How to properly work with UObjects in background threads (GC)

This short article addresses the question of how to work with UObjects in a thread-safe way when passing them to some workers, asynchronous tasks, thread pools, or whatever else using a non-game thread.

One of the crucial issues is garbage collection. When we pass a UObject that is not set to root directly to a background thread, it’s possible that the garbage collector may silently delete the passed UObject. Even if we frequently check the validity of the UObject (IsValidLowLevel()) on a background thread, we cannot guarantee that this object will remain alive, even just a few milliseconds after the check.

UObject* CreatedObject = NewObject<UObject>();
AsyncTask(ENamedThreads::AnyThread, [CreatedObject]()
{
	// Use of CreatedObject...
	// Here we cannot be sure that the CreatedObject will remain alive during the execution of the logic
};

The proper way to handle this is to use FGCObjectScopeGuard, which ensures that the object is not garbage collected within the asynchronous scope:

UObject* CreatedObject = NewObject<UObject>();
AsyncTask(ENamedThreads::AnyThread, [CreatedObject]()
{
	FGCObjectScopeGuard CreatedObjectGuard(CreatedObject);
	// Use of CreatedObject...
	// Here we are sure that the CreatedObject will not be garbage collected within this scope
};

While adding an object to UPROPERTY can prevent it from being garbage collected, it is still possible for its parent object to be collected, leading to the needed object being collected as well. That’s why using FGCObjectScopeGuard is recommended to ensure the object remains alive within the scope, except when executing the logic in the game thread.

Note that the FGCObjectScopeGuard may cause a crash if the object being guarded is forcibly deleted by the garbage collector, which is particularly relevant when exiting the In-Editor Testing mode.