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MimeKit

ImapClientNoOp Method

Ping the IMAP server to keep the connection alive.

Namespace:  MailKit.Net.Imap
Assembly:  MailKit (in MailKit.dll) Version: 2.14.0
Syntax
Exceptions
ExceptionCondition
ObjectDisposedException The ImapClient has been disposed.
ServiceNotConnectedException The ImapClient is not connected.
ServiceNotAuthenticatedException The ImapClient is not authenticated.
OperationCanceledException The operation was canceled via the cancellation token.
IOException An I/O error occurred.
ImapCommandException The server replied to the NOOP command with a NO or BAD response.
ImapProtocolException The server responded with an unexpected token.
Remarks

The NOOP command is typically used to keep the connection with the IMAP server alive. When a client goes too long (typically 30 minutes) without sending any commands to the IMAP server, the IMAP server will close the connection with the client, forcing the client to reconnect before it can send any more commands.

The NOOP command also provides a great way for a client to check for new messages.

When the IMAP server receives a NOOP command, it will reply to the client with a list of pending updates such as EXISTS and RECENT counts on the currently selected folder. To receive these notifications, subscribe to the CountChanged and RecentChanged events, respectively.

For more information about the NOOP command, see rfc3501.

Examples
C#
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 ();
        }
    }
}
See Also