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 "";
}
}
}