抱歉，完全保护应用程序是不可能的。

Here's one idea: you could have a server hosted by your company that all instances of your software need to connect to. Simply having them connect and verify a registration key is not sufficient -- they'll just remove the check. In addition to the key check, you need to also have the server perform some vital task that the client can't perform itself, so it's impossible to remove. This of course would probably mean a lot of heavy processing on the part of your server, but it would make your software difficult to steal, and assuming you have a good key scheme (check ownership, etc), the keys will also be difficult to steal. This is probably more invasive than you want, since it will require your users to be connected to the internet to use your software.

混淆代码!在混淆c#代码中有一个例子。

net反应堆

更新

Jared指出de4dot声称能够反编译它。

.NET Reactor provides complete protection for your sensitive intellectual property by converting your .NET assemblies into unmanaged processes which cannot be understood as CIL, and which no existing tool can decompile. Hackers have no access to any intelligible form of your source. Powerful and flexible, the .NET Reactor licensing features allow you to enforce your license conditions and protect your revenue stream by using hardware and software locks. The license manager can build trial or permanent licenses, in a matter of seconds. A fully documented software development kit (SDK), complete with examples, allows you to call the licensing system directly from your code, allowing you to create custom extensions to the licensing system.

我可以推荐使用混淆器。

只需编写一个好的应用程序和一个简单的保护系统。不管你选择什么保护，它都会被逆转……所以不要浪费太多时间/金钱。