I don't know about MongoDB and C# but on the off chance this is relevant I will relate my challenges with the Google Drive APIs.
LabVIEW requires C# DLL's to be COM enabled. Many are not and so downloading the source code and compiling using Visual Studio with COM enabled used to be enough to fix this. That no longer seems to work possibly for security reasons.
To check if you have this problem. Place a Constructor node
Connectivity => .NET=> Constructor Node
And then browse to your .DLL and see if anything is visible. If nothing shows up you probably have this problem.
I believe that you now have to COM enable using GUID and possibly CLSID which means either editing the packages source code or making creating your own project and file, importing the source code and make your own COM enable routines that front end things you need to call. The advantage of this is it works with updates unless the calls you are using get deprecated.