Can a HashSet<T> store duplicate values?

Short answer: No, HashSet<T> does not allow duplicates. Attempting to add a duplicate value will return false and not change the set.

Example code

bool added = uniqueNumbers.Add(2); // returns false because 2 already exists

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.