Difference between HashSet.IsSuperSetOf and IsProperSuperSetOf?

A superset of set A is a set that contains all of the elements of set A

A proper superset of A is a set that contains all of the elements of A but is not equal to A.

So if A = {1,2,3}, then {1,2,3} is a superset of A but not a proper superset, while {1,2,3,4} is a proper superset.

You can think of it like the difference between > and >=. IsSuperSetOf is doing something like >=, so your set could have exactly the same elements that are in the set you're comparing to. By contrast, a proper super set is kind of like > and has extra elements that the second set doesn't have.

For example, a set is a superset of itself, but it's not a proper superset of itself.