provide a solution for the problem that Microsoft Excel 2010 is not able to open the xlsx file produced on Mono, due to the way the relationship ID is formatted. Mono only produces numbers like 0, 1, 2, etc. Microsoft Office 2010 does not like just a number starting with 0, that Mono would produce see https://github.com/mono/mono/blob/master/mcs/class/WindowsBase/System.IO.Packaging/PackagePart.cs Open XML SDK 2.0 Productivity Tool for Microsoft Office error message: Cannot open the file: Die ID "0" ist keine gültige XSD-ID.