HashSet<T> and choosing the right shape

One of each, no order

var seen = new HashSet<string>();
seen.Add("a");          // True  — new
bool dup = seen.Add("a");   // False — already there
Console.WriteLine(seen.Count);   // 1

Add returns whether anything changed — a one-call duplicate check. Contains is O(1) (hash-based), unlike List.Contains's O(n) scan. Set math is built in: seen.UnionWith(other), IntersectWith, ExceptWith mutate the set; Overlaps/IsSubsetOf answer questions.

The dedup-and-count pair

var distinct = new HashSet<string>(words);   // copies + dedups
Console.WriteLine(distinct.Count);           // number of unique words

Constructing from an existing collection is the fastest dedup in the language — and combining a HashSet (uniqueness) with a Dictionary (counts) covers frequency tables.

Choosing the shape

| Need | Shape | | --- | --- | | Ordered values, duplicates OK, index access | List<T> | | Unique values, fast membership | HashSet<T> | | Look up by a key | Dictionary<TKey,TValue> | | Fixed size, index access | array |

The wrong shape forces manual workarounds — deduping a List by hand, or searching a Dictionary's Values. Decide from the operations you need, not habit: "do I need duplicates? do I need positions? do I need keys?" Those three questions pick the container in almost every beginner program.