{C#}修饰符.readonly

特点

• 属运行时常量，占内存
• 定义时或构造器中赋值，仅修饰字段
• 引用类型时，其子字段可改值，即其地址只读。
• 归创建它的实例独有，共有时加前缀static
• 支持所有数据类型

using System.Collections.Generic;
using static System.Console;

namespace ReadOnlyDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            var demo = new Demo();
            demo.Reset();

            ReadKey();
        }
    }

    public class Demo
    {
        // 值类型，声明时赋值OK
        private readonly string name = "zhang";
        private readonly int age;

        // 引用类型，声明时赋值OK
        private readonly List<Person> persons = new List<Person>
        {
            new Person("张三",10),
            new Person("李四",20),
        };

        // 构造器内赋值OK
        public Demo()
        {
            name = "li";
            age = 15;

            persons = new List<Person>
            {
                new Person("王五",30),
                new Person("曹六",40),
            };
        }

        public void Reset()
        {
            // 只读值类型，报错情形
            // name = "曹六";
            // age = 15;

            // 只读引用类型，许可情形
            persons[0] = new Person("曹六", 25);

            persons[0].Name = "曹六一";
            persons[0].Age = 35;

            // 只读引用类型，报错情形
            // persons = null;
        }
    }

    public class Person
    {
        public string Name;
        public int Age;

        public Person(string name, int age)
        {
            Name = name;
            Age = age;
        }
    }
}