Creating add-in for Excel using C#

(Disclaimer: I develop the Excel-Dna library.)

You should have a look at Excel-Dna - http://excel-dna.net. The library allows managed assemblies written in C#, VB.NET or F# to expose high-performance user-defined functions (UDFs) and macros to Excel through the native .xll interface. The project is open-source and freely allows commercial use.

With Excel-Dna you can create a single .xll add-in file that the user can open as an add-in without any further installation or registration. Excel-Dna add-ins can expose RTD servers and customized Ribbons for Excel 2007 and Excel 2010 without additional registration, so you need no extra setup program.

You will need to create a new Visual Studio project of type Excel 2007 Add-In (or Excel 2003 Add-In). The option for this can be found (in Visual Studio 2008) at:

New Project dialog -> Project types -> Visual C# -> Office

This Add-In will need to be installed on the target machine(s).

The Add-In will be able to hook into the Excel object model and therefore access any loaded spreadsheets etc.

The Add-In will also be able to add buttons to the Excel toolbar/ribbon.

Try looking at VSTO, there's a section explicitly for Excel