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I am using carlosag.excelxmlwriter dll to build Excel files. When trying to build xlsx files, the following error pops up:

Excel can not open the file "test.xlsx" because the file format or file extension is not valid. Verify that file has not been corrupted an file extension matches the format of file.

If I change extension to xls, it runs without issue.

Any suggestions?

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I'm not familiar with that library, but the website does have an FAQ with a contact email address. – dangowans Sep 17 '12 at 22:53
If you aren't committed to the CarlosAg library, some alternatives have been suggested on SO - – dangowans Sep 17 '12 at 22:54

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