(Notes) Notes (2006)

(back) Events vs Delegates (back)

Delegates refer to methods and described as type-safe function pointers (contains type safe AddressOf).

Unlike function pointers, Visual Basic delegates are a reference type based on the class System.Delegate

The AddressOf statement implicitly creates an instance of a delegate that refers to a specific procedure


   1:   AddHandler Button1.Click, New EventHandler(AddressOf Button1_Click) ' the same as line below
   2:   AddHandler Button1.Click, AddressOf Me.Button1_Click ' the same as line above
   3:   'EventHandler(AddressOf Button1_Click) - delegate

   1:   'Delegate Example 1
   2:   Dim inOrder As New InOrderClass
   3:   inOrder.Num1 = 5
   4:   inOrder.Num2 = 4
   5:   
   6:   'Use AddressOf to send a reference to the comparison function you want to use.
   7:   inOrder.ShowInOrder(AddressOf GreaterThan)
   8:   inOrder.ShowInOrder(AddressOf LessThan)
   9:   
  10:   'Use lambda expressions to do the same thing.
  11:   inOrder.ShowInOrder(Function(m, n) m > n)
  12:   inOrder.ShowInOrder(Function(m, n) m < n)
  13:   'Function(m, n) m < n  - Delegate
  14:   
  15:   
  16:    Function GreaterThan(ByVal num1 As Integer, ByVal num2 As Integer) As Boolean
  17:          Return num1 > num2
  18:    End Function
  19:   
  20:    Function LessThan(ByVal num1 As Integer, ByVal num2 As Integer) As Boolean
  21:          Return num1 < num2
  22:    End Function
  23:   
  24:    Class InOrderClass
  25:   
  26:          'Define the delegate function for the comparisons.
  27:          Delegate Function CompareNumbers(ByVal num1 As Integer, ByVal num2 As Integer) As Boolean
  28:   
  29:          'Display properties in ascending or descending order.
  30:          Sub ShowInOrder(ByVal compare As CompareNumbers)
  31:              If compare(_num1, _num2) Then
  32:                  Console.WriteLine(_num1 & "  " & _num2)
  33:              Else
  34:                  Console.WriteLine(_num2 & "  " & _num1)
  35:              End If
  36:          End Sub
  37:          Property Num1() As Integer
  38:          Property Num2() As Integer
  39:     End Class

   1:    'Delegate Example 2 - invoke delegate
   2:   
   3:    Delegate Function MathOperator(ByVal x As Double,  ByVal y As Double) As Double
   4:    Function AddNumbers(ByVal x As Double, ByVal y As Double) As Double
   5:      Return x + y
   6:    End Function
   7:    Function SubtractNumbers(ByVal x As Double, ByVal y As Double) As Double
   8:      Return x - y
   9:    End Function
  10:   
  11:    Sub DelegateCall(ByVal x As Double, ByVal op As MathOperator, ByVal y As Double)
  12:      Dim ret As Double
  13:      ret = op.Invoke(x, y) ' Call the method.
  14:      MsgBox(ret)
  15:    End Sub
  16:   
  17:    Protected Sub Test()
  18:      DelegateCall(5, AddressOf AddNumbers, 3)
  19:      DelegateCall(9, AddressOf SubtractNumbers, 3)
  20:    End Sub

An Event statement implicitly defines a delegate class named EventHandler as a nested class of the class containing the Event statement, and with the same signature as the event. 


   1:    Delegate Sub DelegateType()
   2:    Event AnEvent As DelegateType

   1:    'Event Example 1
   2:   
   3:    'Declare a WithEvents variable.
   4:    Dim WithEvents EClass As New EventClass
   5:   
   6:    'Declare an event handler that handles multiple events.
   7:    Sub EClass_EventHandler() Handles EClass.XEvent, EClass.YEvent
   8:       MsgBox("Received Event.")
   9:    End Sub
  10:   
  11:    Class EventClass
  12:        Public Event XEvent()
  13:        Public Event YEvent()
  14:        'RaiseEvents raises both events.
  15:        Sub FlashEvents()
  16:            RaiseEvent XEvent()
  17:            RaiseEvent YEvent()
  18:        End Sub
  19:    End Class
  20:   
  21:    EClass.FlashEvents()


C#



Using delegates in multithreading.





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