Fix crash after scripting hot-reload in editor due to cached class attribute objects

#1108

Source/Engine/Scripting/ManagedCLR/MCore.h

@@ -45,6 +45,11 @@ public:
     /// </summary>
     static void UnloadEngine();
 
+#if USE_EDITOR
+    // Called by Scripting in a middle of hot-reload (after unloading modules but before loading them again).
+    static void OnMidHotReload();
+#endif
+
 public:
     /// <summary>
     /// Utilities for C# object management.

Source/Engine/Scripting/Runtime/DotNet.cpp

@@ -299,6 +299,17 @@ void MCore::UnloadEngine()
     ShutdownHostfxr();
 }
 
+#if USE_EDITOR
+
+void MCore::OnMidHotReload()
+{
+    // Clear any cached class attributes (see https://github.com/FlaxEngine/FlaxEngine/issues/1108)
+    for (auto e : CachedClassHandles)
+        e.Value->_attributes.Clear();
+}
+
+#endif
+
 MObject* MCore::Object::Box(void* value, const MClass* klass)
 {
     static void* BoxValuePtr = GetStaticMethodPointer(TEXT("BoxValue"));

Source/Engine/Scripting/Runtime/Mono.cpp

@@ -714,6 +714,14 @@ void MCore::UnloadEngine()
 #endif
 }
 
+#if USE_EDITOR
+
+void MCore::OnMidHotReload()
+{
+}
+
+#endif
+
 MObject* MCore::Object::Box(void* value, const MClass* klass)
 {
     return mono_value_box(mono_domain_get(), klass->GetNative(), value);

Source/Engine/Scripting/Runtime/None.cpp

@@ -59,6 +59,14 @@ void MCore::UnloadEngine()
     MRootDomain = nullptr;
 }
 
+#if USE_EDITOR
+
+void MCore::OnMidHotReload()
+{
+}
+
+#endif
+
 MObject* MCore::Object::Box(void* value, const MClass* klass)
 {
     return nullptr;

Source/Engine/Scripting/Scripting.cpp

@@ -665,6 +665,7 @@ void Scripting::Reload(bool canTriggerSceneReload)
     modules.Clear();
     _nonNativeModules.ClearDelete();
     _hasGameModulesLoaded = false;
+    MCore::OnMidHotReload();
 
     // Give GC a try to cleanup old user objects and the other mess
     MCore::GC::Collect();