ImapIdleSessionReadChangesAsync Method

public IAsyncEnumerable<ImapIdleChanges> ReadChangesAsync(
	CancellationToken cancellationToken = default
)

                    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 ImapIdleSessionExample {
    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;
        readonly List<IMessageSummary> messages;
        readonly CancellationTokenSource cancel;
        readonly FetchRequest request;
        readonly ImapClient client;

        public IdleClient (string host, int port, SecureSocketOptions sslOptions, string username, string password)
        {
            this.client = new ImapClient (new ProtocolLogger (Console.OpenStandardError ()));
            this.request = new FetchRequest (MessageSummaryItems.Full | MessageSummaryItems.UniqueId);
            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;

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

                    fetched = await client.Inbox.FetchAsync (startIndex, -1, request, 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<ImapIdleChanges> WaitForChangesAsync (ImapIdleSession session)
        {
            do {
                try {
                    // Note: The session uses IDLE when the server supports it (refreshing it periodically
                    // so that the server doesn't drop the connection) and falls back to polling with NOOP
                    // when it doesn't.
                    return await session.WaitForChangesAsync (cancel.Token);
                } 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);
        }

        void ProcessChanges (ImapIdleChanges changes)
        {
            // Note: Events are reported in the order that the server sent them, so applying
            // them in order keeps our 'messages' cache consistent with the folder.
            foreach (var e in changes.Events) {
                switch (e) {
                case CountChangedIdleEvent countChanged:
                    Console.WriteLine ("{0}: message count changed from {1} to {2}.", e.Folder, countChanged.PreviousCount, countChanged.Count);
                    break;
                case MessageExpungedIdleEvent expunged:
                    if (expunged.Index < messages.Count) {
                        var message = messages[expunged.Index];

                        Console.WriteLine ("{0}: message #{1} has been expunged: {2}", e.Folder, expunged.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 expunged.Index.
                        messages.RemoveAt (expunged.Index);
                    } else {
                        Console.WriteLine ("{0}: message #{1} has been expunged.", e.Folder, expunged.Index);
                    }
                    break;
                case MessagesVanishedIdleEvent vanished:
                    // Note: This is only reported by servers when QRESYNC has been enabled.
                    Console.WriteLine ("{0}: {1} messages have vanished.", e.Folder, vanished.UniqueIds.Count);
                    break;
                case MessageChangedIdleEvent changed:
                    if (changed.Message.Flags.HasValue)
                        Console.WriteLine ("{0}: flags have changed for message #{1} ({2}).", e.Folder, changed.Message.Index, changed.Message.Flags.Value);
                    break;
                case FolderStatusChangedIdleEvent statusChanged:
                    Console.WriteLine ("{0}: folder status changed ({1}).", e.Folder, statusChanged.Item);
                    break;
                }
            }
        }

        async Task IdleAsync ()
        {
            // Note: The default options are suitable for most servers (including GMail), but they
            // can be tweaked by passing an ImapIdleOptions to CreateIdleSession().
            var session = client.CreateIdleSession ();

            // Note: If you don't need to handle reconnecting, this loop can be simplified to:
            //
            // await foreach (var changes in session.ReadChangesAsync (cancel.Token)) { ... }
            try {
                do {
                    var changes = await WaitForChangesAsync (session);

                    ProcessChanges (changes);

                    // Note: Unlike when handling the folder events (such as CountChanged) while
                    // idling, it is safe to send new commands here because the IDLE command has
                    // already been ended by the time WaitForChangesAsync() returns.
                    if (changes.MessagesArrived)
                        await FetchMessageSummariesAsync (true);
                } while (true);
            } catch (OperationCanceledException) {
                // Exit() was called. The IDLE command was ended gracefully, so the client is
                // still connected and usable.
            }
        }

        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;
            }

            await IdleAsync ();

            await client.DisconnectAsync (true);
        }

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

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