If you definitely want to secure something, go for C++, as in the .NET World there is a powerful deobfuscator named de4dot (see here: https://github.com/0xd4d/de4dot) that deobfuscates what most obfuscators on the market produce, event the ones it does not explicitely know.

However, this will only raise the bar, as even in the c++ world, there are powerful tools also (IDA: http://www.hex-rays.com/products/ida/index.shtml).

There are other solutions, like mixed code assemblies where you can have the part you want to hide in native code and the rest in managed. see here for more: Mixed (Native and Managed) Assemblies