(Two related issues) 1) If I call method RunExport(@"C:\Temp\Foo.xml") on an instance of ExportToExcelML, it creates a file called "C:\Temp\Foo.xls" (notice the change of the file extension). 2) But "*.xls" is the wrong file extension, causing Excel to produce a "The file format and extension of 'Foo.xls' don't match..." warning when the file is opened. This warning does not appear if the file is saved as "*.xml". My suggestion: 1) Keep any given file extension in ExportToExcelML.RunExport(..) 2) Adjust the intellisense help text of the RunExport-method accordingly that it mentions the possibility to save the file with extension ".xml" to avoid the said excel warning but with the drawback that this file extension is not exclusively associated with Excel.