1:  using System;
   2:   
   3:  namespace ConsoleApplication4
   4:  {
   5:      class Program
   6:      {
   7:          static void Main(string[] args)
   8:          {
   9:              Circle circle = new Circle();
  10:              circle.Radius = 5;
  11:              double area = AreaCalculator.CalculateArea(circle);
  12:              Console.WriteLine(area);
  13:              Console.ReadLine();
  14:          }
  15:      }
  16:   
  17:      public class Circle
  18:      {
  19:          public double Radius;
  20:      }
  21:   
  22:   
  23:      public class Cylinder
  24:      {
  25:          public double Radius;
  26:          public double Height;
  27:      }
  28:   
  29:      public class Rectangle
  30:      {
  31:          public double Width;
  32:          public double Height;
  33:      }
  34:   
  35:      public class AreaCalculator
  36:      {
  37:          public static double CalculateArea(Circle circle)
  38:          {
  39:              return Math.PI * circle.Radius * circle.Radius;
  40:          }
  41:   
  42:          public static double CalculateArea(Rectangle rectangle)
  43:          {
  44:              return rectangle.Width * rectangle.Height;
  45:          }
  46:   
  47:          public static double CalculateArea(Cylinder cylinder)
  48:          {
  49:              return Math.PI * cylinder.Radius * cylinder.Radius * cylinder.Height;
  50:          }
  51:      }
  52:  }