I was told by my colleague based on one of my classes (it is an instance class) that if you have no fields in your class (backing fields), just make all methods static in th
There's no absolute rule for when a class should be static. It may have no state, but you may need it for reference equality or locking. Classes should be static when their purpose fits it being implemented as a static class. You shouldn't follow hard-and-fast rules in these situations; use what you 'feel' is right.
Having no state makes it a candidate for static-ness, but look at what it's being used for before arbitarily refactoring it.