Securing Assembly

Your only recourse is to obfuscate it, but even that can eventually be reverse engineered. There are A LOT of obfuscation tools out there. Google is your friend.

This problem is not specific to .NET or Java - CPU-specific binaries and even embedded code can be disassembled quite easily.Security by obscurity never works. Obfuscators protect you no better than a sign "beware of the dog" when there's really no dog. Someone determined to read your code will find a way to make an obfuscated disassembly readable. When you give your code to your customers, make sure that its source is completely useless to anyone reading it, i.e. that it does not contain trade secrets, proprietary algorithms that you must protect, or hard-coded passwords or private keys. If possible, move all sensitive code to the server side of which you have undivided control. Otherwise, your only protection is in the hands of the legal system of a country where you distribute your code (which is a fancy way to say that you are mostly unprotected).

Use an obfuscator. Your are wrong to say that obfuscator only renames the API. Some like Crypto Obfuscator has a lot of settings including Method Encryption, Method Call Hiding, String/Constant Encryption, etc besides usual like Symbol Renaming, Control Flow Obfuscation, etc.