Collections & Generics

Collections & Generics

Collections are containers for groups of objects. The most useful ones are generic - they work with any type.

List<T>

A List<T> is like a resizable array:

using System.Collections.Generic;

List<string> cars = new List<string>();
cars.Add("Volvo");
cars.Add("BMW");
Console.WriteLine(cars[0]);   // Volvo
cars.Remove("Volvo");
Console.WriteLine(cars.Count);  // 1

The <T> means generic

The <T> in List<T> is a type parameter. List<int>, List<string>, and List<Car> are all lists, but each holds only its own type. This gives you type safety - the compiler stops you from mixing types.

Dictionary<TKey, TValue>

A dictionary stores key/value pairs for fast lookups:

Dictionary<string, int> ages = new Dictionary<string, int>();
ages["Ada"] = 36;
ages["Grace"] = 85;
Console.WriteLine(ages["Ada"]);   // 36

Other common collections

• HashSet<T> - unique values, no duplicates.
• Queue<T> - first-in, first-out.
• Stack<T> - last-in, first-out.
• LinkedList<T> - fast insertions in the middle.

Arrays vs List

• Array: fixed size, great when you know the count.
• List<T>: grows and shrinks automatically, more flexible.

TL;DR

• List<T> is a resizable, type-safe array.
• <T> makes a collection generic (works with any type).
• Dictionary<TKey, TValue> stores key/value pairs.
• Use using System.Collections.Generic; to access them.