MailFolderCountChanged Event

public event EventHandler<EventArgs> CountChanged

using System;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

using MailKit;
using MailKit.Net.Imap;
using MailKit.Security;

namespace ImapIdleExample {
    class Program
    {
        // Connection-related properties
        const SecureSocketOptions SslOptions = SecureSocketOptions.Auto;
        const string Host = "imap.gmail.com";
        const int Port = 993;

        // Authentication-related properties
        const string Username = "username@gmail.com";
        const string Password = "password";

        public static void Main (string[] args)
        {
            using (var client = new IdleClient (Host, Port, SslOptions, Username, Password)) {
                Console.WriteLine ("Hit any key to end the demo.");

                var idleTask = client.RunAsync ();

                Task.Run (() => {
                    Console.ReadKey (true);
                }).Wait ();

                client.Exit ();

                idleTask.GetAwaiter ().GetResult ();
            }
        }
    }

    class IdleClient : IDisposable
    {
        readonly string host, username, password;
        readonly SecureSocketOptions sslOptions;
        readonly int port;
        List<IMessageSummary> messages;
        CancellationTokenSource cancel;
        CancellationTokenSource done;
        bool messagesArrived;
        ImapClient client;

        public IdleClient (string host, int port, SecureSocketOptions sslOptions, string username, string password)
        {
            this.client = new ImapClient (new ProtocolLogger (Console.OpenStandardError ()));
            this.messages = new List<IMessageSummary> ();
            this.cancel = new CancellationTokenSource ();
            this.sslOptions = sslOptions;
            this.username = username;
            this.password = password;
            this.host = host;
            this.port = port;
        }

        async Task ReconnectAsync ()
        {
            if (!client.IsConnected)
                await client.ConnectAsync (host, port, sslOptions, cancel.Token);

            if (!client.IsAuthenticated) {
                await client.AuthenticateAsync (username, password, cancel.Token);

                await client.Inbox.OpenAsync (FolderAccess.ReadOnly, cancel.Token);
            }
        }

        async Task FetchMessageSummariesAsync (bool print)
        {
            IList<IMessageSummary> fetched = null;

            do {
                try {
                    // fetch summary information for messages that we don't already have
                    int startIndex = messages.Count;

                    fetched = client.Inbox.Fetch (startIndex, -1, MessageSummaryItems.Full | MessageSummaryItems.UniqueId, cancel.Token);
                    break;
                } catch (ImapProtocolException) {
                    // protocol exceptions often result in the client getting disconnected
                    await ReconnectAsync ();
                } catch (IOException) {
                    // I/O exceptions always result in the client getting disconnected
                    await ReconnectAsync ();
                }
            } while (true);

            foreach (var message in fetched) {
                if (print)
                    Console.WriteLine ("{0}: new message: {1}", client.Inbox, message.Envelope.Subject);
                messages.Add (message);
            }
        }

        async Task WaitForNewMessagesAsync ()
        {
            do {
                try {
                    if (client.Capabilities.HasFlag (ImapCapabilities.Idle)) {
                        // Note: IMAP servers are only supposed to drop the connection after 30 minutes, so normally
                        // we'd IDLE for a max of, say, ~29 minutes... but GMail seems to drop idle connections after
                        // about 10 minutes, so we'll only idle for 9 minutes.
                        done = new CancellationTokenSource (new TimeSpan (0, 9, 0));
                        try {
                            await client.IdleAsync (done.Token, cancel.Token);
                        } finally {
                            done.Dispose ();
                            done = null;
                        }
                    } else {
                        // Note: we don't want to spam the IMAP server with NOOP commands, so lets wait a minute
                        // between each NOOP command.
                        await Task.Delay (new TimeSpan (0, 1, 0), cancel.Token);
                        await client.NoOpAsync (cancel.Token);
                    }
                    break;
                } catch (ImapProtocolException) {
                    // protocol exceptions often result in the client getting disconnected
                    await ReconnectAsync ();
                } catch (IOException) {
                    // I/O exceptions always result in the client getting disconnected
                    await ReconnectAsync ();
                }
            } while (true);
        }

        async Task IdleAsync ()
        {
            do {
                try {
                    await WaitForNewMessagesAsync ();

                    if (messagesArrived) {
                        await FetchMessageSummariesAsync (true);
                        messagesArrived = false;
                    }
                } catch (OperationCanceledException) {
                    break;
                }
            } while (!cancel.IsCancellationRequested);
        }

        public async Task RunAsync ()
        {
            // connect to the IMAP server and get our initial list of messages
            try {
                await ReconnectAsync ();
                await FetchMessageSummariesAsync (false);
            } catch (OperationCanceledException) {
                await client.DisconnectAsync (true);
                return;
            }

            // Note: We capture client.Inbox here because cancelling IdleAsync() *may* require
            // disconnecting the IMAP client connection, and, if it does, the `client.Inbox`
            // property will no longer be accessible which means we won't be able to disconnect
            // our event handlers.
            var inbox = client.Inbox;

            // keep track of changes to the number of messages in the folder (this is how we'll tell if new messages have arrived).
            inbox.CountChanged += OnCountChanged;

            // keep track of messages being expunged so that when the CountChanged event fires, we can tell if it's
            // because new messages have arrived vs messages being removed (or some combination of the two).
            inbox.MessageExpunged += OnMessageExpunged;

            // keep track of flag changes
            inbox.MessageFlagsChanged += OnMessageFlagsChanged;

            await IdleAsync ();

            inbox.MessageFlagsChanged -= OnMessageFlagsChanged;
            inbox.MessageExpunged -= OnMessageExpunged;
            inbox.CountChanged -= OnCountChanged;

            await client.DisconnectAsync (true);
        }

        // Note: the CountChanged event will fire when new messages arrive in the folder and/or when messages are expunged.
        void OnCountChanged (object sender, EventArgs e)
        {
            var folder = (ImapFolder) sender;

            // Note: because we are keeping track of the MessageExpunged event and updating our
            // 'messages' list, we know that if we get a CountChanged event and folder.Count is
            // larger than messages.Count, then it means that new messages have arrived.
            if (folder.Count > messages.Count) {
                int arrived = folder.Count - messages.Count;

                if (arrived > 1)
                    Console.WriteLine ("\t{0} new messages have arrived.", arrived);
                else
                    Console.WriteLine ("\t1 new message has arrived.");

                // Note: your first instinct may be to fetch these new messages now, but you cannot do
                // that in this event handler (the ImapFolder is not re-entrant).
                // 
                // Instead, cancel the `done` token and update our state so that we know new messages
                // have arrived. We'll fetch the summaries for these new messages later...
                messagesArrived = true;
                done?.Cancel ();
            }
        }

        void OnMessageExpunged (object sender, MessageEventArgs e)
        {
            var folder = (ImapFolder) sender;

            if (e.Index < messages.Count) {
                var message = messages[e.Index];

                Console.WriteLine ("{0}: message #{1} has been expunged: {2}", folder, e.Index, message.Envelope.Subject);

                // Note: If you are keeping a local cache of message information
                // (e.g. MessageSummary data) for the folder, then you'll need
                // to remove the message at e.Index.
                messages.RemoveAt (e.Index);
            } else {
                Console.WriteLine ("{0}: message #{1} has been expunged.", folder, e.Index);
            }
        }

        void OnMessageFlagsChanged (object sender, MessageFlagsChangedEventArgs e)
        {
            var folder = (ImapFolder) sender;

            Console.WriteLine ("{0}: flags have changed for message #{1} ({2}).", folder, e.Index, e.Flags);
        }

        public void Exit ()
        {
            cancel.Cancel ();
        }

        public void Dispose ()
        {
            client.Dispose ();
            cancel.Dispose ();
        }
    }
}