c# - Using .NET 4.5 code in Unity 5 -

I am working on the game using Unity 3D version 5. The game should already use strict business rules programmed in C # .NET4. Using the 5 engine Xamarin / Mono, we hope to make it useful on Windows, Android, and others ...

I tried to make the engine in DLL and it was in the form of a managed plugin in unity Used in But Unity uses the old version of Mono to compile the whole game and refuses to use the .NET 4.5 DLS.

I really needed to use this code, I thought it was downgrading to .NET 3.5, but this is not an acceptable solution that has already been done on the engine.

Regarding Android, I think use the xamine to get the Netme Android code and use it as the original plugin in Unity. Now I need a solution for Windows. I am looking for ways to use the C # code in C + code to create unmanaged DLLs, but I understand that COM interop, C ++ / CLI or other methods Which way is applicable in the middle.

Does anybody know how to add codes in some / little effort at Unity?