See the question and my original answer on StackOverflow

There are basically two cases to call a .NET DLL from unmanaged code:

  1. The .NET DLL exposes a COM interface. In this case, you can use COM from your C++ code.
  2. The .NET DLL does not expose a COM interface. In this case, you have two possibilities (to make it simple):

    2.a. host the CLR as described here: Loading the Common Language Runtime into a Process
    2.b. write a piece of managed C++ code (another DLL - written in C++/CLI) to wrap the .NET DLL and expose 'old way' DLL exports to unmanaged clients.

I don't specifically know the sharpbox system, but it looks like it's pure .NET and does not expose COM interfaces, so 2.b might be the best way to do it (not so easy...). Maybe it has a REST/Web easier API you could use.

PS: you can also add exports to a .NET DLL. This is described here: Is is possible to export functions from a C# DLL like in VS C++? but it's kinda hacky.