You can't.
There are steps you can take to make it a little more difficult, but ultimately any executable on the local machine is crackable. Eventually, that code has to be converted into native machine code and every application that is runnable is vulnerable.
What you want to do is just make it difficult enough to crack to make it not worth peoples' trouble.
Some suggestions I have for you to help protect your application:
- Obfuscate your code.
Dotfuscator has a free edition and comes with Visual Studio.
- Use
public/private key or asymmetric encryption to generate your product licenses. This ensures that only you can generate your license codes. Even if your application is cracked, you can be sure that they won't be releasing a key generator for your application, because it is impossible to reverse the key generating algorithm.
- Use a third-party packer to pack your .NET executable into an encrypted Win32 wrapper application.
Themida is one of the better ones. This stops people from reflecting your application in .NET Reflector and makes it a pain to unpack for reversing.
- Write your own custom packer. If the third-party packers are too expensive, consider writing your own. Sometimes custom packers can be very effective, because there aren't well published methods on how to unpack them. The tutorial
How to write your own packer gives a ton of good information on writing your own Win32 packer.
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