Unity can't "find" managed class in dll

I've compiled a C++ DLL using Visual Studio 2015 that contains an unmanaged class with a managed class that wraps the unmanaged one.
Now, I can use the class in C# if I add it as a reference to the project in Visual Studio but Unity can't seem to find the managed class in the dll.

Is there anything I need to add or change so that Unity can find the managed class?

This is how the managed class looks in the dll:

Code (CSharp):

1. #using <mscorlib.dll>
2. #include "PortAudioManager.h"
3. using namespace System;
4.  
5. public ref class PortAudioManaged
6. {
7. private:
8.     PortAudioManager* audioManager;
9. public:
10.     PortAudioManaged() : audioManager(new PortAudioManager()) {
11.     }
12.  
13.     virtual ~PortAudioManaged() {
14.         this->!PortAudioManaged();
15.     }
16.  
17.     // = Object.Finalize
18.     !PortAudioManaged() {
19.         delete audioManager;
20.         audioManager = nullptr;
21.     }
22.  
23.     void openStreamManaged() {
24.         audioManager->openStream();
25.     }
26.  
27.     void stopStreamManaged() {
28.         audioManager->stopStream();
29.     }
30. };