I created a DLL using C++ and called it from C# long time ago. C# can call C functions in DLL using marshall. If I remember correctly, C++ DLL exported some C functions, like createObj func create a C++ object and return a pointer to it, freeObj takes the pointer and free the object and other functions that takes that pointer and do something. In C# code, manage returned pointer manually. I also wrote function signatures of exported functions in C# code using marshall that is similar to Nim's importc. I didn't got much problems (if I remember correctly) and it worked.
You probably be able to do similar thing with Nim. DLL written in Nim export C functions using exportc and dynlib pragma. `createNimObj` proc create a ref object, increase ref count of it with `GC_ref` and return to C# code. `freeNimObj` takes that pointer, decrease ref count with `GC_unref`. And other Nim procedures that takes the pointer and do something with the given ref object. You probably need to manage the pointer returned from `createNimObj` manually unless C# has something like destructor. I didn't used C# for long time.
