using System;

class Program
{
    static void Main()
    {
        Console.WriteLine("Hello World");
    }
}

string name = "John";
int age = 30;
double price = 19.99;
bool isActive = true;

// Type inference
var city = "New York";
var score = 95.5;

// Constants
const double PI = 3.14159;

// If statement
if (age >= 18)
{
    Console.WriteLine("Adult");
}

// For loop
for (int i = 0; i < 10; i++)
{
    Console.WriteLine(i);
}

// Foreach
foreach (var item in items)
{
    Console.WriteLine(item);
}

// Switch
switch (day)
{
    case "Monday":
        Console.WriteLine("Start week");
        break;
    default:
        Console.WriteLine("Other day");
        break;
}

// Basic method
void Greet(string name)
{
    Console.WriteLine($"Hello {name}");
}

// Return value
int Add(int a, int b)
{
    return a + b;
}

// Optional parameters
void Print(string text, bool uppercase = false)
{
    Console.WriteLine(uppercase ? text.ToUpper() : text);
}

class Person
{
    // Properties
    public string Name { get; set; }
    public int Age { get; set; }
    
    // Constructor
    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }
    
    // Method
    public void Introduce()
    {
        Console.WriteLine($"I'm {Name}, {Age} years old");
    }
}

// Usage
var person = new Person("Alice", 25);
person.Introduce();

class Animal
{
    public void Eat()
    {
        Console.WriteLine("Eating...");
    }
}

class Dog : Animal
{
    public void Bark()
    {
        Console.WriteLine("Woof!");
    }
}

// Usage
var dog = new Dog();
dog.Eat();
dog.Bark();

interface IShape
{
    double CalculateArea();
}

class Circle : IShape
{
    public double Radius { get; set; }
    
    public double CalculateArea()
    {
        return Math.PI * Radius * Radius;
    }
}

// Usage
IShape circle = new Circle { Radius = 5 };
Console.WriteLine(circle.CalculateArea());

// List
List<string> names = new List<string> { "Alice", "Bob" };
names.Add("Charlie");

// Dictionary
Dictionary<string, int> ages = new Dictionary<string, int>();
ages["Alice"] = 30;
ages["Bob"] = 25;

// LINQ
var adults = names.Where(n => ages[n] >= 18)
                  .OrderBy(n => n)
                  .ToList();

try
{
    int result = 10 / int.Parse("0");
}
catch (DivideByZeroException ex)
{
    Console.WriteLine("Cannot divide by zero");
}
catch (Exception ex)
{
    Console.WriteLine($"Error: {ex.Message}");
}
finally
{
    Console.WriteLine("Cleanup");
}

async Task<string> DownloadDataAsync()
{
    using var client = new HttpClient();
    return await client.GetStringAsync("https://api.example.com/data");
}

// Usage
async Task ProcessDataAsync()
{
    var data = await DownloadDataAsync();
    Console.WriteLine(data);
}

class User
{
    // Auto property
    public string Name { get; set; }
    
    // Read-only
    public DateTime Created { get; } = DateTime.Now;
    
    // Computed
    public bool IsAdult => Age >= 18;
    
    // With validation
    private int age;
    public int Age
    {
        get => age;
        set => age = value >= 0 ? value : 0;
    }
}

class Repository<T>
{
    private List<T> items = new List<T>();
    
    public void Add(T item)
    {
        items.Add(item);
    }
    
    public T Get(int index)
    {
        return items[index];
    }
}

// Usage
var repo = new Repository<string>();
repo.Add("Hello");

// Delegate
public delegate void MessageHandler(string message);

// Usage
void ProcessMessage(string msg)
{
    Console.WriteLine(msg);
}

MessageHandler handler = ProcessMessage;
handler("Hello!");

// Lambda
Action<string> print = msg => Console.WriteLine(msg);
Func<int, int, int> add = (a, b) => a + b;

static class StringExtensions
{
    public static bool IsEmpty(this string str)
    {
        return string.IsNullOrEmpty(str);
    }
}

// Usage
string text = "";
Console.WriteLine(text.IsEmpty()); // True

// Record definition
public record Person(string Name, int Age);

// Usage
var person1 = new Person("John", 30);
var person2 = person1 with { Age = 31 };

// Value equality
Console.WriteLine(person1 == new Person("John", 30)); // True

string GetTypeDescription(object obj) => obj switch
{
    string s => $"String: {s}",
    int i => $"Integer: {i}",
    null => "Null",
    _ => "Unknown"
};

// Usage
Console.WriteLine(GetTypeDescription("hello"));

// Read file
string content = File.ReadAllText("file.txt");

// Write file
File.WriteAllText("output.txt", "Hello World");

// Read lines
string[] lines = File.ReadAllLines("data.csv");

var person = new { Name = "John", Age = 30 };

// Serialize
string json = JsonSerializer.Serialize(person);

// Deserialize
var deserialized = JsonSerializer.Deserialize<Person>(json);

[ApiController]
[Route("api/[controller]")]
public class UsersController : ControllerBase
{
    [HttpGet]
    public IActionResult GetUsers()
    {
        var users = new[] { "Alice", "Bob" };
        return Ok(users);
    }
    
    [HttpGet("{id}")]
    public IActionResult GetUser(int id)
    {
        var user = new { Id = id, Name = "John" };
        return Ok(user);
    }
}