This event is actually a static event, but exposed as an instance event! The actual static list of handlers is of type ‘List’ and in addition to that the ‘FrameworkElement’-instance also keeps an instance of List with all of the attached eventhandlers.
To make it even stranger, this List is not kept as a normal instance field, but rather via a DependencyProperty (I have no idea why!).

Eventually when the event is raised (via unmanaged code) only the static ‘List’ is used. I guess that the actual reference of the eventhandler is also kept in memory to prevent that the “reference-tree” connected to the eventhandler is garbagecollected. This is probably done so to make it behave like a normal instance event instead of a static weak-event, so this means you have to remove/detach the eventhandler to prevent memory leaks (as in a normal event).

The ‘WeakRefSTRUCT’ is used as a helper to be able to remove an eventhandler from both the static and the instance lists more easily I guess.

This is my best guess as to why this ‘WeakRefSTRUCT’ thing is used, so I might be completely wrong about it.
Anyway you should take a look at the code in Reflector to view the entire ‘LayoutUpdated’ code.

And why not use a struct? Mystifying.

I also enjoy that WeakRefSTRUCT, all protestations to the contrary, is not in fact a struct.

it’s in a core piece of Silverlight

that probably explains why there are so many memory leaks with Silverlight, no matter how hard you try to avoid them