Access to the database from MQTT receive message event in .net 6

Question

I use MQTT (https://github.com/dotnet/MQTTnet) in my WebAPI project (.net 6) and need to check in the database when get message from MQTT, but can't and occur error

Cannot access a disposed context instance. A common cause of this error is disposing a context instance that was resolved from dependency injection and then later trying to use the same context instance elsewhere in your application. This may occur if you are calling 'Dispose' on the context instance, or wrapping it in a using statement. If you are using dependency injection, you should let the dependency injection container take care of disposing context instances. Object name: 'DatabaseContext

I use dependency injection and UnitOFWork

Program.cs

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddControllers();
builder.Services.AddDbContext<DatabaseContext>(options => options.UseSqlServer(builder.Configuration.GetSection("ConnectionStrings").GetSection(environment).Value));
builder.Services.AddTransient<IUnitOfWork, UnitOfWork>();
builder.Services.AddTransient<MQTTManager>();
builder.Services.AddEndpointsApiExplorer();
var app = builder.Build();

MQTTCheck(app);

app.MapControllers();
app.Run();

void MQTTCheck(IHost app)
{
    var scopedFactory = app.Services.GetService<IServiceScopeFactory>();
    using (var scope = scopedFactory.CreateScope())
    {
        var service = scope.ServiceProvider.GetService<MQTTManager>();
        service.check();
    }
}

MQTTManager.cs:

public class MQTTManager
    {
        IMqttClient client;
        readonly IUnitOfWork _uow;

        string serverAddress = "XXXXXX";
        int port = 1883;

        public MQTTManager(IUnitOfWork uow)
        {
            _uow = uow;
        }

        public async void check()
        {
            try
            {
                client = new MqttFactory().CreateMqttClient();
                var options = new MqttClientOptionsBuilder()
                    .WithClientId(Guid.NewGuid().ToString())
                    .WithTcpServer(serverAddress, port)
                    .WithCleanSession()
                    .WithWillRetain(false)
                    .Build();

                client.ApplicationMessageReceivedAsync += Client_ApplicationMessageReceivedAsync;

                await client.ConnectAsync(options);
            }
            catch (Exception ex){}
        }

        private Task Client_ApplicationMessageReceivedAsync(MqttApplicationMessageReceivedEventArgs x)
        {
            string topic = x.ApplicationMessage.Topic;
            string receiveMsg = x.ApplicationMessage.ConvertPayloadToString();

            Home home = _uow.HomeRepository.Get(h => h.Name == topic);
            //...

            return Task.CompletedTask;
        }

    }

The error occurs in this line:

Home home = _uow.HomeRepository.Get(h => h.Name == topic);

For example in windows form application when I want to access controls on the form in MQTT receive message event should use this pattern

this.BeginInvoke((MethodInvoker)delegate { label1.Text = "Disconnected"; });

I think that I should use something like this!

Answer 1

I can solve the problem with these changes:

I changed this line in Program.cs

builder.Services.AddTransient<MQTTManager>();

to

builder.Services.AddSingleton<MQTTManager>();

In MQTTManager.cs remove IUnitOfWork and access to the database like below

using (var scope = _factory.CreateScope())
{
   var context = scope.ServiceProvider.GetRequiredService<DatabaseContext>(); 
   var _db = new Lazy<DbSet<Home>>(() => context.Set<Home>());
   Home home = _db.Value.Where(x => x.ClientId == topic).FirstOrDefault();
}