Expose server session items (#1638)

* Expose server session items

* Add support for custom session items for injected application messages

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 packet identifier which was used.
+        ///     Gets the reason string.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
         /// </summary>
-        public ushort PacketIdentifier { get; internal set; }
+        public string ReasonString { get; }
+
+        /// <summary>
+        ///     Gets the user properties which were part of the UNSUBACK packet.
+        ///     <remarks>MQTT 5.0.0+ feature.</remarks>
+        /// </summary>
+        public IReadOnlyCollection<MqttUserProperty> UserProperties { get; }
     }
-}
+}