Can a HashSet<T> store duplicate values?

Answer: No, HashSet&lt;T&gt; does not allow duplicates. Attempting to add a duplicate value will return false and not change the set. bool added = uniqueNumbers.Add(2); // returns false because 2 lready exists

What interviewers expect

• A clear definition tied to Collections in C# Collections projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production C# Collections application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in C# Collections architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.