C# - compare two excel files

When you Google "excel compare sheets", you get only 15 million answers, and you found nothing useful ? Serious ?

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I am looking forward to compare two excel files and put the difference into another sheet.

First you need to define what you want to compare, cells values, formulas, formatting ...

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The main problem i encounter is that i do not know where i should start.

Are you kidding ? There is pretty much one way to do it.
You want to tell your user where are the differences.
For each cells of the 2 first sheets, you want tp write in third sheet if the cells are equal or not.
More or less, you will have:

' not real code
Sheet3.A1=(Sheet1.A1=Sheet2.A1)

details vary on how you want to show differences and what differences you looking for.

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I have searched google/sw develeopers forums and i didn't find something i could use to compare 2 sheets.

Sometimes, you have to work to make ends meet.
When you found a solution in a language, your jobs is to translate in your language and adapt to your criteria.

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Everything i have seen on internet but doesn't apply at all to my code.

Programmer is not just a Copy/Paste job, sometimes you have to create things.

I'm doing something almost exactly like that (WPF application). I'm taking two spreadsheets, and merging them based on a user-specified mapping criteria (with one or more matching column pairs.

I'm using a Wizard-style form to guide the user through the process. My wizard pages are:

- Introduction
- Selecting a merge type (two files, or two worksheets in a single file)
- File (and worksheet) selection
- Column selection
- Matching criteria
- New (merged) file generation/preview/save

You can read your Excel files via the Microsoft.Office.Interop.Excel namespace ()[^], which provides all the classes and methods necessary.