C# 关键字const和readonly对比

C# 关键字const和readonly对比

二者区别

1. const关键字修饰的字段只能在声明的时候初始。readonly关键字修饰的字段可以在声明的时候初始化，也可以在构造器中初始化。
2. const修饰的字段值在编译时确定，readonly修饰的字段值在运行时确定。

public static readonly uint timeStamp = (uint)DateTime.Now.Ticks; 

readonly字段赋值的场合

1. 当变量声明时

public readonly int y = 5;  

1. 该字段为实例字段时，在包含readonly字段声明的类的实例构造器中；或者，该字段为静态字段时，在包含readonly字段声明的静态构造器中。这是能把readonly字段作为out和ref参数传递的唯一情况。

2. 在构造其中赋值。

using System;

namespace readonly关键字
{
    internal class Program
    {
        private static void Main()
        {
            var myClass = new MyClass();

            Console.WriteLine("number1 = {0} , number2 = {1} , number3 = {2} , number4 = {3} , number5 = {4}",
                myClass.Number1, myClass.Number2, MyClass.Number3, myClass.Number4, myClass.Number5);

            Console.ReadKey();
        }
    }

    internal class MyClass
    {
        public readonly int Number1 = 100;//在声明时赋值
        public readonly int Number2;
        public static readonly int Number3;
        public readonly int Number4 = 400;
        public readonly int Number5 = 500;

        static MyClass()
        {
            Number3 = 300;
        }

        public MyClass()
        {
            Number2 = 200;//在构造器中赋值
            ChangeValue(out Number4,ref Number5);
            ChangeValue1(Number4, Number5);//值没有改变
        }

        public void ChangeValue(out int p1, ref int p2)
        {
            p1 = 401;
            p2 = 501;
        }

        public void ChangeValue1(int p1,int p2)
        {
            p1 = 402;
            p2 = 502;
        }
    }

}