// Type: long?, selector function is called public static int Test4g() { Data_Nlong[] source = new Data_Nlong[]{ new Data_Nlong{name="Tim", num=10L}, new Data_Nlong{name="John", num=Int32.MaxValue}, new Data_Nlong{name="Bob", num=null} }; long expected = (long)Int32.MaxValue + 10; var actual = source.Sum((e) => e.num); return ((expected == actual) ? 0 : 1); }