c# - ArgumentNullException was unhandled, out of the blue -

i have been working on project on , off, haven't touched 2 months, today came , unable run due error.

ienumerable<mediatitle> query = mediatitlecollection.oftype<filmmedia>(); 

the debugger stops on line , gives error described in title along

value cannot null. parameter name: source

its frustrating since code working 100% before , dominated error. have tried clean solution without luck.

enumerable.oftype raise exception if collection (mediatitlecollection, in case) null.

its frustrating since code working 100% before , dominated error. have tried clean solution without luck.

you should debug application, , location mediatitlecollection supposed set. being set null or not being initialized properly.