要求模拟string类的部分功能，写一个MyString类，它的部分代码如下所示：
    class MyString : IComparable<MyString>
    {
        private List<char> chs;
        public char this[int index]
        {
            get
            {
                ......
            }
        }
    }
要求添加代码，使得以下测试函数可执行，每个输出操作的结果如注释所示。除了ToString方法外，其余方法实现时不得使用string类的功能。
        static void Main(string[] args)
        {
            MyString str1 = new MyString("Hello");
            MyString str2 = new MyString("Hello world");
            Console.WriteLine(str2.Length);//输出：11
            Console.WriteLine(str2[6]);//输出：w
            Console.WriteLine(str1.ToLower());//输出：hello
            Console.WriteLine(str1.CompareTo(str2.Substring(0, 5)));//输出：0
            Console.WriteLine(str1.CompareTo(str2));//输出：-1
            MyString str3 = str1 + new MyString(". Nice to meet you.");
            Console.WriteLine(str3);//输出：Hello. Nice to meet you.
        }

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

namespace ConsoleApp6
{
    class MyString : IComparable<MyString>
    {
        private List<char> chs;
        public char this[int index]
        {
            get
            {
                if (index < 0 || index >= Length)
                    throw new IndexOutOfRangeException();
                return chs[index];
            }
        }
        public MyString()
        {
            chs = new List<char>();
        }
        public MyString(char[] chars)
        {
            chs = new List<char>(chars);
        }
        public MyString(string str) : this(str.ToCharArray())
        {
        }
        public int Length
        {
            get => chs.Count;
        }
        public MyString Substring(int startIndex, int length)
        {
            if (startIndex < 0 || startIndex >= Length || Length < 0 || startIndex + length >= Length)
                throw new IndexOutOfRangeException();
            MyString newStr = new MyString();
            for (int i = startIndex; i < startIndex + length; i++)
            {
                newStr.chs.Add(chs[i]);
            }
            return newStr;
        }

        public MyString ToLower()
        {
            char[] temp = new char[Length];
            for (int i = 0; i < chs.Count; i++)
            {
                temp[i] = Char.ToLower(chs[i]);
            }

            return new MyString(temp);
        }

        public static MyString operator+(MyString str1, MyString str2)
        {
            str1.chs.AddRange(str2.chs);

            return str1;
        }
        public int CompareTo(MyString other)
        {
            if(other == null)
                throw new ArgumentNullException();
            int size = (Length > other.Length) ? other.Length : Length;
            for (int i = 0; i < chs.Count; i++)
            {
                if (chs[i] > other[i])
                    return 1;
                else if (chs[i] < other[i])
                    return -1;
            }
            if (Length > other.Length)
                return 1;
            else if (Length < other.Length)
                return -1;
            else
                return 0;
        }
        public override string ToString()
        {
            return new string(chs.ToArray());
        }
    }
    class Test
    {
        static void Main(string[] args)
        {
            MyString str1 = new MyString("Hello");
            MyString str2 = new MyString("Hello world");
            Console.WriteLine(str2.Length);//11
            Console.WriteLine(str2[6]);//w
            Console.WriteLine(str1.ToLower());//hello
            Console.WriteLine(str1.CompareTo(str2.Substring(0, 5)));//0
            Console.WriteLine(str1.CompareTo(str2));//-1
            MyString str3 = str1 + new MyString(". Nice to meet you.");
            Console.WriteLine(str3);//Hello. Nice to meet you.
        }
    }
}