Private Function GetNextSerialnumber(ByVal serialnumber As String, _
                               Optional ByRef number As Decimal = -1) As String
 
       Dim frontpart As String = "", i As Integer : number = -1
       If String.IsNullOrWhiteSpace(serialnumber) Then Return Nothing
 
       For i = 0 To serialnumber.Length - 1
           If Char.IsDigit(serialnumber(i)) AndAlso Val(serialnumber(i)) > 0 Then
               If i > 0 Then frontpart = serialnumber.Substring(0, i)
               If Not Decimal.TryParse(serialnumber.Substring(i), number) Then
                   number = -1 : Return Nothing  
                   'Decimal-Überlauf oder falsches Zeichen im Nummernteil
               End If
               Exit For
           End If
       Next i
 
       If number = -1 Then Return Nothing 'Kein Nummernteil
       number += 1 'Nummer hochsetzen
       Return frontpart & number.ToString
 
   End Function