I can't add a comment, because my reputation is to low.
Your problem is not linq itself but how you use it. What you are doing
is printing one line. This line will contain the value of the ToString call
on a list/collection/linq result. I think it might help you if you don't
try to put everything in one line. The error should be obvious if you
extract the linq query from the write line.
I don't have an IDE here at my workplace, so I can't confirm my code
works, but what you want to do is basically:
var addresses = (from myaddresses[x].PostalNr where x =>
myaddresses[x].SortType == "110" )