UnityEngine.dll outside Unity

Hi, i'm working on a visual studio project trying to use UnityEngine.dll as a reference within it, outside the Unity Editor. For what I have seen and read, Unity .dlls are not accessible from outside the engine, i was wondering if there was a turnaround for this, like compiling the project with Mono or accessing directly the Unity .dlls with decompiling tools, if possible. Thanks to all,