Any downsides to locking a collection vs. a syncRoot?

If you expose your collections to the outside world, then, yes this can be a problem. The usual recommendation is to lock on something that you exclusively own and that can never be locked unexpectedly by code that is outside your influence. That's why generally it's probably better to lock on something that you'd never even consider exposing (i.e.

A specific lock object created for that purpose). That way, when your memory fails you, you'll never probably not get unexpected results.

In this case, I would lock on the collection; the purpose for the lock relates directly to the collection and not to any other object, so there is a degree of self-annotation in using it as the lock object.

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