If one of my Excel Services UDFs fail they return error information to the report. I'd also like to have the errors logged. We use log4net with the rest of our product so ideally I'd like to use it here too. So far the only information I've found is:
http://www.thespgeek.com/2011/01/how-to-use-log4net-in-sharepoint-based.html
which doesn't specifically deal with logging from Excel Services UDFs.
Has anyone had success using log4net with Excel Services UDFs?