How to use readonly modifier in the following program?

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The code below is the example from the book "Illustrated C#2012." Can someone help me and make me understand, what should I do to implement triangle or rectangle or circle portion of the class and find out the area?

C#

{
       class Shape
{
  // Keyword Initialized
   // ↓↓
  readonly double PI = 3.1416;
  readonly int NumberOfSides;
  // ↑↑

  // Keyword Not initialized
  public Shape(double side1, double side2) // Constructor
  {
     // Shape is a rectangle
    NumberOfSides = 4;
    // ↑
    // ... Set in constructor
  }
    public Shape(double side1, double side2, double side3) // Constructor
  {
     // Shape is a triangle
     NumberOfSides = 3;
    // ↑
    // ... Set in constructor
   }
   }

So far I was only able to do:
{public static void Main()
{
var s = new Shape(1.0,1.2);
Console.WriteLine(s.NumberOfsides);
}

Posted 31-Mar-14 16:28pm

RalvarezHose

Updated 31-Mar-14 17:14pm

v2

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Comments

PIEBALDconsult 31-Mar-14 23:18pm

That's not even a good example; find a better one.

RalvarezHose 31-Mar-14 23:25pm

That's the one exactly from the book. If you have any good books in mind please let me know.

PIEBALDconsult 31-Mar-14 23:51pm

There's got to be a better book. Making PI a member field is ridiculous.
Even MSDN is probably better: http://msdn.microsoft.com/en-us/library/acdd6hb7.aspx

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Peter Leow · Accepted Answer · 2014-03-31T17:36:00

There are a number of ways to do this,
a better way would be to make this Shape an abstract class with abstract method like area and leave it to the specific shape to implement the actual area formula.
But to stick to your simple example, try this:

C#

using System;

public class Program
{
    class Shape
    {
        readonly double PI = 3.1416;
        readonly int NumberOfSides;

        // add a double member
        private double area;

        public Shape(double side1, double side2) // Constructor
        {
            // Shape is a rectangle
            NumberOfSides = 4;

            // calculate the area
            area = side1 * side2;
        }

        // add an accessor method
        public double GetArea()
        {
            return area;
        }
    }

    public static void Main()
    {
        // instantiate a rectangle object
        Shape rectangle = new Shape(10, 20);

        // get the area from the GetArea() method
        Console.WriteLine(rectangle.GetArea());
    }
}