Structs: value semantics और immutability pattern

using System;

// Simple struct with public fields for demo (okay for small POD data)
public struct Point
{
  public int X;
  public int Y;
}

public class Program
{
  public static void Main(string[] args)
  {
    Point a = new Point();
    a.X = 1; a.Y = 2;

    Point b = a;     // copy the value
    b.X = 9;         // change the copy

    Console.WriteLine("a=(" + a.X + "," + a.Y + ")");
    Console.WriteLine("b=(" + b.X + "," + b.Y + ")"); // different from a
  }
}

using System;

// Immutable struct pattern (C# 6): readonly fields + constructor + With method
public struct Money
{
  private readonly decimal _amount;
  private readonly string _currency;

  public decimal Amount { get { return _amount; } }
  public string Currency { get { return _currency; } }

  public Money(decimal amount, string currency)
  {
    _amount = amount;
    _currency = currency ?? "USD";
  }

  // Copy-on-change: returns a new value with updated amount
  public Money WithAmount(decimal amount)
  {
    return new Money(amount, _currency);
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    Money m1 = new Money(10m, "USD");
    Money m2 = m1.WithAmount(15m); // new value

    Console.WriteLine("m1: " + m1.Amount + " " + m1.Currency);
    Console.WriteLine("m2: " + m2.Amount + " " + m2.Currency);
  }
}

using System;

public struct Counter
{
  public int Value;
}

public class Program
{
  static void IncrementByValue(Counter c)
  {
    c.Value = c.Value + 1; // changes only the local copy
  }

  static void IncrementByRef(ref Counter c)
  {
    c.Value = c.Value + 1; // changes the caller's variable
  }

  public static void Main(string[] args)
  {
    Counter a = new Counter();
    IncrementByValue(a);
    Console.WriteLine("After by-value: " + a.Value); // 0

    IncrementByRef(ref a);
    Console.WriteLine("After by-ref: " + a.Value);   // 1
  }
}

using System;

public interface IShow { string Show(); }

public struct Temperature : IShow
{
  public int C;
  public string Show() { return C + "C"; }
}

public class Program
{
  public static void Main(string[] args)
  {
    Temperature t = new Temperature();
    t.C = 25;

    object boxed = t;              // boxing: copy into object
    Temperature copy = (Temperature)boxed; // unbox (another copy)
    copy.C = 30;

    Console.WriteLine("t = " + t.C);     // 25
    Console.WriteLine("copy = " + copy.C); // 30
  }
}