Fix GC reference tracking for injected classes

Il2CppInterop.Runtime/IL2CPP.cs

@@ -369,6 +369,27 @@ public static void FieldWriteWbarrierStub(IntPtr obj, IntPtr targetAddress, IntP
         *(IntPtr*)targetAddress = value;
     }
 
+    private static volatile bool s_fieldWriteWbarrierAvailable = true;
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static void FieldWriteWbarrier(IntPtr obj, IntPtr targetAddress, IntPtr value)
+    {
+        if (s_fieldWriteWbarrierAvailable)
+        {
+            try
+            {
+                il2cpp_gc_wbarrier_set_field(obj, targetAddress, value);
+                return;
+            }
+            catch (EntryPointNotFoundException)
+            {
+                s_fieldWriteWbarrierAvailable = false;
+            }
+        }
+
+        FieldWriteWbarrierStub(obj, targetAddress, value);
+    }
+
     // IL2CPP Functions
     [DllImport("GameAssembly", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
     public static extern void il2cpp_init(IntPtr domain_name);

Il2CppInterop.Runtime/Injection/ClassInjector.cs

@@ -250,6 +250,7 @@ public static void RegisterTypeInIl2Cpp(Type type, RegisterTypeOptions options)
             .Where(IsFieldEligible)
             .ToArray();
         classPointer.FieldCount = (ushort)fieldsToInject.Length;
+        classPointer.HasReferences = baseClassPointer.HasReferences;
 
         var il2cppFields =
             (Il2CppFieldInfo*)Marshal.AllocHGlobal(classPointer.FieldCount * UnityVersionHandler.FieldInfoSize());
@@ -266,6 +267,9 @@ public static void RegisterTypeInIl2Cpp(Type type, RegisterTypeOptions options)
                 : fieldsToInject[i].FieldType.GenericTypeArguments[0];
             var fieldAttributes = fieldsToInject[i].Attributes;
             var fieldInfoClass = Il2CppClassPointerStore.GetNativeClassPointer(fieldType);
+            if (fieldInfoClass == IntPtr.Zero)
+                throw new Exception($"Type {fieldType} in {type}.{fieldsToInject[i].Name} doesn't exist in Il2Cpp");
+
             if (!_injectedFieldTypes.TryGetValue((fieldType, fieldAttributes), out var fieldTypePtr))
             {
                 var classType =
@@ -283,10 +287,11 @@ public static void RegisterTypeInIl2Cpp(Type type, RegisterTypeOptions options)
             }
 
             fieldInfo.Type = (Il2CppTypeStruct*)fieldTypePtr;
-            if (fieldInfoClass == IntPtr.Zero)
-                throw new Exception($"Type {fieldType} in {type}.{fieldsToInject[i].Name} doesn't exist in Il2Cpp");
+            var fieldIsValueType = IL2CPP.il2cpp_class_is_valuetype(fieldInfoClass);
+            classPointer.HasReferences |= !fieldIsValueType ||
+                                          IL2CPP.il2cpp_class_has_references(fieldInfoClass);
 
-            if (IL2CPP.il2cpp_class_is_valuetype(fieldInfoClass))
+            if (fieldIsValueType)
             {
                 uint _align = 0;
                 var fieldSize = IL2CPP.il2cpp_class_value_size(fieldInfoClass, ref _align);

Il2CppInterop.Runtime/InteropTypes/Fields/Il2CppReferenceField.cs

@@ -28,7 +28,9 @@ public TRefObj Value
 
     public void Set(TRefObj value)
     {
-        *GetPointerToData() = value != null ? value.Pointer : IntPtr.Zero;
+        var targetAddress = (IntPtr)GetPointerToData();
+        IL2CPP.FieldWriteWbarrier(_obj.Pointer, targetAddress,
+            value != null ? value.Pointer : IntPtr.Zero);
     }
 
     public static implicit operator TRefObj(Il2CppReferenceField<TRefObj> _this)

Il2CppInterop.Runtime/InteropTypes/Fields/Il2CppStringField.cs

@@ -30,7 +30,9 @@ public string Value
 
     public void Set(string value)
     {
-        *GetPointerToData() = IL2CPP.ManagedStringToIl2Cpp(value);
+        var targetAddress = (IntPtr)GetPointerToData();
+        IL2CPP.FieldWriteWbarrier(_obj.Pointer, targetAddress,
+            IL2CPP.ManagedStringToIl2Cpp(value));
     }
 
     public static implicit operator string(Il2CppStringField _this)

Il2CppInterop.Runtime/Runtime/VersionSpecific/Class/Interfaces.cs

@@ -31,6 +31,11 @@ public interface INativeClassStruct : INativeStruct
     bool Initialized { get; set; }
     bool InitializedAndNoError { get; set; }
     bool SizeInited { get; set; }
+    bool HasReferences
+    {
+        get => false;
+        set { }
+    }
     bool HasFinalize { get; set; }
     bool IsVtableInitialized { get; set; }