How to identify an excel file(xls, xlsx, csv) which has been converted from .DLL file in c# dot net core

Question

Basically need to identify the excel is originally created from a valid excel. I have tried using Microsoft.Interop.Excel File format. Seems like I cannot use this without Microsoft office being installed. So need another approach for this problem.

EDIT : Basically I want to be able to distinguish a valid excel from invalid one like an excel file which is converted from DLL. Because the file extension will say it is xls. If you try to open that file, it will open an empty workbook without any sheets. But I cannot decide an excel with no sheets is an invalid one.

And the new format is...? When you say "converted from DLL file", do you mean "converted programmatically"? — ProgrammingLlama, Feb 20 '20 at 07:12
So you want to identify if the XLS or XLSX file you're looking at was converted from a CSV, or if the XLSX file you're looking at was converted from a CSV or XLS file, etc? Just with the result of that conversion? — ProgrammingLlama, Feb 20 '20 at 07:41
Basically I want to be able to distinguish a valid excel from invalid one like an excel file which is converted from DLL. Because the file extension will say it is xls. If you try to open that file, it will open an empty workbook without any sheets. But I cannot decide an excel with no sheets is an invalid one. — Sanjay Krishnan C B, Feb 20 '20 at 07:53
Wait, when you say "converted" do you mean "renamed" (like taking a.dll and renaming it to a.xls)? — ProgrammingLlama, Feb 20 '20 at 08:03
That works, when you do that type of file also changes for this case — Sanjay Krishnan C B, Feb 20 '20 at 08:06

score 0 · Answer 1 · answered Feb 20 '20 at 07:11

If you want to idenify wheter file is a valid excel file or not, you can use 'trid file identifier'. TrID - File Identifier.

You can do it own your own also by reading magic number or file signaure bytes. Please go through this question Original file bytes from StreamReader, magic number detection.

How to identify an excel file(xls, xlsx, csv) which has been converted from .DLL file in c# dot net core

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