IMAP IDLE in C# and VB.NET
The IDLE mode is a feature of an IMAP protocol that enables a client to monitor the server's real-time notifications about the selected folder. These notifications may include new messages arriving, erased old messages, flag changes, etc.
Before enabling the IDLE mode, you must select the targeted folder. By keeping track of changes in the number of messages in the folder, you can find out if a new message has arrived or if any message was deleted.
The following example shows how to enable IDLE mode and listen to the server notifications.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using GemBox.Email;
using GemBox.Email.Imap;
class Program
{
static void Main()
{
// If using the Professional version, put your serial key below.
ComponentInfo.SetLicense("FREE-LIMITED-KEY");
using (var imap = new ImapClient("<ADDRESS> (e.g. imap.gmail.com)"))
{
imap.Connect();
imap.Authenticate("<USERNAME>", "<PASSWORD>");
imap.SelectInbox();
using (var listener = new ImapListener(imap))
{
listener.MessagesChanged += OnMessagesChanged;
imap.IdleEnable();
Console.WriteLine("Press any key to exit...");
Console.ReadKey();
imap.IdleDisable();
listener.MessagesChanged -= OnMessagesChanged;
}
}
}
static void OnMessagesChanged(object sender, ImapListenerEventArgs e)
{
foreach (var info in e.NewMessages)
Console.WriteLine($"Message '{info.Uid}' received.");
foreach (var info in e.OldMessages)
Console.WriteLine($"Message '{info.Uid}' deleted.");
}
}
class ImapListener : IDisposable
{
private readonly ImapClient client;
private Dictionary<string, ImapMessageInfo> messages;
private bool running;
private Thread listenerThread;
public event EventHandler<ImapListenerEventArgs> MessagesChanged;
public ImapListener(ImapClient client)
{
this.client = client;
this.messages = this.GetMessages();
this.running = true;
this.listenerThread = new Thread(Listen) { IsBackground = true };
this.listenerThread.Start();
}
private Dictionary<string, ImapMessageInfo> GetMessages()
{
return this.client.ListMessages().ToDictionary(info => info.Uid, info => info);
}
private void Listen()
{
while (this.running)
{
Thread.Sleep(100);
// Compare the previous and current message count of the selected folder.
int comparison = this.client.SelectedFolder.Count.CompareTo(this.messages.Count);
if (comparison == 0)
continue;
var currentMessages = this.GetMessages();
var emptyMessages = Enumerable.Empty<ImapMessageInfo>();
// New message(s) was added.
if (comparison > 0)
{
var newMessages = currentMessages
.Where(message => !this.messages.ContainsKey(message.Key))
.Select(message => message.Value);
this.MessagesChanged?.Invoke(this, new ImapListenerEventArgs(newMessages, emptyMessages));
}
// Old message(s) was deleted.
else
{
var oldMessages = this.messages
.Where(message => !currentMessages.ContainsKey(message.Key))
.Select(message => message.Value);
this.MessagesChanged?.Invoke(this, new ImapListenerEventArgs(emptyMessages, oldMessages));
}
this.messages = currentMessages;
}
}
public void Dispose()
{
this.running = false;
this.listenerThread?.Join(5000);
this.listenerThread = null;
}
}
class ImapListenerEventArgs : EventArgs
{
public IEnumerable<ImapMessageInfo> NewMessages { get; }
public IEnumerable<ImapMessageInfo> OldMessages { get; }
public ImapListenerEventArgs(IEnumerable<ImapMessageInfo> newMessages, IEnumerable<ImapMessageInfo> oldMessages)
{
this.NewMessages = newMessages;
this.OldMessages = oldMessages;
}
}Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Threading
Imports GemBox.Email
Imports GemBox.Email.Imap
Module Program
Sub Main()
' If using the Professional version, put your serial key below.
ComponentInfo.SetLicense("FREE-LIMITED-KEY")
Using imap As New ImapClient("<ADDRESS> (e.g. imap.gmail.com)")
imap.Connect()
imap.Authenticate("<USERNAME>", "<PASSWORD>")
imap.SelectInbox()
Using listener = New ImapListener(imap)
AddHandler listener.MessagesChanged, AddressOf OnMessagesChanged
imap.IdleEnable()
Console.WriteLine("Press any key to exit...")
Console.ReadKey()
imap.IdleDisable()
RemoveHandler listener.MessagesChanged, AddressOf OnMessagesChanged
End Using
End Using
End Sub
Private Sub OnMessagesChanged(sender As Object, e As ImapListenerEventArgs)
For Each info In e.NewMessages
Console.WriteLine($"Message '{info.Uid}' received.")
Next
For Each info In e.OldMessages
Console.WriteLine($"Message '{info.Uid}' deleted.")
Next
End Sub
End Module
Class ImapListener
Implements IDisposable
Private ReadOnly client As ImapClient
Private messages As Dictionary(Of String, ImapMessageInfo)
Private running As Boolean
Private listenerThread As Thread
Public Event MessagesChanged As EventHandler(Of ImapListenerEventArgs)
Public Sub New(client As ImapClient)
Me.client = client
Me.messages = Me.GetMessages()
Me.running = True
Me.listenerThread = New Thread(AddressOf Listen) With {.IsBackground = True}
Me.listenerThread.Start()
End Sub
Private Function GetMessages() As Dictionary(Of String, ImapMessageInfo)
Return Me.client.ListMessages().ToDictionary(Function(info) info.Uid, Function(info) info)
End Function
Private Sub Listen()
While Me.running
Thread.Sleep(100)
' Compare the previous and current message count of the selected folder.
Dim comparison As Integer = Me.client.SelectedFolder.Count.CompareTo(Me.messages.Count)
If comparison = 0 Then Continue While
Dim currentMessages = Me.GetMessages()
Dim emptyMessages = Enumerable.Empty(Of ImapMessageInfo)()
If comparison > 0 Then
' New message(s) was added.
Dim newMessages = currentMessages _
.Where(Function(message) Not Me.messages.ContainsKey(message.Key)) _
.Select(Function(message) message.Value)
RaiseEvent MessagesChanged(Me, New ImapListenerEventArgs(newMessages, emptyMessages))
Else
' Old message(s) was deleted.
Dim oldMessages = Me.messages _
.Where(Function(message) Not currentMessages.ContainsKey(message.Key)) _
.Select(Function(message) message.Value)
RaiseEvent MessagesChanged(Me, New ImapListenerEventArgs(emptyMessages, oldMessages))
End If
Me.messages = currentMessages
End While
End Sub
Public Sub Dispose() Implements IDisposable.Dispose
Me.running = False
Me.listenerThread?.Join(5000)
Me.listenerThread = Nothing
End Sub
End Class
Class ImapListenerEventArgs
Inherits EventArgs
Public ReadOnly Property NewMessages As IEnumerable(Of ImapMessageInfo)
Public ReadOnly Property OldMessages As IEnumerable(Of ImapMessageInfo)
Public Sub New(newMessages As IEnumerable(Of ImapMessageInfo), oldMessages As IEnumerable(Of ImapMessageInfo))
Me.NewMessages = newMessages
Me.OldMessages = oldMessages
End Sub
End Class
