append content to txt file.

 public static void WriteLogs(string log)
        {
            string filePath = HttpContext.Current.Server.MapPath("~\\BllLogs.log");
            List<string> logList = new List<string>();
            if (File.Exists(filePath))
                logList.AddRange(File.ReadAllLines(filePath));
            logList.Add(log);
            File.WriteAllLines(filePath, logList.ToArray());

        }