Constructors

public class Person
{
    public string Name;
    public int Age;
    
    // Constructor - same name as class!
    public Person()  // No return type
    {
        Name = "Unknown";  // Set default values
        Age = 0;
    }
}

// Creating an object - constructor runs!
Person person = new Person();

// Now the object is properly initialized
Console.WriteLine(person.Name);  // "Unknown"
Console.WriteLine(person.Age);   // 0

public class Product
{
    public string Name;
    public decimal Price;
    public int Quantity;
    
    // Default constructor
    public Product()
    {
        Name = "New Product";
        Price = 0.00m;
        Quantity = 0;
    }
}

// Usage
Product p = new Product();
Console.WriteLine(p.Name);     // "New Product"
Console.WriteLine(p.Price);    // 0.00

// This class has NO default constructor
public class Book
{
    public Book(string title)  // Only this constructor exists
    {
        Title = title;
    }
    
    public string Title { get; set; }
}

// ❌ This will NOT work:
// Book b = new Book();  // Error! No default constructor

// ✅ You must use the parameterized constructor:
Book b = new Book("C# Programming");

public class Student
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Course { get; set; }
    
    // Constructor with parameters
    public Student(string name, int age, string course)
    {
        Name = name;
        Age = age;
        Course = course;
    }
}

// Usage - set values when creating!
Student s = new Student("Alice", 20, "Computer Science");
Console.WriteLine(s.Name);    // "Alice"
Console.WriteLine(s.Age);     // 20

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    
    // Constructor 1: Default
    public Product()
    {
        Id = 0;
        Name = "Unknown";
        Price = 0.00m;
    }
    
    // Constructor 2: ID and Name only
    public Product(int id, string name)
    {
        Id = id;
        Name = name;
        Price = 0.00m;
    }
    
    // Constructor 3: All parameters
    public Product(int id, string name, decimal price)
    {
        Id = id;
        Name = name;
        Price = price;
    }
}

// Uses constructor 1 (default)
Product p1 = new Product();
Console.WriteLine(p1.Name);  // "Unknown"

// Uses constructor 2 (ID + Name)
Product p2 = new Product(1, "Laptop");
Console.WriteLine(p2.Name);  // "Laptop"

// Uses constructor 3 (all parameters)
Product p3 = new Product(2, "Mouse", 25.50m);
Console.WriteLine(p3.Price); // 25.50

public class Employee
{
    public int EmployeeId { get; }
    public string Name { get; }
    public string Department { get; }
    public decimal Salary { get; }
    
    // Main constructor - does all the work
    public Employee(int id, string name, string dept, decimal salary)
    {
        EmployeeId = id;
        Name = name;
        Department = dept;
        Salary = salary;
    }
    
    // Chain to the main constructor with default department
    public Employee(int id, string name, decimal salary)
        : this(id, name, "General", salary)  // Calls main constructor
    {
    }
    
    // Chain with default department and salary
    public Employee(int id, string name)
        : this(id, name, "General", 30000)  // Calls main constructor
    {
    }
}

public class Configuration
{
    public static string AppName;
    public static string Version;
    public static DateTime StartTime;
    
    // Static constructor - runs once
    static Configuration()
    {
        AppName = "MyApplication";
        Version = "1.0.0";
        StartTime = DateTime.Now;
        Console.WriteLine("⚙️ Configuration initialized once!");
    }
}

// Usage - static constructor runs the first time
Configuration config = new Configuration();  // Static constructor runs
Console.WriteLine(Configuration.AppName);   // "MyApplication"

public class Singleton
{
    private static Singleton instance;
    
    // Private constructor - nobody can create objects!
    private Singleton()
    {
        Console.WriteLine("🔒 Singleton created!");
    }
    
    // Public method to get the single instance
    public static Singleton GetInstance()
    {
        if (instance == null)
            instance = new Singleton();  // Only class can call private constructor
        return instance;
    }
}

public class BankAccount
{
    public string Owner { get; }
    public decimal Balance { get; private set; }
    
    public BankAccount(string owner, decimal initialDeposit)
    {
        // ✅ Validate Owner
        if (string.IsNullOrWhiteSpace(owner))
            throw new ArgumentException("Owner name cannot be empty!");
        
        // ✅ Validate Initial Deposit
        if (initialDeposit < 0)
            throw new ArgumentException("Initial deposit cannot be negative!");
        
        Owner = owner;
        Balance = initialDeposit;
    }
}