Question

I’m guessing it does NOT, but if someone could confirm.

If I try to intersect two sets:
A (1 million items)
B (1 item)

Does the framework always do A.Contains(B) once, instead of B.Contains(A) one million times?

This is assuming that’s how intersects work beneath the hood, as opposite to some fancy algorithm I am not aware of.

UPDATE:

OK so for c# you should clearly do B.InsersectWith(A), if B << A. Intersect() is defined on IEnumerable and would be a lot less efficient based on the answers below (and MSDN). So the order does matter if you use the best tool, which is IntersectWith().

Answer 1

From the documentation

If the collection represented by the other parameter is a HashSet collection with the same equality comparer as the current HashSet object, this method is an O(n) operation. Otherwise, this method is an O(n + m) operation, where n is Count and m is the number of elements in other.

HashSet.IntersectWith Method

And if you are looking for speed implement (override) GetHashCode if you can derive a meaningful Hash from your data. And override Equal. I do this for any class that will be in a collection.

Object.GetHashCode Method