Some properties are overwritten by the old CDO on reinstancing

[pixeltris]

[UClass, Blueprintable, BlueprintType]
public class SharpActor : AActor
{
	[UProperty, EditAnywhere, BlueprintReadWrite]
	public int SomeValue { get; set; }
	
	public override void Initialize(FObjectInitializer initializer)
	{
		SomeValue = 10;
		AllowTickBeforeBeginPlay = true;
	}
}

• Create the above C# actor.
• Create a blueprint which parents that class.
• Reopen the editor to get a clean state.
• Open the blueprint editor it should have SomeValue as 10 and AllowTickBeforeBeginPlay should be true (checked).

In the C# code change SomeValue to 5 and AllowTickBeforeBeginPlay to false and then recompile the C# code so that it hotreloads. The blueprint will correctly update SomeValue to 5 (unless it has been changed in the details panel to be non-default, which is again correct behavior). However, AllowTickBeforeBeginPlay will still show as being true and will now have the yellow icon/button next to to signify it is no longer the default value. This is incorrect. AllowTickBeforeBeginPlay should now be false and there shouldn't be any default value modified signifier.

This issue stems from the object reinstancing code. Property values from the old CDO are copied over to the new CDO. This occurs in UEngine::CopyPropertiesForUnrelatedObjects.

Look into what determines which values should be copied over, we may need to do custom re-instancing to filter out copying of values which are set to default?

Callstack for the property copy on child blueprints (this happens first):

UE4Editor-Engine.dll!UEngine::CopyPropertiesForUnrelatedObjects(UObject * OldObject, UObject * NewObject, UEngine::FCopyPropertiesForUnrelatedObjectsParams Params) Line 13491	C++
UE4Editor-UnrealEd.dll!ReplaceActorHelper(UObject * OldObject, UClass * OldClass, UObject * & NewUObject, UClass * NewClass, TMap<UObject *,UObject *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UObject *,UObject *,0> > & OldToNewInstanceMap, TMap<UClass *,UClass *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UClass *,UClass *,0> > & InOldToNewClassMap, AActor * OldActor, TMap<FSoftObjectPath,UObject *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<FSoftObjectPath,UObject *,0> > & ReinstancedObjectsWeakReferenceMap, TMap<UObject *,FActorAttachmentData,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UObject *,FActorAttachmentData,0> > & ActorAttachmentData, TArray<FActorReplacementHelper,FDefaultAllocator> & ReplacementActors, bool bPreserveRootComponent, bool & bSelectionChanged) Line 1816	C++
UE4Editor-UnrealEd.dll!FBlueprintCompileReinstancer::ReplaceInstancesOfClass_Inner(TMap<UClass *,UClass *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UClass *,UClass *,0> > & InOldToNewClassMap, UObject * InOriginalCDO, TSet<UObject *,DefaultKeyFuncs<UObject *,0>,FDefaultSetAllocator> * ObjectsThatShouldUseOldStuff, bool bClassObjectReplaced, bool bPreserveRootComponent, bool bArchetypesAreUpToDate) Line 2013	C++
UE4Editor-UnrealEd.dll!FBlueprintCompileReinstancer::BatchReplaceInstancesOfClass(TMap<UClass *,UClass *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UClass *,UClass *,0> > & InOldToNewClassMap, bool bArchetypesAreUpToDate) Line 1471	C++
UE4Editor-Kismet.dll!FBlueprintCompilationManagerImpl::FlushReinstancingQueueImpl() Line 1085	C++
UE4Editor-Kismet.dll!FBlueprintCompilationManagerImpl::CompileSynchronouslyImpl(const FBPCompileRequest & Request) Line 203	C++
UE4Editor-UnrealEd.dll!FKismetEditorUtilities::CompileBlueprint(UBlueprint * BlueprintObj, EBlueprintCompileOptions CompileFlags, FCompilerResultsLog * pResults) Line 745	C++
UE4Editor-UnrealEd.dll!FBlueprintCompileReinstancer::CompileChildren() Line 603	C++
UE4Editor-UnrealEd.dll!FBlueprintCompileReinstancer::ReinstanceObjects(bool bForceAlwaysReinstance) Line 737	C++
UE4Editor-USharp.dll!Export_SharpHotReloadUtils_ReinstanceClass(FSharpHotReloadClassReinstancer * Reinstancer) Line 45	C++
[Managed to Native Transition]	
UnrealEngine.Runtime.dll!UnrealEngine.Runtime.ManagedUnrealTypes.BuildTypes() Line 907	C#
UnrealEngine.Runtime.dll!UnrealEngine.Runtime.ManagedUnrealTypes.Load() Line 63	C#
UnrealEngine.Runtime.dll!UnrealEngine.Runtime.Native.NativeFunctions.OnNativeFunctionsRegistered() Line 189	C#
UnrealEngine.Runtime.dll!UnrealEngine.Runtime.Native.NativeFunctions.RegisterFunctions(System.IntPtr registerFunctionsAddr) Line 99	C#
UnrealEngine.Runtime.dll!UnrealEngine.EntryPoint.DllMain(string arg) Line 70	C#
[Native to Managed Transition]	
[Managed to Native Transition]	
Loader.dll!UnrealEngine.AssemblyLoader.Load() Line 475	C#
[AppDomain (DefaultDomain�, #1) -> AppDomain (Domain30654212 4�, #6)]	
Loader.dll!UnrealEngine.EntryPoint.ReloadAppDomain() Line 249	C#
Loader.dll!UnrealEngine.EntryPoint.ReloadAppDomain.AnonymousMethod__0() Line 195	C#
Loader.dll!UnrealEngine.GameThreadHelper.Tick(float deltaTime) Line 672	C#
[Native to Managed Transition]	
UE4Editor-USharp.dll!FTickerDelegateWrapper::Tick(float DeltaTime) Line 9	C++
UE4Editor-Core.dll!FTicker::Tick(float DeltaTime) Line 98	C++
UE4Editor.exe!FEngineLoop::Tick() Line 3669	C++
UE4Editor.exe!GuardedMain(const wchar_t * CmdLine, HINSTANCE__ * hInInstance, HINSTANCE__ * hPrevInstance, int nCmdShow) Line 166	C++
UE4Editor.exe!WinMain(HINSTANCE__ * hInInstance, HINSTANCE__ * hPrevInstance, char * __formal, int nCmdShow) Line 209	C++
UE4Editor.exe!__scrt_common_main_seh() Line 283	C++
kernel32.dll!00000000778b59cd()	Unknown
ntdll.dll!0000000077a1385d()	Unknown

Callstack for the property copy on instanced objects in the world which can also be instanced C# actors as opposed to blueprint which also exhibit the same issues:

UE4Editor-Engine.dll!UEngine::CopyPropertiesForUnrelatedObjects(UObject * OldObject, UObject * NewObject, UEngine::FCopyPropertiesForUnrelatedObjectsParams Params) Line 13491	C++
UE4Editor-UnrealEd.dll!ReplaceActorHelper(UObject * OldObject, UClass * OldClass, UObject * & NewUObject, UClass * NewClass, TMap<UObject *,UObject *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UObject *,UObject *,0> > & OldToNewInstanceMap, TMap<UClass *,UClass *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UClass *,UClass *,0> > & InOldToNewClassMap, AActor * OldActor, TMap<FSoftObjectPath,UObject *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<FSoftObjectPath,UObject *,0> > & ReinstancedObjectsWeakReferenceMap, TMap<UObject *,FActorAttachmentData,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UObject *,FActorAttachmentData,0> > & ActorAttachmentData, TArray<FActorReplacementHelper,FDefaultAllocator> & ReplacementActors, bool bPreserveRootComponent, bool & bSelectionChanged) Line 1816	C++
UE4Editor-UnrealEd.dll!FBlueprintCompileReinstancer::ReplaceInstancesOfClass_Inner(TMap<UClass *,UClass *,FDefaultSetAllocator,TDefaultMapHashableKeyFuncs<UClass *,UClass *,0> > & InOldToNewClassMap, UObject * InOriginalCDO, TSet<UObject *,DefaultKeyFuncs<UObject *,0>,FDefaultSetAllocator> * ObjectsThatShouldUseOldStuff, bool bClassObjectReplaced, bool bPreserveRootComponent, bool bArchetypesAreUpToDate) Line 2013	C++
UE4Editor-UnrealEd.dll!FBlueprintCompileReinstancer::ReplaceInstancesOfClass(UClass * OldClass, UClass * NewClass, UObject * OriginalCDO, TSet<UObject *,DefaultKeyFuncs<UObject *,0>,FDefaultSetAllocator> * ObjectsThatShouldUseOldStuff, bool bClassObjectReplaced, bool bPreserveRootComponent) Line 1460	C++
UE4Editor-UnrealEd.dll!FBlueprintCompileReinstancer::ReinstanceInner(bool bForceAlwaysReinstance) Line 660	C++
UE4Editor-UnrealEd.dll!FBlueprintCompileReinstancer::ReinstanceObjects(bool bForceAlwaysReinstance) Line 904	C++
UE4Editor-USharp.dll!Export_SharpHotReloadUtils_ReinstanceClass(FSharpHotReloadClassReinstancer * Reinstancer) Line 45	C++
[Managed to Native Transition]	
UnrealEngine.Runtime.dll!UnrealEngine.Runtime.ManagedUnrealTypes.BuildTypes() Line 907	C#
UnrealEngine.Runtime.dll!UnrealEngine.Runtime.ManagedUnrealTypes.Load() Line 63	C#
UnrealEngine.Runtime.dll!UnrealEngine.Runtime.Native.NativeFunctions.OnNativeFunctionsRegistered() Line 189	C#
UnrealEngine.Runtime.dll!UnrealEngine.Runtime.Native.NativeFunctions.RegisterFunctions(System.IntPtr registerFunctionsAddr) Line 99	C#
UnrealEngine.Runtime.dll!UnrealEngine.EntryPoint.DllMain(string arg) Line 70	C#
[Native to Managed Transition]	
[Managed to Native Transition]	
Loader.dll!UnrealEngine.AssemblyLoader.Load() Line 475	C#
[AppDomain (DefaultDomain�, #1) -> AppDomain (Domain30654212 4�, #6)]	
Loader.dll!UnrealEngine.EntryPoint.ReloadAppDomain() Line 249	C#
Loader.dll!UnrealEngine.EntryPoint.ReloadAppDomain.AnonymousMethod__0() Line 195	C#
Loader.dll!UnrealEngine.GameThreadHelper.Tick(float deltaTime) Line 672	C#
[Native to Managed Transition]	
UE4Editor-USharp.dll!FTickerDelegateWrapper::Tick(float DeltaTime) Line 9	C++
UE4Editor-Core.dll!FTicker::Tick(float DeltaTime) Line 98	C++
UE4Editor.exe!FEngineLoop::Tick() Line 3669	C++
UE4Editor.exe!GuardedMain(const wchar_t * CmdLine, HINSTANCE__ * hInInstance, HINSTANCE__ * hPrevInstance, int nCmdShow) Line 166	C++
UE4Editor.exe!WinMain(HINSTANCE__ * hInInstance, HINSTANCE__ * hPrevInstance, char * __formal, int nCmdShow) Line 209	C++
UE4Editor.exe!__scrt_common_main_seh() Line 283	C++
kernel32.dll!00000000778b59cd()	Unknown
ntdll.dll!0000000077a1385d()	Unknown

[pixeltris]

@Algorithman has done some great work looking into this issue. It isn't an issue in UEngine::CopyPropertiesForUnrelatedObjects as described in the original post, but instead an issue in the hotreload default value propagation in UpdateDefaultProperties (#80 also stems from this issue). See the PR he has made for UE4 here https://github.com/EpicGames/UnrealEngine/pull/5830.

There are also issues with reinstancing subobjects in ReplaceObjectHelper (KismetReinstanceUtilities.cpp) which result in non structurally modified subobjects to become corrupt after hotreload (this impacts the C++ hotreload too). This can be tested by defining a component in C++, adding it a blueprint, modifying an exposed property on that component in C++, then hotreloading. It looks like an RF_ArchetypeObject is marked as "pending kill" and the replacement object doesn't work as expected in its place (possibly an object reference isn't being updated?). It should be noted that if you make a structural change, then modify the value in subsequent compiles, it doesn't have the same issue, but there are asserts which fire complaining about archetype object issues (bArchetypeReinstanced). These RF_ArchetypeObject tagged objects are suffixed with _GEN_VARIABLE, none of the other objects create the same issue.

Creating a correct fix for the subobject issue in ReplaceObjectHelper is a bit out of the scope of my knowledge area for how all of this is set up. There are some hacks which should work which involve skipping the RF_ArchetypeObject tagged objects whilst reinstancing, but this will most likely cause issues in other places. I could change USharp to detect a CDO change and force a full recompile if so (this would result in that assert being fired on non-structural changes, but it should work. I'm unsure of the side effects).

---

Some more information on the subobject issue:

After you have made the structural change, the _GEN_VARIABLE (RF_ArchetypeObject) objects no longer show up in the call to ReplaceObjectHelper. This is possibly related to why the bArchetypeReinstanced assert fires. It also explains why there the object doesn't become corrupt on subsequent compiles (because it's no longer being marked as pending kill).

[pixeltris]

Fixed in b79d8d4

The ReplaceObjectHelper issue as described above requires engine source modification, see https://github.com/pixeltris/UnrealEngine/tree/hotreload.