Answer this question: why do you want to locate a specific ResourceDictionary in the first place? Probably, like yours truly, you want to locate a specific resource within that particular ResourceDictionary. Like me you may have written some abomination like this:
Yes, enough to make you reach for the nearest brown paper bag isn’t it.
Good News: You don’t need to know the specific ResourceDictionary to locate your resource. And that’s because Dictionaries, including Merged Dictionaries are gigantic Hash Tables which means that all your resources are uniquely keyed within the Merged Dictionary itself. Which means you don’t need to know which are ResourceDictionary contains your resource. Which means you can simply write:
There. That feels better doesn’t it ?