If they are arrays, I believe you're looking for their Length
property.
mydynamicobject.mylist.Length
Alternatively, I think you might be able to get away with casting mydynamicobject.mylist
to an IEnumerable
and then hand it to IEnueramble.Count
like so:
IEnumerable.Count((IEnumerable)mydynamicobject.mylist);
you could also do as Paolo mentioned:
((ICollection)mydynamicobject.mylist).Count
Although I can't take credit for that one.