ASP NET generated excel file has different format than extension

Question

I have an outdated Asp.net 2.0 web application. I need to create an excel file with a specified table format (the best way if I can load the template and only put data in code behind). I'm trying to utilize this library to accomplish this. CarlosAg.ExcelXmlWriter: 

http://www.carlosag.net/tools/excelxmlwriter/sample

The final file is generated flawlessly, but when I try to access it in MS Excel, a warning window with the message that the file's format is different from the extension appears (xls). I also wonder if I should open it if I don't trust the file's source. The file opened successfully when I selected yes. The same warning—that the file is damaged and cannot be opened—appears on a separate computer running a different version of Excel (no possibility to open). What's remarkable is that I can access that file correctly and without receiving any errors in other office programs, including WPS Office.

    Dim book As Workbook = New Workbook()
    Dim sheet As Worksheet = book.Worksheets.Add("Sample")
    Dim row As WorksheetRow = sheet.Table.Rows.Add()
    row.Cells.Add("Hello World")
    book.Save("D:\Folder\test.xls")

Answer 1

You must include the xlsx file extension.

The legacy (closed) binary format of office files is actually the older Xls file type.

But starting with office 2007, Microsoft switched to using the open XML format for ALL of its office documents (well, ok, outlook pst, and MS Access are exceptions). Try renaming the xlsx document as a zip file if you have any office documents (word, power-point, excel, etc.). You discover that you may now open the zip archive and see, examine, and even edit the XML contained therein.

Therefore, more recent office file formats are actually XML markups, and you can edit them!

In any case, the library you're using unquestionably takes use of the fact that office files are now XML-formatted rather than being stored in a proprietary Microsoft binary format. As a result, these files can now be modified on ANY computer that can handle zip files and text files.

As a result, Excel is alerted to the fact that you are naming the document with a legacy binary file extension (Xls) while opening a plain text XML file. Therefore, you must add the open office XML format extension to that document (xlsx).