Item 2: Prefer readonly to const(Effective C#)

　　Compile-time constants are limited to numbers and strings. Read-only values are also constants, in that they cannot be modified after the constructor has executed.

　　You can use readonly values for instance constants, storing different values for each instance of a class type. Compile-time constants are, by definition,
static constants. 

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Text;
 5 
 6 namespace EffectiveCSharpItem2
 7 {
 8     public class Person 
 9     {
10         public readonly int age;
11 
12         public Person(int age) 
13         {
14             this.age = age;
15         }
16 
17         public int Age 
18         {
19             get { return this.age;  }
20         }
21     }
22 
23     class Program
24     {
25         static void Main(string[] args)
26         {
27             Person p1 = new Person(25);
28             Person p2 = new Person(26);
29             Console.WriteLine(p1.Age);
30             Console.WriteLine(p2.Age);
31 
32             Console.Read();
33         }
34     }
35 }