System.Net.Mail.POP3Client

using System;
using System.Net.Sockets;
using System.IO;
using System.Text.RegularExpressions;
using System.Net.Security;
using System.Security.Cryptography;
using System.Text;
using System.Linq;
using System.Collections.Generic;

namespace System.Net.Mail{
    public struct MailboxStatus{
        public int Messages{ get; set; }
        public int Size{ get; set; }
        public override string ToString(){
            return string.Format("[MailboxStatus: Messages={0}, Size={1}]", Messages, Size);
        }
    }
    public struct MessageInfo{
        public int MessageNumber{ get; set; }
        public int Size{ get; set; }
        public override string ToString(){
            return string.Format("[MessageInfo: MessageNumber={0}, Size={1}]", MessageNumber, Size);
        }
    }
    public class POP3Error:Exception{
        public override string Message{ get{return _Message;} }
        private string _Message;
        public POP3Error(string Message){
            _Message = Message;
        }
        public override string ToString(){
            return string.Format("[POP3Error: Message={0}]", Message);
        }
    }
    public class POP3Client{
        private TcpClient tcpClient;
        private TextWriter sender;
        private TextReader receiver;
        private string APOPChallange;
        public POP3Client(){
            APOPChallange = "";
        }
        public void Connect(string Server, int Port = 110, bool UseSSL = false){
            tcpClient = new TcpClient();
            tcpClient.Connect(Server,Port);
            Stream stream = tcpClient.GetStream();
            if(UseSSL){
                stream = new SslStream(stream);
                ((SslStream)stream).AuthenticateAsClient(Server);
            }
            sender = new StreamWriter(stream);
            receiver = new StreamReader(stream);
            string Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
            APOPChallange = Regex.Match(Line, "^\\+OK (<[^>]+>)").Groups[1].Value;
        }
        public bool AuthenticateUser(string Username, string Password){
            string Line;
            if(!String.IsNullOrEmpty(APOPChallange)){
                string APOPResponse = string.Join("", MD5.Create().ComputeHash(Encoding.UTF8.GetBytes(APOPChallange + Password)).Select(b => string.Format("{0:x2}", b)));
                sender.WriteLine("APOP {0} {1}", Username, APOPResponse);
                sender.Flush();
                Line = receiver.ReadLine();
                if(!Line.ToUpper().StartsWith("+OK")){
                    return false;
                }
                return true;
            }
            else{
                sender.WriteLine("USER {0}", Username);
                sender.Flush();
                Line = receiver.ReadLine();
                if(!Line.ToUpper().StartsWith("+OK")){
                    return false;
                }
                sender.WriteLine("PASS {0}", Password);
                sender.Flush();
                Line = receiver.ReadLine();
                if(!Line.ToUpper().StartsWith("+OK")){
                    throw new Exception(Line);
                }
                return true;
            }
        }
        public MailboxStatus Status(){
            string Line;
            sender.WriteLine("STAT");
            sender.Flush();
            Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
            Match match = Regex.Match(Line, "^\\+OK[ \t]+([0-9]+)[ \t]+([0-9]+)");
            if(match.Groups.Count == 3){
                int Messages, Size;
                if(int.TryParse(match.Groups[1].Value, out Messages) && int.TryParse(match.Groups[2].Value, out Size)){
                    return new MailboxStatus{Messages = Messages, Size = Size};
                }
                throw new FormatException("Invalid status response");
            }
            throw new FormatException("Invalid status response");
        }
        public List<MessageInfo> List(int MessageNumber = 0){
            if(MessageNumber <= 0){
                string Line;
                sender.WriteLine("LIST");
                sender.Flush();
                Line = receiver.ReadLine();
                if(!Line.ToUpper().StartsWith("+OK")){
                    throw new POP3Error(Line);
                }
                List<MessageInfo> Messages = new List<MessageInfo>();
                while((Line = receiver.ReadLine()) != null){
                    if(Line == "."){
                        break;
                    }
                    Match match = Regex.Match(Line, "^\\+OK[ \t]+([0-9]+)[ \t]+([0-9]+)");
                    if(match.Groups.Count == 3){
                        int Size;
                        if(int.TryParse(match.Groups[1].Value, out MessageNumber) && int.TryParse(match.Groups[2].Value, out Size)){
                            Messages.Add(new MessageInfo{MessageNumber = MessageNumber, Size = Size});
                        }
                        else{
                            throw new FormatException("Invalid list response");
                        }
                    }
                    else{
                        throw new FormatException("Invalid list response");
                    }
                }
                return Messages;
            }
            else{
                string Line;
                sender.WriteLine("LIST {0}",MessageNumber);
                sender.Flush();
                Line = receiver.ReadLine();
                if(!Line.ToUpper().StartsWith("+OK")){
                    throw new POP3Error(Line);
                }
                Match match = Regex.Match(Line, "^\\+OK[ \t]+([0-9]+)[ \t]+([0-9]+)");
                if(match.Groups.Count == 3){
                    int Size;
                    if(int.TryParse(match.Groups[1].Value, out MessageNumber) && int.TryParse(match.Groups[2].Value, out Size)){
                        return new List<MessageInfo>(new MessageInfo[]{new MessageInfo{MessageNumber = MessageNumber, Size = Size}});
                    }
                    throw new FormatException("Invalid list response");
                }
                throw new FormatException("Invalid list response");
            }
        }
        public string Retrieve(int MessageNumber){
            string Line;
            sender.WriteLine("RETR {0}",MessageNumber);
            sender.Flush();
            Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
            string MailContent = "";
            while((Line = receiver.ReadLine())!=null){
                if(Line == "."){
                    break;
                }
                MailContent += Line + Environment.NewLine;
            }
            return MailContent;
        }
        public string Retrieve(int MessageNumber, int Lines){
            string Line;
            sender.WriteLine("TOP {0} {1}",MessageNumber,Lines);
            sender.Flush();
            Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
            string MailContent = "";
            while((Line = receiver.ReadLine())!=null){
                if(Line == "."){
                    break;
                }
                MailContent += Line + Environment.NewLine;
            }
            return MailContent;
        }
        public List<string> Capabilities(){
            string Line;
            sender.WriteLine("CAPA");
            sender.Flush();
            Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
            List<string> capas = new List<string>();
            while((Line = receiver.ReadLine())!=null){
                if(Line == "."){
                    break;
                }
                capas.Add(Line);
            }
            return capas;
        }
        public string UniqueID(int MessageNumber){
            string Line;
            sender.WriteLine("UIDL {0}",MessageNumber);
            sender.Flush();
            Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
            string uidl = Regex.Match(Line, "^\\+OK[ \t][0-9]+[ \t](.+)").Groups[1].Value;
            if(string.IsNullOrEmpty(uidl)){
                throw new POP3Error("Invalid uidl response");
            }
            return uidl;
        }
        public void Reset(){
            string Line;
            sender.WriteLine("RSET");
            sender.Flush();
            Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
        }
        public void Delete(int MessageNumber){
            string Line;
            sender.WriteLine("DELE {0}", MessageNumber);
            sender.Flush();
            Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
        }
        public void Quit(){
            string Line;
            sender.WriteLine("QUIT");
            sender.Flush();
            Line = receiver.ReadLine();
            if(!Line.ToUpper().StartsWith("+OK")){
                throw new POP3Error(Line);
            }
            receiver.Close();
            sender.Close();
            tcpClient.Close();
        }
    }
}