Office 365: get files in the same directory as the Excel document through VSTO

I have a VSTO in Ms Excel / Word that verifies the current location of the active document (Dim MyPath as String = Application.ActiveWorkbook.FullName), and then extract the list of files located in the same folder that contains that document (For Each MyFile As FileInfo in New DirectoryInfo(MyPath).GetFiles)

This works well when the file is on a local drive. How can I adapt the code to work for files stored in the Office 365 online version of SharePoint?

Even some recommended readings would be useful, since I'm not sure where to start …

The code in C # or VB is fine.