怎样用程序修改记事本中的指定行的字符串?

for example

using System;
using System.IO;
using System.Text;

string fileName = "test.txt";
String sReplacement = "hello world";
int n = -1; // line number to replace

StringBuilder sb = new StringBuilder();
StreamReader sr = new StreamReader(fileName,Encoding.ASCII);

int nLine=0;
while (sr.Peek() > -1)
{
nLine++;
String s = sr.ReadLine();
if (nLine == n)
sb.Append(sReplacement+"\r\n");
else
sb.Append(s+"\r\n");
}

sr.Close();

StreamWriter sw = new StreamWriter(fileName,false, Encoding.ASCII);
sw.Write(sb.ToString());
sw.Close();