What are the differences between the data structures Dictionary and HashSet in terms of their usage and internal implementation?

Dictionary and HashSet are data structures in C# based on hash tables, but with different purposes and structures:

• Dictionary<TKey, TValue> — a collection of key-value pairs. It allows quick lookup of a value by a unique key. Internally, it uses hashing of keys for fast access.

• HashSet<T> — a collection of unique elements without key-value pairs. It is used to store a set of unique objects and for quick search, addition, or removal of elements.

Main differences:

• Dictionary stores pairs, HashSet — only keys (elements).
• Dictionary allows retrieving a value by key, HashSet checks for the presence of an element.

Example:

var dict = new Dictionary<int, string>();
dict[1] = "one";

var set = new HashSet<int>();
set.Add(1);

bool hasKey = dict.ContainsKey(1); // true
bool hasElement = set.Contains(1); // true

Thus, choose Dictionary when you need to map a key to a value, and HashSet when uniqueness and quick search of elements are important.