Server extensions for https://github.com/chkr1011/MQTTnet, exposes the session manager and sessions via reflection. Remember to lock the session dictionary before using it!!

MQTTnetExtensions.cs

using MQTTnet.Server;
using System;
using System.Collections.Generic;
using System.Reflection;

namespace MQTTnet.Server.Extensions
{
    public static class MQTTnetExtensions
    {
        private static FieldInfo MqttClientSessionsManager_FI = typeof(MqttServer).GetField("_clientSessionsManager", BindingFlags.Instance | BindingFlags.NonPublic);
        private static FieldInfo MqttClientSessions_FI = typeof(MqttClientSessionsManager).GetField("_sessions", BindingFlags.Instance | BindingFlags.NonPublic);

        public static bool WithSession(this IMqttServer self, string sessionId, Action<MqttClientSession> callback)
        {
            var sessManager = self.GetMqttClientSessionsManager();
            if (sessManager is null) return false;
            var sessions = sessManager.GetMqttClientSessions();
            if (sessions is null) return false;

            lock(sessions) {
                if (!sessions.ContainsKey(sessionId)) return false;
                callback?.Invoke(sessions[sessionId]);
            }

            return true;
        }

        public static MqttClientSessionsManager GetMqttClientSessionsManager(this IMqttServer self)
        {
            return MqttClientSessionsManager_FI.GetValue(self) as MqttClientSessionsManager;
        }

        public static Dictionary<string, MqttClientSession> GetMqttClientSessions(this MqttClientSessionsManager self)
        {
            return MqttClientSessions_FI.GetValue(self) as Dictionary<string, MqttClientSession>;
        }
    }
}