How to determine in code that the object is part of prefab in prefab mode

Hi,

I want to initialize a variable If its part of a prefab and to clear it if its not part of a prefab. (called from OnValidate)

This should do it but has an issue:

Code (csharp):

1. if( PrefabUtility.IsPartOfPrefabAsset(gameObject) ) ...

The problem I'm seeing is that when in prefab mode, in OnValidate the above code returns False. I presume this is because the root element when in prefab mode is not it self actually a (sub)prefab .. but is there wany way I know know the root element Is in fact it self in a prefab because thats what I am editing!

Thanks