I have to refactor a large C# application, and I found a lot of functions that are never used. How can I check for unused code, so I can remove all the unused functions?
Yes, ReSharper does this. Right click on your solution and selection "Find Code Issues". One of the results is "Unused Symbols". This will show you classes, methods, etc., that aren't used.
It's a great question, but be warned that you're treading in dangerous waters here. When you're deleting code you will have to make sure you're compiling and testing often.
One great tool come to mind:
NDepend - this tool is just amazing. It takes a little while to grok, and after the first 10 minutes I think most developers just say "Screw it!" and delete the app. Once you get a good feel for NDepend, it gives you amazing insight to how your apps are coupled. Check it out: http://www.ndepend.com/. Most importantly, this tool will allow you to view methods which do not have any direct callers. It will also show you the inverse, a complete call tree for any method in the assembly (or even between assemblies).
Whatever tool you choose, it's not a task to take lightly. Especially if you're dealing with public methods on library type assemblies, as you may never know when an app is referencing them.