C# Collections

var numbers = new List<int> { 1, 2, 3, 4, 5 };

// Adding and removing
numbers.Add(6);           // add one element
numbers.AddRange(new[] { 7, 8 }); // add multiple
numbers.Remove(3);        // remove first occurrence of 3
numbers.RemoveAt(0);      // remove by index
numbers.Clear();          // remove everything

// Querying
numbers.Count;            // number of elements
numbers.Contains(5);      // true or false
numbers.IndexOf(5);       // position, or -1
numbers.First();          // first element (throws if empty)
numbers.FirstOrDefault(); // first element, or default (0 for int)
numbers.Last();           // last element

var cities = new List<string> { "Kathmandu", "Pokhara", "Chitwan" };

// foreach
foreach (var city in cities)
{
    Console.WriteLine(city);
}

// for (when you need the index)
for (int i = 0; i < cities.Count; i++)
{
    Console.WriteLine($"{i}: {cities[i]}");
}

numbers.Sort();                    // ascending
numbers.Sort((a, b) => b.CompareTo(a)); // descending

// LINQ ordering (does not modify the original)
var sorted = cities.OrderBy(c => c).ToList();
var desc = cities.OrderByDescending(c => c).ToList();

var prices = new Dictionary<string, double>
{
    ["Notebook"] = 150,
    ["Pen"] = 25,
    ["Bag"] = 800,
};

// Accessing
double penPrice = prices["Pen"]; // 25

// Safe access (avoids KeyNotFoundException)
if (prices.TryGetValue("Book", out double price))
{
    Console.WriteLine(price);
}
else
{
    Console.WriteLine("Book not found");
}

// Adding and removing
prices["Eraser"] = 10;
prices.Remove("Pen");

// Iterating
foreach (var (item, cost) in prices)  // deconstruction
{
    Console.WriteLine($"{item}: {cost}");
}

int[] nums = { 10, 20, 30, 40, 50 };

Array.Sort(nums);
Array.Reverse(nums);
Array.IndexOf(nums, 30);
Array.Resize(ref nums, 10);

// Queue — First In, First Out (FIFO)
var queue = new Queue<string>();
queue.Enqueue("First");
queue.Enqueue("Second");
string next = queue.Dequeue(); // "First"

// Stack — Last In, First Out (LIFO)
var stack = new Stack<string>();
stack.Push("Bottom");
stack.Push("Top");
string top = stack.Pop(); // "Top"

var unique = new HashSet<string> { "apple", "banana", "apple" };
Console.WriteLine(unique.Count); // 2 — duplicates removed

unique.Add("cherry");
unique.Contains("banana"); // true