EMOS-TestApp/TestApp/Program.cs

using MQTTnet;
using MQTTnet.Client;
using NModbus;
using System.Net.Sockets;

namespace TestApp
{
    internal class Program
    {
        private static IMqttClient _MqttClient;
        private static Dictionary<UInt64, UInt64> _CurrentValues = new Dictionary<ulong, ulong>();

        static async Task Main(string[] args)
        {
            await ConnectMQTT();

            while (true)
            {
                try
                {
                    ConnectToModBus();
                }
                catch (Exception ex)
                {
                    Console.WriteLine($"Error connecting to ModBus slave: {ex.Message}");
                    Thread.Sleep(5000);
                }
            }
        }

        private static void ConnectToModBus()
        {
            var factory = new ModbusFactory();
            var tcpClient = new TcpClient("192.168.188.21", 502);
            var master = factory.CreateMaster(tcpClient);
            Console.WriteLine("ModBus TCP Connection established!");

            while (tcpClient.Connected)
            {
                try
                {
                    var currentWirkleistung = _CurrentValues.GetValueOrDefault(1099528667391UL) / 1000; 

                    ushort input = master.ReadHoldingRegisters(1, 1000, 1)[0];
                    ushort output = (ushort)(input + currentWirkleistung);
                    master.WriteMultipleRegisters(1, 1100, new ushort[] { output });
                    Thread.Sleep(100);
                }
                catch (Exception e)
                {
                    Console.WriteLine($"Error updating ModBus registers: {e.Message}");
                    Thread.Sleep(1000);
                }
            }
        }

        private static async Task ConnectMQTT() {
            var factory = new MqttFactory();

            _MqttClient = factory.CreateMqttClient();

            var clientOptions = new MqttClientOptionsBuilder()
                .WithTcpServer("localhost")
                .Build();

            Console.WriteLine("Connecting to local MQTT ...");
            await _MqttClient.ConnectAsync(clientOptions);
            Console.WriteLine("Connected to local MQTT!");

            _MqttClient.ApplicationMessageReceivedAsync += Client_ApplicationMessageReceivedAsync;

            var subscribeOptions = factory.CreateSubscribeOptionsBuilder()
                .WithTopicFilter(
                    f => {
                        f.WithTopic("gdr/local/values/smart-meter");
                    }
                )
                .Build();

            Console.WriteLine("Subscribing to smart-meter values ...");
            var response = await _MqttClient.SubscribeAsync(subscribeOptions);
            Console.WriteLine("Subscribed to smart-meter values!");
        }

        private static Task Client_ApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs arg)
        {
            switch (arg.ApplicationMessage.Topic) {
                case "gdr/local/values/smart-meter":
                    ParseGDRs(arg.ApplicationMessage.Payload);
                    break;
            }

            return Task.CompletedTask;
        }

        private static void ParseGDRs(byte[] payload)
        {
            var gdrs = GDRs.Parser.ParseFrom(payload);

            if (!gdrs.GDRs_.TryGetValue("smart-meter", out GDR smartMeterGDR)) {
                return;
            }

            foreach (var value in smartMeterGDR.Values) {
                _CurrentValues[value.Key] = value.Value;
            }
        }
    }
}