Add support for custom session items for injected application messages

.github/workflows/ReleaseNotes.md

@@ -5,3 +5,4 @@
 * [Server] Exposed MQTT v5 sent properties from the affected client in _ClientDisconnectedAsync_ event.
 * [Server] Fixed wrong client ID passed to _InterceptingUnsubscriptionEventArgs_ (#1631, thanks to @ghord). 
 * [Server] Exposed socket settings for TCP keep alive in TCP options (#1544).
+* [Server] Exposed server session items at server level and allow custom session items for injected application messages (#1638).

Source/MQTTnet/Adapter/MqttPacketInspector.cs

@@ -82,12 +82,7 @@ namespace MQTTnet.Adapter
         {
             try
             {
-                var eventArgs = new InspectMqttPacketEventArgs
-                {
-                    Buffer = buffer,
-                    Direction = direction
-                };
-
+                var eventArgs = new InspectMqttPacketEventArgs(direction, buffer);
                 _asyncEvent.InvokeAsync(eventArgs).GetAwaiter().GetResult();
             }
             catch (Exception exception)

Source/MQTTnet/Client/Publishing/MqttClientPublishResult.cs

@@ -9,9 +9,18 @@ namespace MQTTnet.Client
 {
     public sealed class MqttClientPublishResult
     {
+        public MqttClientPublishResult(ushort? packetIdentifier, MqttClientPublishReasonCode reasonCode, string reasonString, IReadOnlyCollection<MqttUserProperty> userProperties)
+        {
+            PacketIdentifier = packetIdentifier;
+            ReasonCode = reasonCode;
+            ReasonString = reasonString;
+            UserProperties = userProperties;
+        }
+
         /// <summary>
         ///     Returns if the overall status of the publish is a success. This can be the reason code _Success_ or
-        ///     _NoMatchingSubscribers_. _NoMatchingSubscribers_ usually indicates only that no other client is interested in the topic but overall transmit
+        ///     _NoMatchingSubscribers_. _NoMatchingSubscribers_ usually indicates only that no other client is interested in the
+        ///     topic but overall transmit
         ///     to the server etc. was a success.
         /// </summary>
         public bool IsSuccess => ReasonCode == MqttClientPublishReasonCode.Success || ReasonCode == MqttClientPublishReasonCode.NoMatchingSubscribers;
@@ -19,19 +28,19 @@ namespace MQTTnet.Client
         /// <summary>
         ///     Gets the packet identifier which was used for this publish.
         /// </summary>
-        public ushort? PacketIdentifier { get; set; }
+        public ushort? PacketIdentifier { get; }
 
         /// <summary>
         ///     Gets or sets the reason code.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public MqttClientPublishReasonCode ReasonCode { get; set; } = MqttClientPublishReasonCode.Success;
+        public MqttClientPublishReasonCode ReasonCode { get; } = MqttClientPublishReasonCode.Success;
 
         /// <summary>
         ///     Gets or sets the reason string.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public string ReasonString { get; set; }
+        public string ReasonString { get; }
 
         /// <summary>
         ///     Gets or sets the user properties.
@@ -40,8 +49,8 @@ namespace MQTTnet.Client
         ///     As long as you don’t exceed the maximum message size, you can use an unlimited number of user properties to add
         ///     metadata to MQTT messages and pass information between publisher, broker, and subscriber.
         ///     The feature is very similar to the HTTP header concept.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public IReadOnlyCollection<MqttUserProperty> UserProperties { get; internal set; }
+        public IReadOnlyCollection<MqttUserProperty> UserProperties { get; }
     }
 }
\ No newline at end of file

Source/MQTTnet/Client/Publishing/MqttClientPublishResultFactory.cs

@@ -10,25 +10,23 @@ namespace MQTTnet.Client
 {
     public sealed class MqttClientPublishResultFactory
     {
-        static readonly MqttClientPublishResult EmptySuccessResult = new MqttClientPublishResult();
         static readonly IReadOnlyCollection<MqttUserProperty> EmptyUserProperties = new List<MqttUserProperty>();
+        static readonly MqttClientPublishResult AtMostOnceSuccessResult = new MqttClientPublishResult(null, MqttClientPublishReasonCode.Success, null, EmptyUserProperties);
 
         public MqttClientPublishResult Create(MqttPubAckPacket pubAckPacket)
         {
             // QoS 0 has no response. So we treat it as a success always.
             if (pubAckPacket == null)
             {
-                return EmptySuccessResult;
+                return AtMostOnceSuccessResult;
             }
 
-            var result = new MqttClientPublishResult
-            {
+            var result = new MqttClientPublishResult(
+                pubAckPacket.PacketIdentifier,
                 // Both enums have the same values. So it can be easily converted.
-                ReasonCode = (MqttClientPublishReasonCode)(int)pubAckPacket.ReasonCode,
-                PacketIdentifier = pubAckPacket.PacketIdentifier,
-                ReasonString = pubAckPacket.ReasonString,
-                UserProperties = pubAckPacket.UserProperties ?? EmptyUserProperties
-            };
+                (MqttClientPublishReasonCode)(int)pubAckPacket.ReasonCode,
+                pubAckPacket.ReasonString,
+                pubAckPacket.UserProperties ?? EmptyUserProperties);
 
             return result;
         }
@@ -37,43 +35,28 @@ namespace MQTTnet.Client
         {
             if (pubRecPacket == null || pubCompPacket == null)
             {
-                return new MqttClientPublishResult
-                {
-                    ReasonCode = MqttClientPublishReasonCode.UnspecifiedError
-                };
+                var packetIdentifier = pubRecPacket?.PacketIdentifier ?? pubCompPacket?.PacketIdentifier;
+                return new MqttClientPublishResult(packetIdentifier, MqttClientPublishReasonCode.ImplementationSpecificError, null, EmptyUserProperties);
             }
 
-            MqttClientPublishResult result;
-
             // The PUBCOMP is the last packet in QoS 2. So we use the results from that instead of PUBREC.
             if (pubCompPacket.ReasonCode == MqttPubCompReasonCode.PacketIdentifierNotFound)
             {
-                result = new MqttClientPublishResult
-                {
-                    PacketIdentifier = pubCompPacket.PacketIdentifier,
-                    ReasonCode = MqttClientPublishReasonCode.UnspecifiedError,
-                    ReasonString = pubCompPacket.ReasonString,
-                    UserProperties = pubCompPacket.UserProperties ?? EmptyUserProperties
-                };
-
-                return result;
+                return new MqttClientPublishResult(
+                    pubCompPacket.PacketIdentifier,
+                    MqttClientPublishReasonCode.UnspecifiedError,
+                    pubCompPacket.ReasonString,
+                    pubCompPacket.UserProperties ?? EmptyUserProperties);
             }
 
-            result = new MqttClientPublishResult
-            {
-                PacketIdentifier = pubCompPacket.PacketIdentifier,
-                ReasonCode = MqttClientPublishReasonCode.Success,
-                ReasonString = pubCompPacket.ReasonString,
-                UserProperties = pubCompPacket.UserProperties ?? EmptyUserProperties
-            };
-
+            var reasonCode = MqttClientPublishReasonCode.Success;
             if (pubRecPacket.ReasonCode != MqttPubRecReasonCode.Success)
             {
                 // Both enums share the same values.
-                result.ReasonCode = (MqttClientPublishReasonCode)pubRecPacket.ReasonCode;
+                reasonCode = (MqttClientPublishReasonCode)pubRecPacket.ReasonCode;
             }
 
-            return result;
+            return new MqttClientPublishResult(pubCompPacket.PacketIdentifier, reasonCode, null, pubCompPacket.UserProperties ?? EmptyUserProperties);
         }
     }
 }
\ No newline at end of file

Source/MQTTnet/Client/Subscribing/MqttClientSubscribeResult.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using MQTTnet.Packets;
 
@@ -9,23 +10,34 @@ namespace MQTTnet.Client
 {
     public sealed class MqttClientSubscribeResult
     {
-        public IReadOnlyCollection<MqttClientSubscribeResultItem> Items { get; internal set; }
+        public MqttClientSubscribeResult(ushort packetIdentifier, IReadOnlyCollection<MqttClientSubscribeResultItem> items, string reasonString, IReadOnlyCollection<MqttUserProperty> userProperties)
+        {
+            PacketIdentifier = packetIdentifier;
+            Items = items ?? throw new ArgumentNullException(nameof(items));
+            ReasonString = reasonString;
+            UserProperties = userProperties ?? throw new ArgumentNullException(nameof(userProperties));
+        }
+
+        /// <summary>
+        /// Gets the result for every topic filter item.
+        /// </summary>
+        public IReadOnlyCollection<MqttClientSubscribeResultItem> Items { get; }
         
         /// <summary>
         /// Gets the user properties which were part of the SUBACK packet.
-        /// MQTTv5 only.
+        /// <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public IReadOnlyCollection<MqttUserProperty> UserProperties { get; internal set; }
+        public IReadOnlyCollection<MqttUserProperty> UserProperties { get; }
         
         /// <summary>
         /// Gets the reason string.
-        /// MQTTv5 only.
+        /// <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public string ReasonString { get; internal set; }
+        public string ReasonString { get; }
 
         /// <summary>
         /// Gets the packet identifier which was used.
         /// </summary>
-        public ushort PacketIdentifier { get; internal set; }
+        public ushort PacketIdentifier { get; }
     }
 }
\ No newline at end of file

Source/MQTTnet/Client/Subscribing/MqttClientSubscribeResultFactory.cs

@@ -16,42 +16,35 @@ namespace MQTTnet.Client
 
         public MqttClientSubscribeResult Create(MqttSubscribePacket subscribePacket, MqttSubAckPacket subAckPacket)
         {
-            if (subscribePacket == null) throw new ArgumentNullException(nameof(subscribePacket));
-            if (subAckPacket == null) throw new ArgumentNullException(nameof(subAckPacket));
-            
+            if (subscribePacket == null)
+            {
+                throw new ArgumentNullException(nameof(subscribePacket));
+            }
+
+            if (subAckPacket == null)
+            {
+                throw new ArgumentNullException(nameof(subAckPacket));
+            }
+
             // MQTTv5.0.0 handling.
             if (subAckPacket.ReasonCodes.Any() && subAckPacket.ReasonCodes.Count != subscribePacket.TopicFilters.Count)
             {
-                throw new MqttProtocolViolationException(
-                    "The reason codes are not matching the topic filters [MQTT-3.9.3-1].");
+                throw new MqttProtocolViolationException("The reason codes are not matching the topic filters [MQTT-3.9.3-1].");
             }
-            
+
             var items = new List<MqttClientSubscribeResultItem>();
             for (var i = 0; i < subscribePacket.TopicFilters.Count; i++)
             {
                 items.Add(CreateSubscribeResultItem(i, subscribePacket, subAckPacket));
             }
-            
-            var result = new MqttClientSubscribeResult
-            {
-                PacketIdentifier = subAckPacket.PacketIdentifier,
-                ReasonString = subAckPacket.ReasonString,
-                UserProperties = subAckPacket.UserProperties ?? EmptyUserProperties,
-                Items = items
-            };
-            
-            return result;
+
+            return new MqttClientSubscribeResult(subAckPacket.PacketIdentifier, items, subAckPacket.ReasonString, subAckPacket.UserProperties ?? EmptyUserProperties);
         }
 
         static MqttClientSubscribeResultItem CreateSubscribeResultItem(int index, MqttSubscribePacket subscribePacket, MqttSubAckPacket subAckPacket)
         {
-            var resultCode = (MqttClientSubscribeResultCode) subAckPacket.ReasonCodes[index];
-
-            return new MqttClientSubscribeResultItem
-            {
-                TopicFilter = subscribePacket.TopicFilters[index],
-                ResultCode = resultCode,
-            };
+            var resultCode = (MqttClientSubscribeResultCode)subAckPacket.ReasonCodes[index];
+            return new MqttClientSubscribeResultItem(subscribePacket.TopicFilters[index], resultCode);
         }
     }
 }
\ No newline at end of file

Source/MQTTnet/Client/Subscribing/MqttClientSubscribeResultItem.cs

@@ -2,22 +2,29 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using MQTTnet.Packets;
 
 namespace MQTTnet.Client
 {
     public sealed class MqttClientSubscribeResultItem
     {
+        public MqttClientSubscribeResultItem(MqttTopicFilter topicFilter, MqttClientSubscribeResultCode resultCode)
+        {
+            TopicFilter = topicFilter ?? throw new ArgumentNullException(nameof(topicFilter));
+            ResultCode = resultCode;
+        }
+
         /// <summary>
         /// Gets or sets the topic filter.
         /// The topic filter can contain topics and wildcards.
         /// </summary>
-        public MqttTopicFilter TopicFilter { get; internal set; }
+        public MqttTopicFilter TopicFilter { get; }
 
         /// <summary>
         /// Gets or sets the result code.
-        /// Hint: MQTT 5 feature only.
+        /// <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public MqttClientSubscribeResultCode ResultCode { get; internal set; }
+        public MqttClientSubscribeResultCode ResultCode { get; }
     }
 }

Source/MQTTnet/Client/Unsubscribing/MqttClientUnsubscribeResult.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using MQTTnet.Packets;
 
@@ -9,23 +10,38 @@ namespace MQTTnet.Client
 {
     public sealed class MqttClientUnsubscribeResult
     {
-        public IReadOnlyCollection<MqttClientUnsubscribeResultItem> Items { get; internal set; }
+        public MqttClientUnsubscribeResult(
+            ushort packetIdentifier,
+            IReadOnlyCollection<MqttClientUnsubscribeResultItem> items,
+            string reasonString,
+            IReadOnlyCollection<MqttUserProperty> userProperties)
+        {
+            PacketIdentifier = packetIdentifier;
+            Items = items ?? throw new ArgumentNullException(nameof(items));
+            ReasonString = reasonString;
+            UserProperties = userProperties ?? throw new ArgumentNullException(nameof(userProperties));
+        }
 
         /// <summary>
-        /// Gets the user properties which were part of the UNSUBACK packet.
-        /// MQTTv5 only.
+        ///     Gets the result for every topic filter item.
         /// </summary>
-        public IReadOnlyCollection<MqttUserProperty> UserProperties { get; internal set; }
-        
+        public IReadOnlyCollection<MqttClientUnsubscribeResultItem> Items { get; }
+
         /// <summary>
-        /// Gets the reason string.
-        /// MQTTv5 only.
+        ///     Gets the packet identifier which was used.
         /// </summary>
-        public string ReasonString { get; internal set; }
-        
+        public ushort PacketIdentifier { get; }
+
+        /// <summary>
+        ///     Gets the reason string.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+        /// </summary>
+        public string ReasonString { get; }
+
         /// <summary>
-        /// Gets the packet identifier which was used.
+        ///     Gets the user properties which were part of the UNSUBACK packet.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public ushort PacketIdentifier { get; internal set; }
+        public IReadOnlyCollection<MqttUserProperty> UserProperties { get; }
     }
-}
+}
\ No newline at end of file

Source/MQTTnet/Client/Unsubscribing/MqttClientUnsubscribeResultFactory.cs

@@ -12,17 +12,23 @@ namespace MQTTnet.Client
     public sealed class MqttClientUnsubscribeResultFactory
     {
         static readonly IReadOnlyCollection<MqttUserProperty> EmptyUserProperties = new List<MqttUserProperty>();
-        
+
         public MqttClientUnsubscribeResult Create(MqttUnsubscribePacket unsubscribePacket, MqttUnsubAckPacket unsubAckPacket)
         {
-            if (unsubscribePacket == null) throw new ArgumentNullException(nameof(unsubscribePacket));
-            if (unsubAckPacket == null) throw new ArgumentNullException(nameof(unsubAckPacket));
+            if (unsubscribePacket == null)
+            {
+                throw new ArgumentNullException(nameof(unsubscribePacket));
+            }
+
+            if (unsubAckPacket == null)
+            {
+                throw new ArgumentNullException(nameof(unsubAckPacket));
+            }
 
             // MQTTv3.1.1 has no reason code at all!
             if (unsubAckPacket.ReasonCodes != null && unsubAckPacket.ReasonCodes.Count != unsubscribePacket.TopicFilters.Count)
             {
-                throw new MqttProtocolViolationException(
-                    "The return codes are not matching the topic filters [MQTT-3.9.3-1].");
+                throw new MqttProtocolViolationException("The return codes are not matching the topic filters [MQTT-3.9.3-1].");
             }
 
             var items = new List<MqttClientUnsubscribeResultItem>();
@@ -30,33 +36,21 @@ namespace MQTTnet.Client
             {
                 items.Add(CreateUnsubscribeResultItem(i, unsubscribePacket, unsubAckPacket));
             }
-            
-            var result = new MqttClientUnsubscribeResult
-            {
-                PacketIdentifier = unsubAckPacket.PacketIdentifier,
-                ReasonString = unsubAckPacket.ReasonString,
-                UserProperties = unsubAckPacket.UserProperties ?? EmptyUserProperties,
-                Items = items
-            };
-            
-            return result;
+
+            return new MqttClientUnsubscribeResult(unsubscribePacket.PacketIdentifier, items, unsubAckPacket.ReasonString, unsubAckPacket.UserProperties ?? EmptyUserProperties);
         }
-        
+
         static MqttClientUnsubscribeResultItem CreateUnsubscribeResultItem(int index, MqttUnsubscribePacket unsubscribePacket, MqttUnsubAckPacket unsubAckPacket)
         {
             var resultCode = MqttClientUnsubscribeResultCode.Success;
-            
+
             if (unsubAckPacket.ReasonCodes != null)
             {
                 // MQTTv3.1.1 has no reason code and no return code!.
-                resultCode = (MqttClientUnsubscribeResultCode) unsubAckPacket.ReasonCodes[index];
+                resultCode = (MqttClientUnsubscribeResultCode)unsubAckPacket.ReasonCodes[index];
             }
-            
-            return new MqttClientUnsubscribeResultItem
-            {
-                TopicFilter = unsubscribePacket.TopicFilters[index],
-                ResultCode = resultCode
-            };
+
+            return new MqttClientUnsubscribeResultItem(unsubscribePacket.TopicFilters[index], resultCode);
         }
     }
 }
\ No newline at end of file

Source/MQTTnet/Client/Unsubscribing/MqttClientUnsubscribeResultItem.cs

@@ -2,20 +2,28 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
+
 namespace MQTTnet.Client
 {
     public sealed class MqttClientUnsubscribeResultItem
     {
+        public MqttClientUnsubscribeResultItem(string topicFilter, MqttClientUnsubscribeResultCode resultCode)
+        {
+            TopicFilter = topicFilter ?? throw new ArgumentNullException(nameof(topicFilter));
+            ResultCode = resultCode;
+        }
+
         /// <summary>
-        /// Gets or sets the topic filter.
-        /// The topic filter can contain topics and wildcards.
+        ///     Gets or sets the result code.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public string TopicFilter { get; internal set; }
-        
+        public MqttClientUnsubscribeResultCode ResultCode { get; }
+
         /// <summary>
-        /// Gets or sets the result code.
-        /// Hint: MQTT 5 feature only.
+        ///     Gets or sets the topic filter.
+        ///     The topic filter can contain topics and wildcards.
         /// </summary>
-        public MqttClientUnsubscribeResultCode ResultCode { get; internal set; }
+        public string TopicFilter { get; }
     }
 }
\ No newline at end of file

Source/MQTTnet/Diagnostics/PacketInspection/InspectMqttPacketEventArgs.cs

@@ -8,8 +8,14 @@ namespace MQTTnet.Diagnostics
 {
     public sealed class InspectMqttPacketEventArgs : EventArgs
     {
-        public MqttPacketFlowDirection Direction { get; internal set; }
+        public InspectMqttPacketEventArgs(MqttPacketFlowDirection direction, byte[] buffer)
+        {
+            Direction = direction;
+            Buffer = buffer ?? throw new ArgumentNullException(nameof(buffer));
+        }
 
-        public byte[] Buffer { get; set; }
+        public byte[] Buffer { get; }
+
+        public MqttPacketFlowDirection Direction { get; }
     }
 }
\ No newline at end of file

Source/MQTTnet/Server/Events/PreparingSessionEventArgs.cs

@@ -10,7 +10,7 @@ namespace MQTTnet.Server
 {
     public sealed class PreparingSessionEventArgs : EventArgs
     {
-        public string Id { get; internal set; }
+        public string Id { get; set; }
 
         // TODO: Allow adding of packets to the queue etc.
 

Source/MQTTnet/Server/Events/ValidatingConnectionEventArgs.cs

@@ -9,7 +9,6 @@ using System.Security.Cryptography.X509Certificates;
 using System.Text;
 using MQTTnet.Adapter;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
@@ -20,10 +19,11 @@ namespace MQTTnet.Server
     {
         readonly MqttConnectPacket _connectPacket;
 
-        public ValidatingConnectionEventArgs(MqttConnectPacket connectPacket, IMqttChannelAdapter clientAdapter)
+        public ValidatingConnectionEventArgs(MqttConnectPacket connectPacket, IMqttChannelAdapter clientAdapter, IDictionary sessionItems)
         {
             _connectPacket = connectPacket ?? throw new ArgumentNullException(nameof(connectPacket));
             ChannelAdapter = clientAdapter ?? throw new ArgumentNullException(nameof(clientAdapter));
+            SessionItems = sessionItems ?? throw new ArgumentNullException(nameof(sessionItems));
         }
 
         /// <summary>
@@ -34,13 +34,13 @@ namespace MQTTnet.Server
 
         /// <summary>
         ///     Gets or sets the authentication data.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public byte[] AuthenticationData => _connectPacket.AuthenticationData;
 
         /// <summary>
         ///     Gets or sets the authentication method.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public string AuthenticationMethod => _connectPacket.AuthenticationMethod;
 
@@ -99,7 +99,7 @@ namespace MQTTnet.Server
         ///     Gets or sets the reason code. When a MQTTv3 client connects the enum value must be one which is
         ///     also supported in MQTTv3. Otherwise the connection attempt will fail because not all codes can be
         ///     converted properly.
-        ///     MQTTv5 only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public MqttConnectReasonCode ReasonCode { get; set; } = MqttConnectReasonCode.Success;
 
@@ -114,19 +114,19 @@ namespace MQTTnet.Server
 
         /// <summary>
         ///     Gets the request problem information.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public bool RequestProblemInformation => _connectPacket.RequestProblemInformation;
 
         /// <summary>
         ///     Gets the request response information.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public bool RequestResponseInformation => _connectPacket.RequestResponseInformation;
 
         /// <summary>
         ///     Gets or sets the response authentication data.
-        ///     MQTTv5 only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public byte[] ResponseAuthenticationData { get; set; }
 
@@ -137,14 +137,14 @@ namespace MQTTnet.Server
         ///     As long as you don’t exceed the maximum message size, you can use an unlimited number of user properties to add
         ///     metadata to MQTT messages and pass information between publisher, broker, and subscriber.
         ///     The feature is very similar to the HTTP header concept.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public List<MqttUserProperty> ResponseUserProperties { get; set; }
 
         /// <summary>
         ///     Gets or sets the server reference. This can be used together with i.e. "Server Moved" to send
         ///     a different server address to the client.
-        ///     MQTTv5 only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public string ServerReference { get; set; }
 
@@ -158,7 +158,7 @@ namespace MQTTnet.Server
         /// <summary>
         ///     Gets or sets a key/value collection that can be used to share data within the scope of this session.
         /// </summary>
-        public IDictionary SessionItems { get; internal set; }
+        public IDictionary SessionItems { get; }
 
         /// <summary>
         ///     Gets or sets the topic alias maximum.
@@ -169,7 +169,7 @@ namespace MQTTnet.Server
 
         [Obsolete("This property name has a typo. Use 'UserName' instead. This one will be removed soon.")]
         public string Username => _connectPacket.Username;
-        
+
         public string UserName => _connectPacket.Username;
 
         /// <summary>
@@ -179,7 +179,7 @@ namespace MQTTnet.Server
         ///     As long as you don’t exceed the maximum message size, you can use an unlimited number of user properties to add
         ///     metadata to MQTT messages and pass information between publisher, broker, and subscriber.
         ///     The feature is very similar to the HTTP header concept.
-        ///     Hint: MQTT 5 feature only.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
         public List<MqttUserProperty> UserProperties => _connectPacket.UserProperties;
 

Source/MQTTnet/Server/InjectedMqttApplicationMessage.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections;
 
 namespace MQTTnet.Server
 {
@@ -13,8 +14,15 @@ namespace MQTTnet.Server
             ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
         }
 
-        public string SenderClientId { get; set; } = string.Empty;
+        public MqttApplicationMessage ApplicationMessage { get; }
+
+        /// <summary>
+        ///     Gets or sets the session items which should be used for all event handlers which are involved in message
+        ///     processing.
+        ///     If _null_ is specified the singleton session items from the server are used instead.
+        /// </summary>
+        public IDictionary CustomSessionItems { get; set; }
 
-        public MqttApplicationMessage ApplicationMessage { get; set; }
+        public string SenderClientId { get; set; } = string.Empty;
     }
 }
\ No newline at end of file

Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs

@@ -669,11 +669,9 @@ namespace MQTTnet.Server
 
         async Task<ValidatingConnectionEventArgs> ValidateConnection(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter)
         {
-            var eventArgs = new ValidatingConnectionEventArgs(connectPacket, channelAdapter)
-            {
-                SessionItems = new ConcurrentDictionary<object, object>()
-            };
-
+            // TODO: Load session items from persisted sessions in the future.
+            var sessionItems = new ConcurrentDictionary<object, object>();
+            var eventArgs = new ValidatingConnectionEventArgs(connectPacket, channelAdapter, sessionItems);
             await _eventContainer.ValidatingConnectionEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
 
             // Check the client ID and set a random one if supported.

Source/MQTTnet/Server/MqttServer.cs

@@ -232,9 +232,11 @@ namespace MQTTnet.Server
                 throw new NotSupportedException("Injected application messages must contain a topic. Topic alias is not supported.");
             }
 
+            var sessionItems = injectedApplicationMessage.CustomSessionItems ?? ServerSessionItems;
+            
             return _clientSessionsManager.DispatchApplicationMessage(
                 injectedApplicationMessage.SenderClientId,
-                ServerSessionItems,
+                sessionItems,
                 injectedApplicationMessage.ApplicationMessage,
                 cancellationToken);
         }