কালেকশনস এবং জেনেরিকস (Collections & Generics)

// Create a dictionary: country code → country name
var countries = new Dictionary<string, string>
{
    ["US"] = "United States",
    ["JP"] = "Japan",
    ["BR"] = "Brazil"
};

// Add more entries
countries["DE"] = "Germany";
countries["IN"] = "India";

// Look up by key
Console.WriteLine($"JP = {countries["JP"]}");

// Safe lookup (avoid KeyNotFoundException)
if (countries.TryGetValue("FR", out string? name))
    Console.WriteLine($"FR = {name}");
else
    Console.WriteLine("FR not found!");

// Check if key exists
Console.WriteLine($"Has US? {countries.ContainsKey("US")}");

// Iterate over all pairs
Console.WriteLine($"\nAll {countries.Count} countries:");
foreach (var pair in countries)
    Console.WriteLine($"  {pair.Key} → {pair.Value}");

JP = Japan
FR not found!
Has US? True

All 5 countries:
  US → United States
  JP → Japan
  BR → Brazil
  DE → Germany
  IN → India

var visited = new HashSet<string> { "Paris", "Tokyo", "London" };

// Add — returns false if already exists
bool added1 = visited.Add("Sydney");  // true — new city
bool added2 = visited.Add("Paris");   // false — already there!
Console.WriteLine($"Added Sydney: {added1}");
Console.WriteLine($"Added Paris again: {added2}");

// Super fast contains check
Console.WriteLine($"Been to Tokyo? {visited.Contains("Tokyo")}");

// Set operations
var wishlist = new HashSet<string> { "Rome", "Tokyo", "Cairo" };

// Intersection — cities in BOTH sets
var both = new HashSet<string>(visited);
both.IntersectWith(wishlist);
Console.Write("In both: ");
foreach (var city in both) Console.Write($"{city} ");
Console.WriteLine();

// Except — in wishlist but not visited
wishlist.ExceptWith(visited);
Console.Write("Still need to visit: ");
foreach (var city in wishlist) Console.Write($"{city} ");
Console.WriteLine();

Added Sydney: True
Added Paris again: False
Been to Tokyo? True
In both: Tokyo 
Still need to visit: Rome Cairo 

// Queue — first in, first out
var printQueue = new Queue<string>();
printQueue.Enqueue("Report.pdf");
printQueue.Enqueue("Photo.jpg");
printQueue.Enqueue("Resume.docx");

Console.WriteLine($"Next to print: {printQueue.Peek()}");
Console.WriteLine($"Queue size: {printQueue.Count}");

while (printQueue.Count > 0)
{
    string job = printQueue.Dequeue();
    Console.WriteLine($"  Printing: {job}");
}

// Stack — last in, first out
var undoStack = new Stack<string>();
undoStack.Push("Typed 'Hello'");
undoStack.Push("Made text bold");
undoStack.Push("Changed font to Arial");

Console.WriteLine($"\nLast action: {undoStack.Peek()}");

Console.WriteLine("Undoing:");
while (undoStack.Count > 0)
{
    string action = undoStack.Pop();
    Console.WriteLine($"  Undo: {action}");
}

Next to print: Report.pdf
Queue size: 3
  Printing: Report.pdf
  Printing: Photo.jpg
  Printing: Resume.docx

Last action: Changed font to Arial
Undoing:
  Undo: Changed font to Arial
  Undo: Made text bold
  Undo: Typed 'Hello'

// Generic method — works with any type
static T GetFirst<T>(List<T> items)
{
    if (items.Count == 0)
        throw new InvalidOperationException("List is empty!");
    return items[0];
}

// Generic class — a simple wrapper
class Pair<T1, T2>
{
    public T1 First { get; }
    public T2 Second { get; }

    public Pair(T1 first, T2 second)
    {
        First = first;
        Second = second;
    }

    public override string ToString() => $"({First}, {Second})";
}

// Using the generic method
var names = new List<string> { "Anika", "Rafi", "Sadia" };
var numbers = new List<int> { 42, 17, 99 };

Console.WriteLine($"First name: {GetFirst(names)}");
Console.WriteLine($"First number: {GetFirst(numbers)}");

// Using the generic class
var coordinate = new Pair<double, double>(40.7128, -74.0060);
var entry = new Pair<string, int>("Score", 100);

Console.WriteLine($"NYC: {coordinate}");
Console.WriteLine($"Entry: {entry}");

First name: Anika
First number: 42
NYC: (40.7128, -74.006)
Entry: (Score, 100)