How can I achieve deferred initialization on a Readonly Hashtable

In my question here I got a lot of good responses & suggestions.

In this nested internal class scenario I'm seeking a deferred initialization for a Readonly Hashtable with the purpose of giving an encapsulating-outside actor a chance to also pass a Hashtable (empty or pre-initialized). I realize I can do this with an overloaded constructor.

My question is what happens if the external reference that was passed in is not a Readonly and I copy the reference over to my Readonly Hashtable

Can you provide some guidance, a sample on how I can achieve this, so that it can address cases where external actors provide pre-initialized Hashtables as well.