小弟最近在学习C#语言 遇到了一个这样的题目 让小弟把"abc hello bcd" 变成"abc landy bcd"结果 运行结果总是不正确
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace practise
{
    class Program
    {
        static void Main(string[] args)
        {
            string str = "abc hello bcd";
            char[] chars = str.ToCharArray();
            int index = str.IndexOf("hello");
                Console.WriteLine(index);
                Console.ReadLine();

                if (index != -1)
                {

                    chars[index] = 'l';
                    chars[index++] = 'a';
                    chars[index++] = 'n';
                    chars[index++] = 'd';
                    chars[index++] = 'y';
                    str = new string(chars);
                    Console.WriteLine(str);
                    Console.ReadLine();
                }                                                                  
        }
    }
}}结果为"abc andyo bcd" 小弟 怎么也想不通 还请各位DX帮帮忙啊