From 3b5667c8827a6ad93b3229ee7fcda2457b850db2 Mon Sep 17 00:00:00 2001
From: logicaloud <47704763+logicaloud@users.noreply.github.com>
Date: Tue, 13 Jun 2023 07:52:54 +1200
Subject: [PATCH] Fix #1761: MQTTRetainedMessageManager, return task with NULL
 result instead of NULL (#1762)

* Return task with NULL result instead of NULL

* move Task.FromResult call out of lock

* Update ReleaseNotes.md

---------

Co-authored-by: Christian <6939810+chkr1011@users.noreply.github.com>
---
 .github/workflows/ReleaseNotes.md             |  3 ++-
 .../MqttRetainedMessageManager_Tests.cs       | 25 +++++++++++++++++++
 .../Internal/MqttRetainedMessagesManager.cs   |  4 +--
 3 files changed, 29 insertions(+), 3 deletions(-)
 create mode 100644 Source/MQTTnet.Tests/Server/MqttRetainedMessageManager_Tests.cs

diff --git a/.github/workflows/ReleaseNotes.md b/.github/workflows/ReleaseNotes.md
index 24a926c6..6c670b92 100644
--- a/.github/workflows/ReleaseNotes.md
+++ b/.github/workflows/ReleaseNotes.md
@@ -1 +1,2 @@
-* [Client] Fixed _PlatformNotSupportedException_ when using Blazor (#1755, thanks to @Nickztar).
\ No newline at end of file
+* [Client] Fixed _PlatformNotSupportedException_ when using Blazor (#1755, thanks to @Nickztar).
+* [Server] Fixed _NullReferenceException_ in retained messages management (#1762, thanks to @logicaloud).
diff --git a/Source/MQTTnet.Tests/Server/MqttRetainedMessageManager_Tests.cs b/Source/MQTTnet.Tests/Server/MqttRetainedMessageManager_Tests.cs
new file mode 100644
index 00000000..36032ed7
--- /dev/null
+++ b/Source/MQTTnet.Tests/Server/MqttRetainedMessageManager_Tests.cs
@@ -0,0 +1,25 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class MqttRetainedMessageManager_Tests
+    {
+        [TestMethod]
+        public async Task MqttRetainedMessageManager_GetUndefinedTopic()
+        {
+            var logger = new Mockups.TestLogger();
+            var eventContainer = new MQTTnet.Server.MqttServerEventContainer();
+            var retainedMessagesManager = new MQTTnet.Server.MqttRetainedMessagesManager(eventContainer, logger);
+            var task = retainedMessagesManager.GetMessage("undefined");
+            Assert.IsNotNull(task, "Task should not be null");
+            var result = await task;
+            Assert.IsNull(result, "Null result expected");
+        }
+    }
+}
diff --git a/Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs b/Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs
index 4dc0b2c0..c23dfeac 100644
--- a/Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs
+++ b/Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs
@@ -134,9 +134,9 @@ namespace MQTTnet.Server
                 {
                     return Task.FromResult(message);
                 }
-
-                return null;
             }
+
+            return Task.FromResult<MqttApplicationMessage>(null);
         }
 
         public async Task ClearMessages()