Collections and LINQ

C# provides a rich set of collection types and LINQ (Language Integrated Query) for querying and transforming data. Together, they form one of the most powerful features of the language.

---

Common Collection Types

Arrays

Fixed-size, strongly typed collections:

int[] numbers = { 1, 2, 3, 4, 5 };
string[] names = new string[3];
names[0] = "Alice";

// Multi-dimensional
int[,] matrix = { { 1, 2 }, { 3, 4 } };

// Jagged (array of arrays)
int[][] jagged = new int[3][];
jagged[0] = new[] { 1, 2, 3 };

List

A resizable, generic collection (the most commonly used):

var fruits = new List<string> { "Apple", "Banana", "Cherry" };
fruits.Add("Date");
fruits.Remove("Banana");
fruits.Insert(1, "Blueberry");

Console.WriteLine(fruits.Count);       // 3
Console.WriteLine(fruits.Contains("Apple")); // true

Dictionary<TKey, TValue>

Key-value pairs with fast lookup:

var capitals = new Dictionary<string, string>
{
    ["UK"] = "London",
    ["France"] = "Paris",
    ["Germany"] = "Berlin"
};

// Safe lookup
if (capitals.TryGetValue("UK", out string? capital))
{
    Console.WriteLine(capital); // London
}

Other Collections

Type	Description	Use Case
HashSet<T>	Unique elements, fast contains	Membership testing
Queue<T>	First-in, first-out	Processing queues
Stack<T>	Last-in, first-out	Undo operations, parsing
LinkedList<T>	Doubly linked list	Frequent insertions/removals
SortedSet<T>	Sorted unique elements	Ordered unique data
SortedDictionary<K,V>	Sorted key-value pairs	Ordered lookup
ConcurrentDictionary<K,V>	Thread-safe dictionary	Multi-threaded access

---

Collection Initialisation

// Traditional
var list = new List<int> { 1, 2, 3 };

// Collection expressions (C# 12+)
List<int> list2 = [1, 2, 3];
int[] array = [4, 5, 6];

// Spread operator (C# 12+)
List<int> combined = [..list2, ..array, 7, 8, 9];

---

What is LINQ?

LINQ (Language Integrated Query) lets you query collections using SQL-like syntax directly in C#:

var numbers = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// Query syntax
var evenNumbers = from n in numbers
                  where n % 2 == 0
                  orderby n descending
                  select n;

// Method syntax (more common)
var evenNumbers2 = numbers
    .Where(n => n % 2 == 0)
    .OrderByDescending(n => n)
    .ToList();

---

Essential LINQ Methods