Code:
public void WriteLine(String sFilename, int iLine, string sLines, bool bReplace)
    {
        string sContent = "";
        string[] delimiterstring = { "\r\n" };
 
        if (File.Exists(sFilename))
        {
            StreamReader myFile = new StreamReader(sFilename, System.Text.Encoding.Default);
            sContent = myFile.ReadToEnd();
            myFile.Close();
        }

        string[] sCols = sContent.Split(delimiterstring, StringSplitOptions.None);
 
        if (sCols.Length >= iLine)
        {
            if (!bReplace)
                sCols[iLine - 1] = sLines + "\r\n" + sCols[iLine - 1];
            else
                sCols[iLine - 1] = sLines;
 
            sContent = "";
            for (int x = 0; x < sCols.Length-1; x++)
            {
                sContent += sCols[x] + "\r\n";
            }
            sContent += sCols[sCols.Length-1];

        }
        else
        {
            for (int x = 0; x < iLine - sCols.Length; x++)
                sContent += "\r\n";
 
            sContent += sLines;
        }
 

        StreamWriter mySaveFile = new StreamWriter(sFilename);
        mySaveFile.Write(sContent);
        mySaveFile.Close();
    }
}