mycorerepository/MyCore/Controllers/MQTTController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using MQTTnet;
using MQTTnet.Client;

namespace MyCore.Controllers
{
    [Authorize(Roles = "Admin")]
    [Route("api/mqtt")]
    [ApiController]
    public class MQTTController : Controller
    {
        /// <summary>
        /// It's a mqtt publish test ! :) 
        /// </summary>
        [AllowAnonymous]
        [HttpGet]
        public void GetToPublishMqtt()
        {
            Client_Publish();
        }

        public async Task Client_Publish()
        {
            var server = new MqttFactory().CreateMqttServer();

            try
            {
                var client1 = new MqttFactory().CreateMqttClient();
                await client1.ConnectAsync(new MqttClientOptionsBuilder().WithTcpServer("192.168.0.8").Build());
                var message = new MqttApplicationMessageBuilder().WithPayload("It's a test").WithTopic("IpAddress").WithRetainFlag().Build();
                await client1.PublishAsync(message);

                await Task.Delay(500);

            }
            finally
            {
                await server.StopAsync();
            }
        }
    }
}