>Hi All, I've been tasked with writing an app that can extract data from an .xlsx excel file and generate a .csv file from it , I've achieved it using interop but as ever my clients have introduced some business rules which blow the interop method out of the equation so I will have to find a way of reading the excel file row by row and cell by cell, I've googled and found a lot of free libraries such as NPOI which seems fairly good but the examples they provide are mainly for creating the files not processing them so I thought I would ask you people if you had any recommendations as to how to approach this.

I don't understand why you can't use interop to read the .xllsx file - can you explain ?

BTW I've found this has a couple of neat methods for serializing/deserializing C# object lists to CSV:
http://www.codeproject.com/Articles/566656/CSV-Serializer-for-NET