in pairs). CType operators are Widening (implicit) or Narrowing (explicit) conversions. AndAlso and OrElse are formed from And, Or, IsTrue and IsFalse.

Not described

Known, not yet covered

• Reference: overload resolution (the rules for choosing among overloads, optional and ParamArray arguments, OverloadResolutionPriority of version 17.13), partial methods, interface implementation clauses (Implements on a method) and Handles MyBase / Handles Me are not described in full.
• Reference: the marshalling attributes of Declare and DllImport, COM events and the application-framework events are not described.

See also

• Classes, modules and interfaces – where procedures live
• Generics, lambdas and delegates – generic procedures and anonymous procedures