Private _ID As String = ""
     Public Property ID(Optional ByVal holen As Boolean = False) As String
      Get
        Return _ID
      End Get
      Set(ByVal value As String)
           _ID = value
           if holen then  Me.Text = getForeignKey(_ID)
       End Set
    End Property

meineTextBox.DataBindings.Add("ID", _BindingSource, "CustomerID")