Private Sub ActivateTextbox(ByVal bEnable As Boolean)
        Try
            For Each cTextbox As TextBox In Me.Controls
                If TypeOf cTextbox Is TextBox Then
                    Dim oTextbox As TextBox = CType(cTextbox, TextBox)
                    If bEnable = True Then
                        oTextbox.BorderStyle = _
                          System.Windows.Forms.BorderStyle.Fixed3D
                        oTextbox.BackColor = System.Drawing.Color.White
                        oTextbox.ReadOnly = False
                    Else
                        oTextbox.BorderStyle = _
                          System.Windows.Forms.BorderStyle.FixedSingle
                        oTextbox.BackColor = Me.BackColor
                        oTextbox.ReadOnly = True
                    End If
                End If
            Next
        Catch ex As Exception
            MessageBox.Show(ex.Message)
        End Try
          End Sub

Das Objekt des Typs System.Windows.Forms.Label kann nicht in Typ _
  System.Windows.Forms.TextBox umgewandelt werden.

Eine Ausnahme (erste Chance) des Typs "System.InvalidCastException" ist in _
  Rechnungsmanager.exe aufgetreten.