How can I find out the external dependencies in a piece of C# code to restrict what it calls?

I'm extending the functionality of a platform that I'm building to include user written components. I could develop a language and write a compiler but since .Net makes it so easy to build and link code I'm exploring just using .net. It would mean compiling them on the fly, which is fine, but I need to know that they're not making any System.IO calls to create files, for instance, or using [unsafe] code to trash memory or calling sockets to communicate with the world. Insuring against bad actors may be too difficult but its at least work looking at. How do I find out what external dependencies a piece of code has?

thanks, john