what's the differences between readonly & const in C# 在C#中readonly和const的区别
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变量和readonly变量的值一旦被初始化其值在运行时就不能再改变。但是，他们之间的区别在于他们被初始化的时机是不一样的。
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常量必须在其声明时被初始化，因为其值在编译而相应对象构造之前就被设定。但是 ，readonly可以被动态设定，其既可以在初始化时设定，也可以在构造函数中设定（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;
}
}