Public Function AdjustFilename(ByVal filename As String, _
                      Optional ByVal ReplaceChar As Char = "-"c) As String
 
   If String.IsNullOrWhiteSpace(filename) Then Return Nothing
 
   'Array der unzulässigen Zeichen im Dateinamen
   Dim ifnc As Char() = IO.Path.GetInvalidFileNameChars
 
   If ifnc.Contains(ReplaceChar) Then
      Throw New ArgumentException("Invalid ReplaceChar")
   End If
 
   Dim FileCharList As New System.Text.StringBuilder(filename)
 
   For i As Integer = 0 To FileCharList.Length - 1
      'unzulässige Zeichen ggf. ersetzen 
      If ifnc.Contains(FileCharList(i)) Then _
         FileCharList(i) = ReplaceChar
   Next i
 
   Return FileCharList.ToString
End Function