Collections

Arrays are nice, but they have a problem: once created, their size is fixed. What if you don't know how many items you'll have? That's where collections come in — they grow and shrink as needed.

List<T>

List<T> is a dynamic array. The T is the type of elements it holds. You can add, remove, and access items just like an array, but it resizes automatically.

List names = new List();

names.Add("Alice");
names.Add("Bob");
names.Add("Charlie");

Console.WriteLine(names[0]);
Console.WriteLine("Count: " + names.Count);
    

Working with Lists

Lists give you useful methods: Add, Remove, Contains, Insert, Sort, and more.

List fruits = new List();
fruits.Add("Apple");
fruits.Add("Banana");
fruits.Add("Cherry");

fruits.Remove("Banana");
fruits.Insert(1, "Blueberry");

foreach (string fruit in fruits)
{
    Console.WriteLine(fruit);
}
    

Dictionary<TKey, TValue>

A dictionary stores key-value pairs. You look up a value by its key, like a real dictionary. Keys must be unique.

Dictionary ages = new Dictionary();

ages["Alice"] = 30;
ages["Bob"] = 25;
ages["Charlie"] = 35;

Console.WriteLine(ages["Alice"]);

foreach (KeyValuePair entry in ages)
{
    Console.WriteLine(entry.Key + ": " + entry.Value);
}
    

Use ContainsKey to check if a key exists before accessing it, or use TryGetValue to avoid exceptions.

Choosing a Collection

• List<T> — ordered items, access by index, duplicates allowed
• Dictionary<TKey, TValue> — fast lookup by key, unique keys
• HashSet<T> — unique elements, no ordering, fast membership checks
• Queue<T> — first-in, first-out (like a line)
• Stack<T> — last-in, first-out (like a pile)