Got it to work - I am such a flippn n00b.
I put this in the wrong place and "MyFileName" wasnt populated all the way yet. Its derived from quite a few variables in a loop and I put this function about 10 lines to early. UGGH - i really need to tidy my project up! lol
Kinda silly that you would get that error for an invalid filename.
5 A first chance exception of type 'System.ArgumentException' occurred in mscorlib.dll