Value kontra reference; literals; var-inferens; const/readonly

using System;

// Value type (struct) copies by value
public struct PointS
{
  public int X;
  public int Y;
}

// Reference type (class) copies the reference
public class PointC
{
  public int X;
  public int Y;
}

// Readonly demo class
public class Config
{
  public readonly string Name;
  public Config(string name)
  {
    Name = name; // can set here
  }
}

public class Program
{
  // Compile-time constant
  private const double Pi = 3.14159;

  public static void Main(string[] args)
  {
    // 1) Value vs reference
    PointS a = new PointS { X = 1, Y = 2 };
    PointS b = a;         // copy value
    b.X = 99;
    Console.WriteLine("Value struct a.X = " + a.X); // 1 (unchanged)
    Console.WriteLine("Value struct b.X = " + b.X); // 99

    PointC ac = new PointC { X = 1, Y = 2 };
    PointC bc = ac;       // copy reference
    bc.X = 77;
    Console.WriteLine("Ref class ac.X = " + ac.X);  // 77 (same object)
    Console.WriteLine("Ref class bc.X = " + bc.X);  // 77

    // 2) Literals & suffixes
    float f = 3.14f;
    decimal price = 10m;
    long big = 123L;
    Console.WriteLine("f=" + f + " price=" + price + " big=" + big);

    // 3) var inference (type is obvious)
    var sum = a.X + a.Y; // int
    Console.WriteLine("sum=" + sum);

    // 4) const vs readonly
    Config cfg = new Config("Demo");
    Console.WriteLine("Pi=" + Pi + " Name=" + cfg.Name);
  }
}