How to Programmatically Create OOXML Strict Documents - excel

Question

It is possible to manually save an Excel spreadsheet as a "Strict Open XML" file type in Excel when using "Save as" instead of saving it as the default workbook OOXML file, which is a "Transitional" variant of the OOXML standard. The extension is .xlsx for both Strict and Transitional file format variants of the OOXML standard.
How can I do the same thing programmatically through automated workflows in e.g. C#? The purpose is to bulk convert Transitional Excel files to Strict Excel files.
I have found these code snippets part of the Office XML SDK:
https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.conformanceclass?view=openxml-2.8.1
https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.workbook.conformance?view=openxml-2.8.1#documentformat-openxml-spreadsheet-workbook-conformance
Can I use them or other ways of doing bulk conversion from Transitional to Strict?
UPDATE: I am in dialogue with the developers of Open XML SDK in this issue.

Answer 1

The only way possible, that I have found after extensive research is to use Excel.Interop and have your Excel installation handle the conversion in the background. It is not a pretty solution, because it is Excel dependent, but it is a programmatic approach.
You can find the Excel.Interop package here: https://www.nuget.org/packages/Microsoft.Office.Interop.Excel
I had an issue, where the package would not find my Excel installation, so I had to make a new reference to the Excel Interop DLL on your computer.
Add reference > Browse > C:\Windows\assembly\GAC\Microsoft.Office.Interop.Excel and pick the DLL from the folder in here. I also added the DLL from the subfolder in this path C:\Windows\assembly\GAC_MSIL\office
Conversion code
using Excel = Microsoft.Office.Interop.Excel;
void Convert_Transitional_to_Strict(string input_filepath, string output_filepath)
{
Excel.Application app = new Excel.Application(); // Create Excel instance
app.DisplayAlerts = false; // Don't display any Excel prompts
Excel.Workbook excelWorkbook = app.Workbooks.Open(input_filepath); // Create workbook instance and open Excel Workbook for conversion
excelWorkbook.SaveAs(output_filepath, 61); // Save file as .xlsx Strict
excelWorkbook.Close(); // Close the Workbook
app.Quit(); // Quit Excel Application
}
PS. If anyone finds a programmatic approach using Open XML SDK or another C# framework, do still post your answer.