It seems like this sequence of events might be possible, resulting in creation of an DefaultAssemblyResolver instance that's never disposed:

1. Thread 1 sees assembly_resolver.value == null
2. Thread 2 sees assembly_resolver.value == null
3. Thread 1 enters the lock block and sets assembly_resolver to a new instance
4. Thread 2 enters the lock block and re-sets assembly_resolver to a new instance

The instance in step 3 is now lost and was never disposed.