With a list you can do:
list.AddRange(otherCollection);
There is no add range method in a HashSet.
What is the best way to add anoth
For HashSet, the name is UnionWith.
This is to indicate the distinct way the HashSet works. You cannot safely Add a set of random elements to it like in Collections, some elements may naturally evaporate.
I think that UnionWith takes its name after "merging with another HashSet", however, there's an overload for IEnumerable too.