How can I compile raw C# code at runtime? So if I want to add security by removing important pieces of code and downloading it via a server, and then execute it from within my application?
Do you want to download the source code, or a compiled version?
You can download the MSIL and use Assembly.Load(byte[]) to load it for execution.
But none of these methods add security. If you send the code to the user's computer in ANY format, they can capture a copy. Never saving it to disk just adds a small speedbump.