These are not the Droids you are looking for - A guide to obfuscation
If you are keen to protect your code and your assets Obfuscation is a word you should learn, no matter if you are doing Silverlight or XNA Projects. Obfuscation is a technique to make it hard for people to break open your compiled code and copy all your hard work, it does this by wrapping your project in another layer that only exposes just enough to make the project runnable.
