what’s the differences between readonly & const in C# 在C#中readonly和const的區別

1. the similar feature 相似性
   grammar:
    public const int x=100;
    public readonly int x;    
    const variable and readonly variable can not be modified once they are initialized in run-time. However，the main differences between them is the time to be initialized!
const變數和readonly變數的值一旦被初始化其值在運行時就不能再改變。但是，他們之間的區別在於他們被初始化的時機是不一樣的。

2 the differences
    const variable must be initialized when it's be declared, because  its value is set in compile-time before the respective object is constructed. However, readonly can be initialized dynamically. that is to say, its value can be set via class constructor or vairable initiazer（no other feasible ways）.
    const常量必須在其聲明時被初始化，因為其值在編譯而相應物件建構之前就被設定。但是 ，readonly可以被動態設定，其既可以在初始化時設定，也可以在建構函式中設定（其他方式均不可）。
   e.g
    public class A
    {
        public const m_x=100;

        //public const m_x=DataTime.Now.Tricks;
        //error!DataTime.Now.Tricks can not give a exact value for const variable in compile-time.
        public readonly long  m_y=DataTime.Now.Tricks;
        //it can be initialized via viriable initiazer

        public readonly int  m_z;
        public A()
        {
        m_z=DataTime.Now.Tricks;
        //it also can be initialized via constructor;
        }
    }