What types of collections exist in C#?

C# offers a variety of collection types for storing and managing data, located in different namespaces (mainly System.Collections, System.Collections.Generic, System.Collections.Concurrent, System.Collections.Immutable). They can be classified by several main features:

1. By type of stored data:

   • Untyped: Store objects of type object (System.Collections). Require casting and may be less performant.
   • Typed (Generic): Store objects of a specific type (System.Collections.Generic). Provide type safety and better performance.

2. By data structure:

   • Lists: Sequential collections, access elements by index.
     • ArrayList (untype)
     • List<T> (typed)
     • LinkedList<T> (typed doubly linked list)
   • Arrays: Fixed size, access by index.
     • Array (base class)
     • T[] (syntactic sugar for arrays)
   • Dictionaries: Key-value pair collections, access by key.
     • Hashtable (untype)
     • Dictionary<TKey, TValue> (typed)
     • SortedList (untype, sorted by key)
     • SortedList<TKey, TValue> (typed, sorted by key)
     • SortedDictionary<TKey, TValue> (typed, sorted by key, uses red-black tree)
   • Sets: Collections of unique elements.
     • HashSet<T> (typed, unordered)
     • SortedSet<T> (typed, ordered)
   • Queues: Collections following FIFO principle.
     • Queue (untype)
     • Queue<T> (typed)
   • Stacks: Collections following LIFO principle.
     • Stack (untype)
     • Stack<T> (typed)

3. By thread safety:

   • Non-thread-safe: Most standard collections (System.Collections, System.Collections.Generic). Require external synchronization for multithreading.
   • Thread-safe (Concurrent): Designed for safe operation in multithreaded environments without external locking (System.Collections.Concurrent).
     • ConcurrentBag<T>
     • ConcurrentDictionary<TKey, TValue>
     • ConcurrentQueue<T>
     • ConcurrentStack<T>

4. By mutability:

   • Mutable: Most collections allow adding, removing, and modifying elements.
   • Immutable: Ensure immutability after creation (System.Collections.Immutable). Any modification operation returns a new collection.
     • ImmutableArray<T>
     • ImmutableList<T>
     • ImmutableDictionary<TKey, TValue>
     • ImmutableHashSet<T>
     • ImmutableSortedSet<T>
     • ImmutableQueue<T>
     • ImmutableStack<T>

The choice of a specific collection type depends on the task requirements: data type, need for index or key access, element uniqueness, storage order, performance, thread safety, and mutability. The most commonly used in modern C# are the typed collections from the System.Collections.Generic namespace.