[.NET][C#] Manager with singleton for manipulate the Windows Hosts File

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Text.RegularExpressions;

namespace Framework
{
    public class ManagerHostFile
    {
        #region singleton

        private static ManagerHostFile instance = null;
        private static readonly object padlock = new object();

        ManagerHostFile()
        {
        }

        public static ManagerHostFile Instance
        {
            get
            {
                lock (padlock)
                {
                    if (instance == null)
                    {
                        instance = new ManagerHostFile();
                    }

                    hostList = ParseHostsFile(ref noHosts);
                    return instance;
                }
            }
        }
        #endregion singleton

        private IList<stHost> hostList = new IList<stHost>();

        #region General Methods

        private string GetHostsPath()
        {
            // Gets dynamic hosts path from system folder
            string systemPath = Environment.GetFolderPath(Environment.SpecialFolder.System);
            string hostsPath = Path.Combine(systemPath, @"drivers\etc");
            return hostsPath;
        }

        private string GetHostsFilename()
        {
            // Creates full host filename
            string hostsFileName = Path.Combine(GetHostsPath(), "hosts");
            return hostsFileName;
        }

        private bool StartWithNumber(string str)
        {
            // Check if string starts with an ip number
            Regex regex = new Regex(@"^\d{1}");
            return regex.IsMatch(str);
        }

        private bool StartWithNumberCommented(string str)
        {
            // Check if strings starts with a commented ip number
            Regex regex = new Regex(@"^#{1}\d{1}");
            return regex.IsMatch(str);
        }

        #endregion

        private struct stHost
        {
            public string HostDescription;
            public string HostComment;
            public string IP;
            public bool Selected;
        }


	///
	/// Parse Windows Host File
	///
        private IList<stHost> ParseHostsFile(ref bool noHosts)
        {
            noHosts = true;
            IList<string> lineList = new List<string>();
            IList<stHost> hostList = new List<stHost>();

            try
            {
                // Reads all lines from hosts file
                FileInfo fileInfo = new FileInfo(GetHostsFilename());
                StreamReader sr = fileInfo.OpenText();
                while (sr.EndOfStream == false)
                {
                    string line = sr.ReadLine();
                    line = line.Trim();

                    //if (string.IsNullOrEmpty(line) == false)
                    {
                        lineList.Add(line.Replace("\t", " ").Replace("\r", " ").Replace("\n", " "));
                    }
                }
                sr.Close();

                // Creates a host list
                for (int f = 0; f < lineList.Count; f++)
                {
                    // If we found an host entry with a comment in the line before, ex:
                    //
                    // # DIT - x.acme.com       [host comment]
                    // #38.25.63.10				acme          [host ip]   [host address]
                    //
                    if (lineList[f].StartsWith("# ") &&
                        (f + 1) < lineList.Count &&
                        (StartWithNumberCommented(lineList[f + 1]) || StartWithNumber(lineList[f + 1])))
                    {
                        // Get host comment
                        string hostComment = lineList[f].Replace("# ", "");

                        // Retrieve host details line
                        string hostDetailsLine = lineList[f + 1];

                        // Get host IP
                        string hostIp = hostDetailsLine.Replace("#", "");
                        if (hostIp.IndexOf(" ") > -1)
                        {
                            hostIp = hostIp.Substring(0, hostIp.IndexOf(" "));
                        }

                        // Get host address
                        string hostAddress = string.Empty;
                        if ((hostDetailsLine.IndexOf(" ") > -1) && hostDetailsLine.IndexOf(" ") < hostDetailsLine.Length)
                        {
                            hostAddress = hostDetailsLine.Substring(hostDetailsLine.IndexOf(" ")).Trim();
                        }

                        // Creates the host description with comment, addresss and IP
                        string hostDescription = hostComment + " [" + hostAddress + " at " + hostIp + "]";

                        // Check if host is not commented out
                        bool selected = StartWithNumber(hostDetailsLine);


                        // Search for repeated IP entries
                        bool hostAlreadyExists = false;
                        foreach (stHost hostItemSearch in hostList)
                        {
                            if (hostItemSearch.IP == hostIp)
                            {
                                hostAlreadyExists = true;
                            }
                        }

                        // Make sure we don´t add repeated IP entries
                        // And also only entries with host IP and host address
                        if (hostAlreadyExists == false &&
                            string.IsNullOrEmpty(hostIp) == false &&
                            string.IsNullOrEmpty(hostAddress) == false)
                        {
                            // Create and add new host object to the list
                            stHost hostItem = new stHost();
                            hostItem.HostDescription = hostDescription;
                            hostItem.HostComment = hostComment;
                            hostItem.IP = hostIp;
                            hostItem.Selected = selected;
                            hostList.Add(hostItem);

                            // If we found a single selected host, no hosts menu option can´t be enabled
                            if (selected)
                            {
                                noHosts = false;
                            }
                        }
                    }
                    else if ((StartWithNumberCommented(lineList[f]) || StartWithNumber(lineList[f])) &&
                        (f - 1) >= 0 &&
                        lineList[f - 1].StartsWith("# ") == false)
                    {
                        // Parse strings in both formats:
                        //
                        // #102.54.94.97 rhino.acme.com # Rhino server     [host ip]  [host address] [host comment]
                        //
                        // #10.4.2.4 www.test1234567.com        [host ip]  [host address]
                        //

                        // Retrieve host ip
                        string hostIp = lineList[f].Replace("#", "");
                        if (hostIp.IndexOf(" ") > -1)
                        {
                            hostIp = hostIp.Substring(0, hostIp.IndexOf(" "));
                        }

                        // Gets host address and comment
                        string hostFullText = string.Empty;
                        if (lineList[f].IndexOf(" ") > -1)
                        {
                            hostFullText = lineList[f].Substring(lineList[f].IndexOf(" ") + 1).TrimStart();
                        }

                        // Gets only host address
                        string hostAddress = hostFullText;
                        if (hostFullText.IndexOf(" ") > -1)
                        {
                            hostAddress = hostFullText.Substring(0, hostFullText.IndexOf(" ")).Trim();
                        }

                        // Gets only host comment
                        string hostComment = string.Empty;
                        if (hostFullText.IndexOf("#") > -1 && ((hostFullText.IndexOf("#") + 1) < hostFullText.Length))
                        {
                            hostComment = hostFullText.Substring(hostFullText.IndexOf("#") + 1).Trim();
                        }

                        // Creates the host description with comment, addresss and IP
                        string hostDescription = hostComment + " [" + hostAddress + " at " + hostIp + "]";

                        // Check if host is not commented out
                        bool selected = StartWithNumber(lineList[f]);


                        // Search for repeated IP entries
                        bool hostAlreadyExists = false;
                        foreach (stHost hostItemSearch in hostList)
                        {
                            if (hostItemSearch.IP == hostIp)
                            {
                                hostAlreadyExists = true;
                            }
                        }

                        // Make sure we don´t add repeated IP entries
                        // And also only entries with host IP and host address
                        if (hostAlreadyExists == false &&
                            string.IsNullOrEmpty(hostIp) == false &&
                            string.IsNullOrEmpty(hostAddress) == false)
                        {
                            // Create and add new host object to the list
                            stHost hostItem = new stHost();
                            hostItem.HostDescription = hostDescription;
                            hostItem.HostComment = hostComment;
                            hostItem.IP = hostIp;
                            hostItem.Selected = selected;
                            hostList.Add(hostItem);

                            // If we found a single selected host, no hosts menu option can´t be enabled
                            if (selected)
                            {
                                noHosts = false;
                            }
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }

            return hostList;
        }

        /// <summary>
        /// Update HostFile
        /// </summary>
        /// <param name="ipHost"></param>
        public void UpdateHosts(string ipHost)
        {
            try
            {
                StringBuilder sbHostsNew = new StringBuilder();

                // Read hosts file
                FileInfo fileInfo = new FileInfo(GetHostsFilename());
                StreamReader sr = fileInfo.OpenText();
                while (sr.EndOfStream == false)
                {
                    string line = sr.ReadLine();

                    // Check if we found selected host commented out
                    if (string.IsNullOrEmpty(ipHost) == false &&
                        line.Contains(ipHost) &&
                        line.StartsWith("#" + ipHost))
                    {
                        // Uncomment selected host
                        line = line.Substring(1);
                    }
                    else if (((string.IsNullOrEmpty(ipHost) == false) && (line.Contains(ipHost) == false)) ||
                               string.IsNullOrEmpty(ipHost) == true)
                    {
                        // Comment out other entries except already uncommend host entry
                        if (StartWithNumber(line))
                        {
                            line = "#" + line;
                        }
                    }
                    sbHostsNew.Append(line + "\r\n");
                }
                sr.Close();

                // Saves new host file and prevent file watcher from raising event
                TextWriter tw = new StreamWriter(GetHostsFilename());
                tw.Write(sbHostsNew.ToString());
                tw.Flush();
                tw.Close();
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
    }
}