I created a sample workbook in excel, Open'd it up with openXML sdk 2.0 productivity tool.
Reflected the code to get a base to work off of.
Created my own excel file using the c# code.
Loading the new excel file into openXML sdk 2.0 productivity tool. validates with 0 errors. I validated all elements individually as well.
But the file does not open in Excel, "Excel cannot open the file 'Tenant1.xlsx' because the file format or file extension is not valid. Verify that t he file has not been corrupted and that the file extension matches the format of the file.
below is a link to the xlsx file for reference.
If anyone has any help with this or has seen this problem before.
You seem to have used the cell value for both the value and the "r" attribute of the cell. You should be using the cell name (e.g. A1) for that attribute.
The Productivity Tool will validate the XML structure, but not the data.