Welcome back! Today, we're honing in on C#'s HashSet — a cornerstone of efficient collection manipulation. C#'s HashSet resembles a mathematical set; it ensures uniqueness by preventing duplicates, similar to how a club assigns unique membership IDs to each member. Throughout the session, you'll see how HashSet simplifies problems involving ensuring uniqueness and checking for overlaps. Let's explore how HashSet can transform lengthy, cumbersome operations into efficient, elegant code.

Imagine you're developing a feature for a social media platform that requires user groups to be exclusive — you need to ensure that users can't belong to more than one group at a time. It's like organizing events where a guest should not appear on the lists for two different parties at the same venue — an overlap would be a significant issue.

Initially, you might consider checking for overlap by comparing each member of one group with every member of the other — a somewhat cumbersome O(n * m) operation. If you have hundreds or thousands of users in each group, the time it would take to compare them all grows exponentially. This approach is impractical and resource-intensive, especially on the scale of a social media platform with potentially millions of users.