[原创]一个查找并且替换的算法

namespace ConsoleApp
{
 
    class Program
    {

        public static void Main()
        {
            string input=Console.ReadLine();
            string output = Program.Replace (input, "ab", "00");
            System.Console.ReadLine();

        }

        private static string Replace(string source, string a, string b)
        {
            string result = source;
            int position = 0;//发现的位置
            int len = 0;//
            int num = 0;//记录发现的要替换的个数

            for (int i = 0; i < result.Length - 1; i++)
            {
                if (result[i] == a[0])
                {
                    position = i;
                    len = 0;
                    //int j = 0;

                    while (len<=a.Length-1 && result[position + len] == a[len])
                    {
                        len++;
                    }

                    if (len == a.Length)
                    {
                        Console.WriteLine("find a positon");
                        num++;
                        i = position+len;
                        Console.WriteLine("the posion is " + position + "\n the num is " + num);
                        continue;
                    }

                    if (a.Length == b.Length)
                    {

                        for (int j = position; j < a.Length - 1; j++)
                        {
                            result[j] = a[j];
                        }
                    }
                    else if (a.Length > b.Length)
                    {
 
                    }
                    else
                    {

                    }
                }
            }
            return "";
        }
    }
}

暂时实现了查找符合条件的个数和位置，替换的时候我想这样
if (a.Length == b.Length)
                    {

                        for (int j = position; j < a.Length - 1; j++)
                        {
                            result[j] = a[j];
                        }
                    }
                    else if (a.Length > b.Length)
                    {
 
                    }
                    else
                    {

                    }
要考虑三种情况，要查找的和要替换的字符串的长度相等，大于，小于。
相等就可以直接替换相应位置的字符就可以了，可是发现错误
////..............................
 无法对属性或索引器“string.this[int]”赋值 -- 它是只读的  
//.....................
我的思路是搞一个数组存放没有替换的和替换了的所有字符，不知各位有没有更好的呢，在这里先谢谢你的回复！！