Public Class Form2
    Private aa As New A
    Private bb As New B
 
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button3.Click
        aa.Decrease(bb)
        Label2.Text = bb.Punkte.ToString
    End Sub
 
    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button4.Click
        bb.Decrease(aa)
        Label1.Text = aa.Punkte.ToString
    End Sub
End Class
 
Public MustInherit Class Base
    Protected Friend pPunkte As Integer = 10
 
    MustOverride Property Punkte() As Integer
 
    MustOverride Function Decrease(ByRef value As Base) As Integer
End Class
 
Public Class A
    Inherits Base
 
    Public Overrides Function Decrease(ByRef value As Base) As Integer
        value.pPunkte -= 1
    End Function
 
    Public Overrides Property Punkte() As Integer
        Get
            Return Me.pPunkte
        End Get
        Set(ByVal value As Integer)
            pPunkte = value
        End Set
    End Property
End Class
 
Public Class B
    Inherits Base
 
    Public Overrides Function Decrease(ByRef value As Base) As Integer
        value.pPunkte -= 1
    End Function
 
    Public Overrides Property Punkte() As Integer
        Get
            Return Me.pPunkte
        End Get
        Set(ByVal value As Integer)
            pPunkte = value
        End Set
    End Property
End Class