VSTO does not support yet the introduction of UDFs, which is a kind of crazy!
However there are few work around.
UDFs as Automation Add-in
This technique is good if you want to make the function available for every workbook in the Excel Application. It is based on the development of an "Excel Automation Add-in".
This consist of developing a kind of C# COM exposed class.
It was a good idea that the author included in the example the GUID identifiers to avoid registry bloating, however I don't know why he did not defined an Internface and let the class implement it, which is the standard way to develop COM exposed class in .NET
UDFs in Code behind files
This technique is good when you want to define workbook level UDFs. It is based on a COM exposed class and some VBA wrapper that you need to write at workbook level. The only problem I had was to the the Exel.Application.Run command work fine. My mistake was due to the fact that I was adding the VBA code the the Excel file that shows up after building and running the program. I did not realized that each time I run, the original .xls file used by the solution was overwriting the one I was editing.
Given the fact that VSTO does not suggest to save aa .XLA, a .XLS files based on VSTO solution, it is still quite hard to deploy an add-in that can use and excel file as data storage.
For example I developed some years ago an Excel Add-in .XLA to bootstrap the Interest Rate Swap Curve in US and Euro area, and I was using a spreadsheet in the .xls file to store data. The add-in makes available to every workbook a set of functions to compute some fixed income analytics : PV, duration, factor analysis...
Of course if this is the idea, the code-behind pattern VSTO solution is ruled out (it cannot make available function to everywork book that runs in an Excel application).
We are left with the Automation add-in solution, which is basically a COM Exposed Class. However, the automation add-in does not have any worksheet to store the data. To be honest one could also develop a COM based add-in (which I do not cover here), but I think it would not sort the problem out.
If anybody has any idea on how to develop .xla kind of add-in, ie a VSTO solution that allows to
1) make UDFs available at application level
2) use an Excel file to store some data
as an .xla solution does, please let me know.