只读字段的使用

<pre class="csharp" name="code">using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace ConsoleApplication3{    class CircleArea    {        readonly double radius;        const double PI = 3.1415926;        public CircleArea(double r)        {            radius = r;//通过构造函数为只读字段赋值        }        public double GetArea()        {            return radius * radius * PI;        }    }    class Program    {        static void Main(string[] args)        {            CircleArea c = new CircleArea(2.0);            Console.WriteLine("圆的面积是：" + c.GetArea());            Console.ReadLine();        }    }}