using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net.Mail;
using System.Threading.Tasks;
using DnsClient;
using Microsoft.VisualBasic.FileIO;

namespace EmailVerification
{
    class Program
    {
        static void Main(string[] args)
        {

            const string sourceFile = @"c:\subscribers.csv";
            const string destinationFile = @"c:\invalid-subscribers.txt";

            var spamEmailAddresses = new List<string>();

            try
            {
                Console.WriteLine("Source File set to " + sourceFile);
                Console.WriteLine("Destination File set to " + destinationFile);

                Console.WriteLine("Parsing the source file ...");

                using (var parser = new TextFieldParser(sourceFile))
                {
                    var headerRow = true;
                    parser.Delimiters = new[] { "," };
                    while (!parser.EndOfData)
                    {
                        var row = parser.ReadFields();
                        if (row == null) continue;

                        if (headerRow)
                        {
                            headerRow = false;
                        }
                        else
                        {
                            var email = row[1];

                            try
                            {
                                var valid = IsValid(email);

                                if (!valid)
                                {
                                    Console.WriteLine(email + " has been flagged");

                                    spamEmailAddresses.Add(email);
                                }
                            }
                            catch (FormatException ex)
                            {
                                Console.WriteLine("Exception : " + email + " : " + ex.Message);

                                spamEmailAddresses.Add(email);
                            }
                        }
                    }
                }

                Console.WriteLine("Writing to destination file");

                using (var writer = new StreamWriter(destinationFile, false))
                {
                    foreach (var spamEmailAddress in spamEmailAddresses)
                    {
                        writer.WriteLine(spamEmailAddress);
                    }
                }

                Console.WriteLine("Finished writing to destination file");
            }
            catch (Exception ex)
            {
                Console.WriteLine("Exception : " + ex.Message);
            }

            Console.ReadKey();
        }

        #region Sync methods

        static public bool IsValid(string email)
        {
            try
            {
                var mailAddress = new MailAddress(email);
                var host = mailAddress.Host;

                return CheckDnsEntries(host);
            }
            catch (FormatException)
            {
                return false;
            }
        }

        static public bool CheckDnsEntries(string host)
        {
            try
            {
                var lookupClient = new LookupClient
                {
                    Timeout = TimeSpan.FromSeconds(5)
                };
                var result = lookupClient
                    .Query(host, QueryType.ANY);
                var records = result
                    .Answers
                    .Where(record => record.RecordType == DnsClient.Protocol.ResourceRecordType.A ||
                                     record.RecordType == DnsClient.Protocol.ResourceRecordType.AAAA ||
                                     record.RecordType == DnsClient.Protocol.ResourceRecordType.MX);
                return records.Any();
            }
            catch (DnsResponseException)
            {
                return false;
            }
        }

        #endregion

        #region Async methods

        static public Task<bool> IsValidAsync(string email)
        {
            try
            {
                var mailAddress = new MailAddress(email);
                var host = mailAddress.Host;

                return CheckDnsEntriesAsync(host);
            }
            catch (FormatException)
            {
                return Task.FromResult(false);
            }
        }

        static public async Task<bool> CheckDnsEntriesAsync(string host)
        {
            try
            {
                var lookupClient = new LookupClient
                {
                    Timeout = TimeSpan.FromSeconds(5)
                };
                var result = await lookupClient
                    .QueryAsync(host, QueryType.ANY)
                    .ConfigureAwait(false);
                var records = result
                    .Answers
                    .Where(record => record.RecordType == DnsClient.Protocol.ResourceRecordType.A ||
                                     record.RecordType == DnsClient.Protocol.ResourceRecordType.AAAA ||
                                     record.RecordType == DnsClient.Protocol.ResourceRecordType.MX);
                return records.Any();
            }
            catch (DnsResponseException)
            {
                return false;
            }
        }

        #endregion
    }
}