What collections do you know in C#?

In C#, there are several categories of collections:

1. Non-generic: Work with the object type and require casting.

   • ArrayList - dynamic array.
   • Hashtable - key-value pair collection based on hash table.
   • Stack - stack collection (LIFO).
   • Queue - queue collection (FIFO).
   • SortedList - key-value pair collection sorted by key.

2. Generic: Provide type safety and better performance.

   • List<T> - type-safe dynamic array.
   • Dictionary<TKey, TValue> - type-safe key-value pair collection based on hash table.
   • Stack<T> - type-safe stack collection (LIFO).
   • Queue<T> - type-safe queue collection (FIFO).
   • HashSet<T> - collection of unique elements without a specific order.
   • SortedList<TKey, TValue> - type-safe key-value pair collection sorted by key.
   • SortedDictionary<TKey, TValue> - type-safe key-value pair collection sorted by key in the form of a binary search tree.
   • LinkedList<T> - doubly linked list.

3. Thread-safe: For use in multithreaded environments.

   • ConcurrentBag<T> - unordered thread-safe collection of elements.
   • ConcurrentDictionary<TKey, TValue> - thread-safe dictionary.
   • ConcurrentQueue<T> - thread-safe queue (FIFO).
   • ConcurrentStack<T> - thread-safe stack (LIFO).
   • BlockingCollection<T> - implements blocking add and take operations.

4. Specialized:

   • ObservableCollection<T> - collection that notifies about changes (for data binding).
   • BitArray - compact representation of bits.
   • NameValueCollection - collection of key-value pairs, keys are not unique.

Most of them implement various interfaces such as IEnumerable, ICollection, IList, IDictionary.

Example of using List<T>:

// Creating a list of strings
using System.Collections.Generic;

List<string> names = new List<string>();

// Adding elements
names.Add("Alice");
names.Add("Bob");
names.Add("Charlie");

// Accessing an element
string firstPerson = names[0]; // Alice

// Removing an element
names.Remove("Bob");

// Iterating over the list
foreach (string name in names)
{
    // Do something with name
}

Example of using Dictionary<TKey, TValue>:

// Creating a dictionary
using System.Collections.Generic;

Dictionary<int, string> students = new Dictionary<int, string>();

// Adding elements
students.Add(101, "Alice");
students.Add(102, "Bob");

// Accessing an element by key
string studentName = students[101]; // Alice

// Checking if a key exists
if (students.ContainsKey(103))
{
    // Key exists
}

// Iterating over the dictionary
foreach (KeyValuePair<int, string> student in students)
{
    // Access student.Key and student.Value
}