Dependency injection in a class not working on a worker service

Question

Is there a way to DI something into a class?

For example,

I have a .net 6 worker service that looks like the following:

Startup:

IConfiguration builder =
    new ConfigurationBuilder().SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json", false, true)
    .AddEnvironmentVariables()
    .Build();

IHost host = Host.CreateDefaultBuilder(args)
    .ConfigureServices(services =>
    {
        services.AddHostedService<Worker>();
        services.AddSingleton(mapper);

        services.AddScoped<MpwContext>(
            serviceProvider =>
            {
                return new MyDBContext(builder.GetSection("MyDBConnectionString").Value.ToString());
            });
    }

Worker.cs:

      public Worker(ILogger<Worker> logger, IConfiguration configuration, IMapper mapper)
        {
            //All the constructor stuff here
        }

        protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        {
            var partService = new PartService();

            await partService.DoSomething();
        }

PartService:

       public PartService(IMapper mapper, MyDbContext myDbContext)
        {
            _mapper = mapper;
            _myDbContext = myDbContext;
        }

        public async Task DoSomething()
        {
        }

The issue I am facing is where I say: var partService = new PartService(); It is telling me that I am missing all the paramters in the constructor.

There is no argument given that corresponds to the required field MyDbContext and mapper....

I know with mvc the constructors pick up on the DI stuff on its own, how can I do that in this case?

The Databases are scoped so I cannot consume them directly in the worker, so I need to DI them in to each respective service. But its asking for them on the object creation.

Answer 1

You've almost got it.

It's normal for background services to be singletons (and thus only allow singleton dependencies). By default, there's no DI scope within ExecuteAsync , but you can create your own :

private readonly IServiceProvider _serviceProvider;

public Worker(IServiceProvider serviceProvider, ILogger<Worker> logger, IConfiguration configuration, IMapper mapper)
{
  _serviceProvider = serviceProvider;
  ...
}

protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
  using var scope = _serviceProvider.CreateScope();

  // Not working yet:
  var partService = new PartService();
  await partService.DoSomething();
}

The other thing you need to know is that the .NET DI is rather simple-minded. The common pattern is to explicitly register every single type you need, including types that only have a single constructor:

services.AddHostedService<Worker>();
services.AddSingleton(mapper);

services.AddScoped<MpwContext>(
    serviceProvider =>
    {
      return new MyDBContext(builder.GetSection("MyDBConnectionString").Value.ToString());
});
services.AddScoped<PartService>();

Now that you've defined PartService as a scoped service, you can ask the DI to create an instance for you:

protected override async Task ExecuteAsync(CancellationToken stoppingToken)
{
  using var scope = _serviceProvider.CreateScope();

  var partService = scope.ServiceProvider.GetRequiredService<PartService>();
  await partService.DoSomething();
}