在电子邮件技术中，IDLE是RFC 2177中描述的一项IMAP功能，它允许客户端向服务器表明它已准备好接受实时通知。
Internet消息访问协议IMAP4协议，它要求客户端轮询服务器来更改所选中的文件夹（如拉取新邮件、删除邮件），如果能让服务器推送通知客户端，告知客户端有新邮件的话会更方便客户端，尤其是在手机端的时候，大量的轮询查询服务器会耗费电量和流量，用户是不太允许这样做的，而且也不是很及时的收到邮件。
考虑到这种情况，其实IMAP4的扩展协议中是支持这个推送模式，即IMAP的IDLE模式。
首先我们用CAPABILITY 命令查询一下是否支持IDLE模式，因为并不是所有邮箱多支持的。
如qq邮箱：["CAPABILITY", "IMAP4", "IMAP4rev1", "IDLE", "XAPPLEPUSHSERVICE", "AUTH=LOGIN", "NAMESPACE", "CHILDREN", "ID", "UIDPLUS"]就支持这种模式，而163邮箱：["CAPABILITY", "IMAP4rev1", "XLIST", "SPECIAL-USE", "ID", "LITERAL+", "STARTTLS", "XAPPLEPUSHSERVICE", "UIDPLUS", "X-CM-EXT-1"]并不支持。
我们就用qq邮箱来测试一下，测试前请开通QQ邮箱的imap协议功能保持连接正常，关于IDLE命令的使用，需要先登录验证后，选中文件夹之后才可以使用，具体测试如下图：
查看命令可以知道，每次收到新邮件这会更改EXISTS的数量，这样就收到一个邮件通知，然后通过这个通知在去拉取邮件就可以。这是基本原理，具体到具体应用，由于我一直使用mailkit来获取邮件，而mailkit本身也是支持这种模式的。
mailkit具体代码如下：
1 namespace TestMailKit 2 { 3public partial class Form2 : Form 4{ 5 public Form2() 6 { 7 InitializeComponent(); 8 } 9 10 private void button1_Click(object sender, EventArgs e) 11 { 12 TodoMail(); 13 } 14 15 public static void TodoMail() 16 { 17 try 18 { 19 using (var client = new ImapClient(new ProtocolLogger(Console.OpenStandardError()))) 20 { 21 client.Connect("", 993, true); 22 if (client.AuthenticationMechanisms.Contains("XOAUTH2")) 23client.AuthenticationMechanisms.Remove("XOAUTH2"); 24 client.Authenticate("110xxxxx31@", "******chfcf"); 25 26 client.Inbox.Open(FolderAccess.ReadOnly); 27 28 // Get the summary information of all of the messages (suitable for displaying in a message list). 29 var messages = client.Inbox.Fetch(0, -1, MessageSummaryItems.Full | MessageSummaryItems.UniqueId).ToList(); 30 31 // Keep track of messages being expunged so that when the CountChanged event fires, we can tell if it's 32 // because new messages have arrived vs messages being removed (or some combination of the two). 33 client.Inbox.MessageExpunged += (sender, e) => 34 { 35var folder = (ImapFolder)sender; 36 37if (e.Index < messages.Count) 38{ 39 var message = messages[e.Index]; 40 41 Console.WriteLine("{0}: expunged message {1}: Subject: {2}", folder, e.Index, message.Envelope.Subject); 42 43 // Note: If you are keeping a local cache of message information 44 // (e.g. MessageSummary data) for the folder, then you'll need 45 // to remove the message at e.Index. 46 messages.RemoveAt(e.Index); 47} 48else 49{ 50 Console.WriteLine("{0}: expunged message {1}: Unknown message.", folder, e.Index); 51} 52 }; 53 54 // Keep track of changes to the number of messages in the folder (this is how we'll tell if new messages have arrived). 55 client.Inbox.CountChanged += (sender, e) => 56 { 57// Note: the CountChanged event will fire when new messages arrive in the folder and/or when messages are expunged. 58var folder = (ImapFolder)sender; 59 60Console.WriteLine("The number of messages in {0} has changed.", folder); 61 62// Note: because we are keeping track of the MessageExpunged event and updating our 63// 'messages' list, we know that if we get a CountChanged event and folder.Count is 64// larger than messages.Count, then it means that new messages have arrived. 65if (folder.Count > messages.Count) 66{ 67 Console.WriteLine("{0} new messages have arrived.", folder.Count - messages.Count); 68 69 // Note: your first instict may be to fetch these new messages now, but you cannot do 70 // that in an event handler (the ImapFolder is not re-entrant). 71 // 72 // If this code had access to the 'done' CancellationTokenSource (see below), it could 73 // cancel that to cause the IDLE loop to end. 74} 75 }; 76 77 // Keep track of flag changes. 78 client.Inbox.MessageFlagsChanged += (sender, e) => 79 { 80var folder = (ImapFolder)sender; 81 82Console.WriteLine("{0}: flags for message {1} have changed to: {2}.", folder, e.Index, e.Flags); 83 }; 84 85 Console.WriteLine("Hit any key to end the IDLE loop."); 86 using (var done = new CancellationTokenSource()) 87 { 88// Note: when the 'done' CancellationTokenSource is cancelled, it ends to IDLE loop. 89var thread = new Thread(IdleLoop); 90 91thread.Start(new IdleState(client, done.Token)); 92 93Console.ReadKey(); 94done.Cancel(); 95thread.Join(); 96 } 97 98 if (client.Inbox.Count > messages.Count) 99 {100Console.WriteLine("The new messages that arrived during IDLE are:");101foreach (var message in client.Inbox.Fetch(messages.Count, -1, MessageSummaryItems.Full | MessageSummaryItems.UniqueId))102 Console.WriteLine("Subject: {0}", message.Envelope.Subject);103 }104 105 client.Disconnect(true);106 }107 }108 catch (Exception ex)109 {110 Console.WriteLine(ex.Message);111 }112 }113 114 static void IdleLoop(object state)115 {116 var idle = (IdleState)state;117 118 lock (idle.Client.SyncRoot)119 {120 // Note: since the IMAP server will drop the connection after 30 minutes, we must loop sending IDLE commands that121 // last ~29 minutes or until the user has requested that they do not want to IDLE anymore.122 //123 // For GMail, we use a 9 minute interval because they do not seem to keep the connection alive for more than ~10 minutes.124 while (!idle.IsCancellationRequested)125 {126 // Note: Starting with .NET 4.5, you can make this simpler by using the CancellationTokenSource .ctor that127 // takes a TimeSpan argument, thus eliminating the need to create a timer.128 using (var timeout = new CancellationTokenSource())129 {130using (var timer = new System.Timers.Timer(9 * 60 * 1000))131{132 // End the IDLE command when the timer expires.133 timer.Elapsed += (sender, e) => timeout.Cancel();134 timer.AutoReset = false;135 timer.Enabled = true;136 137 try138 {139 // We set the timeout source so that if the idle.DoneToken is cancelled, it can cancel the timeout140 idle.SetTimeoutSource(timeout);141 142 if (idle.Client.Capabilities.HasFlag(ImapCapabilities.Idle))143 {144 // The Idle() method will not return until the timeout has elapsed or idle.CancellationToken is cancelled145 idle.Client.Idle(timeout.Token, idle.CancellationToken);146 }147 else148 {149 // The IMAP server does not support IDLE, so send a NOOP command instead150 idle.Client.NoOp(idle.CancellationToken);151 152 // Wait for the timeout to elapse or the cancellation token to be cancelled153 WaitHandle.WaitAny(new[] { timeout.Token.WaitHandle, idle.CancellationToken.WaitHandle });154 }155 }156 catch (OperationCanceledException)157 {158 // This means that idle.CancellationToken was cancelled, not the DoneToken nor the timeout.159 break;160 }161 catch (ImapProtocolException)162 {163 // The IMAP server sent garbage in a response and the ImapClient was unable to deal with it.164 // This should never happen in practice, but it's probably still a good idea to handle it.165 //166 // Note: an ImapProtocolException almost always results in the ImapClient getting disconnected.167 break;168 }169 catch (ImapCommandException)170 {171 // The IMAP server responded with "NO" or "BAD" to either the IDLE command or the NOOP command.172 // This should never happen... but again, we're catching it for the sake of completeness.173 break;174 }175 finally176 {177 // We're about to Dispose() the timeout source, so set it to null.178 idle.SetTimeoutSource(null);179 }180}181 }182 }183 }184 }185 186}187 188class IdleState189{190 readonly object mutex = new object();191 CancellationTokenSource timeout;192 193 /// <summary>194 /// Get the cancellation token.195 /// </summary>196 /// <remarks>197 /// <para>The cancellation token is the brute-force approach to cancelling the IDLE and/or NOOP command.</para>198 /// <para>Using the cancellation token will typically drop the connection to the server and so should199 /// not be used unless the client is in the process of shutting down or otherwise needs to200 /// immediately abort communication with the server.</para>201 /// </remarks>202 /// <value>The cancellation token.</value>203 public CancellationToken CancellationToken { get; private set; }204 205 /// <summary>206 /// Get the done token.207 /// </summary>208 /// <remarks>209 /// <para>The done token tells the <see cref="Program.IdleLoop"/> that the user has requested to end the loop.</para>210 /// <para>When the done token is cancelled, the <see cref="Program.IdleLoop"/> will gracefully come to an end by211 /// cancelling the timeout and then breaking out of the loop.</para>212 /// </remarks>213 /// <value>The done token.</value>214 public CancellationToken DoneToken { get; private set; }215 216 /// <summary>217 /// Get the IMAP client.218 /// </summary>219 /// <value>The IMAP client.</value>220 public ImapClient Client { get; private set; }221 222 /// <summary>223 /// Check whether or not either of the CancellationToken's have been cancelled.224 /// </summary>225 /// <value><c>true</c> if cancellation was requested; otherwise, <c>false</c>.</value>226 public bool IsCancellationRequested227 {228 get229 {230 return CancellationToken.IsCancellationRequested || DoneToken.IsCancellationRequested;231 }232 }233 234 /// <summary>235 /// Initializes a new instance of the <see cref="IdleState"/> class.236 /// </summary>237 /// <param name="client">The IMAP client.</param>238 /// <param name="doneToken">The user-controlled 'done' token.</param>239 /// <param name="cancellationToken">The brute-force cancellation token.</param>240 public IdleState(ImapClient client, CancellationToken doneToken, CancellationToken cancellationToken = default(CancellationToken))241 {242 CancellationToken = cancellationToken;243 DoneToken = doneToken;244 Client = client;245 246 // When the user hits a key, end the current timeout as well247 doneToken.Register(CancelTimeout);248 }249 250 /// <summary>251 /// Cancel the timeout token source, forcing ImapClient.Idle() to gracefully exit.252 /// </summary>253 void CancelTimeout()254 {255 lock (mutex)256 {257 if (timeout != null)258 timeout.Cancel();259 }260 }261 262 /// <summary>263 /// Set the timeout source.264 /// </summary>265 /// <param name="source">The timeout source.</param>266 public void SetTimeoutSource(CancellationTokenSource source)267 {268 lock (mutex)269 {270 timeout = source;271 272 if (timeout != null && IsCancellationRequested)273 timeout.Cancel();274 }275 }276}277 278 }
mailkit IDLE模式
通过上面的代码即可订阅邮件通知，方便的获取邮件。
