From c97bd823348d0cd79b4bca541343fee4b52ffdcd Mon Sep 17 00:00:00 2001
From: zhaowgit <zhaowms@live.com>
Date: Sat, 18 Oct 2025 01:04:11 +0800
Subject: [PATCH] =?UTF-8?q?Issues=20#2091=20Improve=20the=20performance=20?=
 =?UTF-8?q?of=20RetainedMessages=20without=20subs=E2=80=A6=20(#2093)?=
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* Issues #2091 Improve the performance of RetainedMessages without subscribing to RetainedMessageChangedEvent.

* Fix build errors

* Update release notes

---------

Co-authored-by: Christian <6939810+chkr1011@users.noreply.github.com>
---
 .../Internal/MqttRetainedMessagesManager.cs           | 11 ++++++++---
 Source/ReleaseNotes.md                                |  1 +
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/Source/MQTTnet.Server/Internal/MqttRetainedMessagesManager.cs b/Source/MQTTnet.Server/Internal/MqttRetainedMessagesManager.cs
index 41dacb9b..9b55369d 100644
--- a/Source/MQTTnet.Server/Internal/MqttRetainedMessagesManager.cs
+++ b/Source/MQTTnet.Server/Internal/MqttRetainedMessagesManager.cs
@@ -9,8 +9,8 @@ namespace MQTTnet.Server.Internal;
 
 public sealed class MqttRetainedMessagesManager
 {
-    readonly Dictionary<string, MqttApplicationMessage> _messages = new Dictionary<string, MqttApplicationMessage>(4096);
-    readonly AsyncLock _storageAccessLock = new AsyncLock();
+    readonly Dictionary<string, MqttApplicationMessage> _messages = new(4096);
+    readonly AsyncLock _storageAccessLock = new();
 
     readonly MqttServerEventContainer _eventContainer;
     readonly MqttNetSourceLogger _logger;
@@ -61,6 +61,7 @@ public sealed class MqttRetainedMessagesManager
 
             lock (_messages)
             {
+                var hasHandlers = _eventContainer.RetainedMessageChangedEvent.HasHandlers;
                 var payload = applicationMessage.Payload;
                 var hasPayload = payload.Length > 0;
 
@@ -86,10 +87,14 @@ public sealed class MqttRetainedMessagesManager
                         }
                     }
 
+                    if (saveIsRequired && hasHandlers)
+                    {
+                        messagesForSave = new List<MqttApplicationMessage>(_messages.Values);
+                    }
                     _logger.Verbose("Client '{0}' set retained message for topic '{1}'.", clientId, applicationMessage.Topic);
                 }
 
-                if (saveIsRequired)
+                if (saveIsRequired && hasHandlers)
                 {
                     messagesForSave = new List<MqttApplicationMessage>(_messages.Values);
                 }
diff --git a/Source/ReleaseNotes.md b/Source/ReleaseNotes.md
index 40e90033..961b7027 100644
--- a/Source/ReleaseNotes.md
+++ b/Source/ReleaseNotes.md
@@ -1,2 +1,3 @@
 * Core: Used new language features across the entire library
 * Core: Performance improvements
+* Server: Improved performance of retained messages when no event handler is attached (#2093, thanks to @zhaowgit)