Porting Unity 2017 UWP c# project to 2020 UWP c++ can't find UnityEngine

Have a project I wrote against 2017 that generated c# code. I want to move it to the most recent version and continue development. I see that I have to move to c++ for the generated project, no problem I am well versed in c++ and UWP.

I have most of the code moved over but an running into an issue in that I can't find the UnityEngine namespace equivalent. And then as a result UnityEngine.GameObject. The code I am trying to port is:

Code (CSharp):

1. namespace UnityXAML
2. {
3.     public delegate void UnityEvent(object arg);
4.     public sealed class Communications
5.     {
6.         /// Called to send a message to the unity components
7.         public static void SendMessageToUnity(string msg)
8.         {
9.             UnityEngine.GameObject gameObject = UnityEngine.GameObject.Find("Camera");
10.             if (gameObject != null)
11.             {
12.                 gameObject.GetComponent<ButtonHandlers>().ShowFeedback(msg);
13.             }
14.             else
15.             {
16.                 throw new Exception("Camera not found, have you exported the correct scene?");
17.             }
18.         }
19.  
20.         /// Connects the event delegates to be able to receive events from unity.
21.         public static void SetEventCallback(UnityEvent e)
22.         {
23.             UnityEngine.GameObject gameObject = UnityEngine.GameObject.Find("Camera");
24.             if (gameObject != null)
25.             {
26.                 var bh = gameObject.GetComponent<ButtonHandlers>();
27.                 if (bh != null)
28.                 {
29.                     bh.onEvent = new ButtonHandlers.OnEvent(e);
30.                 }
31.             }
32.             else
33.             {
34.                 throw new Exception("Camera not found, have you exported the correct scene?");
35.             }
36.         }
37.     }
38. }
39.  

 

I created a communications.cpp and added it to assets\shims.

When I build the project I get a file in IL2CppOutputProject(desktop) with source\cppplugins\communications.cpp

Now how am I supposed to reference this from my UWP project? I can't add the IL2CppOutputProject project as a dependency because it is incompatible (according to Visual Studio 2019).

 

So within the shim code does the c++ file have access to the unity engine?
If say I want to be able to change a unity buttons text from within my xaml project do I need to:

1. Store a static GameObject pointer in the shim file? Where do I get the pointer definition?
2. On unity startup tell the shim file "here is the button", which is then stored in the shim
3. From xaml call the shim UpdateButtonText(string foo)
4. Use the object pointer and manipulate the text (how would I get access to the internals of that button).

Or do I need to

1. Store a callback in the shim with a string parameter
2. On unity startup register a callback from one of my c# scripts that takes a string param
3. From xaml call the shim UpdateButtonText(string foo)
4. Use the stored callback to forward the string to the c# which can then manipulate the gameobject

 

I was thinking that was going to be the case:
In my ButtonHandlers.cs handler I have:

public class ButtonHandlers : MonoBehaviour {
delegate void ShimCallbackDelegate(int number);

void ShimCallback(int number) // member method

In start:
#if ENABLE_WINMD_SUPPORT && UNITY_WSA
SetupAnswerEventCallback(ShimCallback);
#endif



In my shim I have
typedef void(__stdcall* AnswerEventCallback)(int value);
extern "C"
{
// Called from within unity
void __stdcall SetupAnswerEventCallback(AnswerEventCallback callback);
}


When I start my application I get a message in the development console:
"NotSupportedException: IL2CPP does not support marshaling delegates that point to instance methods to native code. The method we're attempting to marshal is:ButtonHandlers: ShimCallback."

 

Ok, got that side working, and if I use the callback from within the c# it works to add a string to my feedback object (which is a unity.ui.text).

From the c++ side, I am now calling the shim:
void MainPage::OnSendToUnityClick(Platform::Object^ sender, Windows::UI::Xaml::RoutedEventArgs^ e)
{
// Validate unity is up and running as expected
if (UnityPlayer::AppCallbacks::Instance->IsInitialized())
{
// Unity interactions must occur on the App thread
auto dispatcher = CoreWindow::GetForCurrentThread()->Dispatcher;
ThreadPool::RunAsync(ref new WorkItemHandler([this, dispatcher](IAsyncAction^)
{
CallUnity();
}));
}
}

Which then does:
// This will be called from the desktop code as a hook to talk to unity
__declspec(dllexport) void __stdcall CallUnity()
{
CriticalSection::Lock lock(s_CriticalSection);
auto callback = s_AnswerEventCallback;
if (callback != nullptr)
{
callback(2112);
}
}

But it is then crashing down in the IL2CPP code (this works when called from the unity script):
VirtActionInvoker1< String_t* >::Invoke(75 /* System.Void UnityEngine.UI.Text::set_text(System.String) */, L_3, L_6);

SetupActivatedEventCallback(Exception thrown at 0x00007FF9EBD2D759 in NewUnityWebViewTest.exe: Microsoft C++ exception: Il2CppExceptionWrapper at memory location 0x000000F1D67CCAC0.
Unhandled exception at 0x00007FF96F58736C (ucrtbased.dll) in NewUnityWebViewTest.exe: An invalid parameter was passed to a function that considers invalid parameters fatal.