How do you perform a union or intersection between two HashSet<T> objects?

Short answer: Union: Combines all unique elements from both sets set1.UnionWith(set2); Intersection: Keeps only elements present in both sets set1.IntersectWith(set2); Example: HashSet<int> set1 = new HashSet<int> { 1, 2, 3 };

Example code

HashSet<int> set2 = new HashSet<int> { 3, 4, 5 };
set1.UnionWith(set2); // set1 = {1, 2, 3, 4, 5}
set1.IntersectWith(set2); // set1 = {3, 4, 5}

Real-world example (ShopNest)

When applying a coupon, ShopNest keeps used coupon codes in a HashSet<string> so “already used?” checks stay fast and unique.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.