Implement new managed client.

Source/MQTTnet.Extensions.ManagedClient/New/Events/ApplicationMessageEnqueueingEventArgs.cs

@@ -0,0 +1,21 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// 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.Extensions.ManagedClient
+{
+    public sealed class ApplicationMessageEnqueueingEventArgs : EventArgs
+    {
+        public ApplicationMessageEnqueueingEventArgs(ManagedMqttApplicationMessage applicationMessage, ushort packetIdentifier)
+        {
+            ApplicationMessage = applicationMessage ?? throw new ArgumentNullException(nameof(applicationMessage));
+            PacketIdentifier = packetIdentifier;
+        }
+
+        public ManagedMqttApplicationMessage ApplicationMessage { get; }
+
+        public ushort PacketIdentifier { get; }
+    }
+}