Visual Basic

public void Foo()
{
  // pre tag sample
}

'The delegate that handles the communications between classes.
Public Delegate Sub AreaChangedHandler(ByVal sender As PartObject)

Public Class PartsCollection
    'simplified collection
    'not intended to demonstrate a true collection class
    Dim m_collection As New ArrayList
    Dim m_Area As Double
    Shared Event AreaChanged As AreaChangedHandler
    Public Event PartAdded(ByVal part As PartObject)
    ReadOnly Property Area() As Double
        Get
            Dim obj As PartObject
            Dim totalArea As Single
            For Each obj In m_collection
                totalArea += obj.Area
            Next
            Return totalArea
        End Get

    End Property

    Friend Sub Add(ByVal part As PartObject)
        m_collection.Add(part)
        RaiseEvent PartAdded(part)
    End Sub

    Shared Sub OnAreaChanged(ByVal sender As PartObject)
        RaiseEvent AreaChanged(sender)
    End Sub

End Class

Public Class PartObject
    Dim m_Width As Single
    Dim m_Height As Single
    'Shared delegate that invokes OnAreaChanged method in PartsCollection
    'without having to create an instance of PartsCollection
    Private Shared delegateAreaChanged As New AreaChangedHandler(AddressOf PartsCollection.OnAreaChanged)

    Property Height() As Single
        Get
            Return m_Height
        End Get
        Set(ByVal Value As Single)
            m_Height = Value
            Call AreaChanged()
        End Set
    End Property

    Property Width() As Single
        Get
            Return m_Width
        End Get
        Set(ByVal Value As Single)
            m_Width = Value
            Call AreaChanged()
        End Set
    End Property

    ReadOnly Property Area() As Single
        Get
            Return m_Height * m_Width
        End Get
    End Property

    Protected Overridable Sub AreaChanged()
        delegateAreaChanged.Invoke(Me)
    End Sub
End Class