String Builder vs Lists

Question

I am reading in multiple files in with millions of lines and I am creating a list of all line numbers that have a specific issue. For example if a specific field is left bla

Answer 1

My end goal is to out put a message like "Specific field is blank on 1-32, 40, 45, 47, 49-51

If that's the end goal, no point in going through an intermediary representation such as a List - just go with a StringBuilder. You will save on memory and CPU that way.