ASP.NET Core comes with a thin but rich and powerful built-in dependency injection mechanism we can use to inject instances to controllers and some other parts of web applications. Although constructor injection is the most famous dependency injection method for controllers, there is also dependency injection to controller actions available. This post introduces controller action injection and shows how to benefit from it.

Constructor Injection With Controllers

Controller action injection is a great feature to use with controllers where most of the actions need some specific service that others don't need. In ASP.NET Core, it's traditional to use constructor injection. Let's take a look at the following controller.

public class HomeController : Controller
{
    private readonly ICustomerDAO _customerDao;
    private readonly ISmsService _smsService;
    private readonly IVehicleTelemetryService _telemetryService;
    private readonly IMailService _mailService;

    public HomeController(ICustomerDAO customerDao, 
                            ISmsService smsService, 
                            IMailService mailService, 
                            IVehicleTelemetryService telemetryService)
    {
        _customerDao = customerDao;
        _smsService = smsService;
        _mailService = mailService;
        _telemetryService = telemetryService;
    }

    public IActionResult Index(int page = 1)
    {
        var customers = _customerDao.ListCustomers(page: 1, pageSize: 25);

        return View(customers);
    }

    public IActionResult SendSms(int id, string text)
    {
        var customer = _customerDao.GetById(id);

        _smsService.Send(customer.Phone, text);
    }

    public IActionResult SendMail(int id, string subject, string body)
    {
        var customer = _customerDao.GetById(id);

        _mailService.Send(customer.Email, subject, body);
    }

    public IActionResult GetCoordinates(int id)
    {
        var coordinates = _telemetryService.GetCoordinates(id);

        return Json(coordinates);
    }
}

It seems all okay but when we think a few steps further we can see that part of this class can easily grow messy, as, over time, we'll probably inject more services into the controller.

public class HomeController : Controller
{
    private readonly ICustomerDAO _customerDao;
    private readonly ISmsService _smsService;
    private readonly IVehicleTelemetryService _telemetryService;
    private readonly IMailService _mailService;
    /*
        More injected services here 
    */

    public HomeController(ICustomerDAO customerDao,
                            ISmsService smsService,
                            IMailService mailService,
                            /* More injected services here */
                            IVehicleTelemetryService telemetryService)
    {
        _customerDao = customerDao;
        _smsService = smsService;
        _mailService = mailService;
        _telemetryService = telemetryService;
        /*
            More injected services here 
        */
    }

    // Controller actions follow
}

But at the same time, we have services used only by one controller action and there is no reason to have these services available in the class scope. The action scope would be enough for us.

Injecting Services to the Controller Action

The solution to our problem is called controller action injection. It means we can inject services into controller actions instead of controller constructors. When we add some service to an action parameters list we confuse the MVC, as it doesn't know, by default, that we want something from the services. We have to apply FromServicesAttribute to the action parameters to tell MVC that this parameter is coming from dependency-injection. As a sample, let's use controller action injection with the SendSms() method of our sample controller.

public IActionResult SendSms(int id, string text, [FromServices]ISmsService smsService)
{
    var customer = _customerDao.GetById(id);

    smsService.Send(customer.Phone, text);
}

ButSendSms() is not the only action that needs an instance of some service that the actions don't use. We also have the SendMail() and GetCoordinates() actions. After moving to the controller action injection, our class looks smaller and cleaner.

public class HomeController : Controller
{
    private readonly ICustomerDAO _customerDao;

    public HomeController(ICustomerDAO customerDao)
    {
        _customerDao = customerDao;
    }

    public IActionResult Index(int page = 1)
    {
        var customers = _customerDao.ListCustomers(page: 1, pageSize: 25);

        return View(customers);
    }

    public IActionResult SendSms(int id, string text, [FromServices]ISmsService smsService)
    {
        var customer = _customerDao.GetById(id);

        smsService.Send(customer.Phone, text);
    }

    public IActionResult SendMail(int id, string subject, string body, [FromServices]IMailService mailService)
    {
        var customer = _customerDao.GetById(id);

        mailService.Send(customer.Email, subject, body);
    }

    public IActionResult GetCoordinates(int id, [FromServices]IVehicleTelemetryService telemetryService)
    {
        var coordinates = telemetryService.GetCoordinates(id);

        return Json(coordinates);
    }
}

We leave ICustomerDAO at the class level and inject it using controller constructor injection because this class is used by multiple controller actions. Our class is now cleaner and we don't have A growing pile of constructor arguments anymore. It also makes it easier to write tests for controllers, as we don't have to initialize controllers with all the dependencies it could possibly be using.

Wrapping Up

Controller action injection is a useful feature that helps us keep services used by one action away from the class scope. This way we don't pile injected services to class level and our controller classes are smaller. If service is used by more than one or few actions it is a good idea to have reference to it at the class level. Otherwise, we can keep controller classes smaller by using controller action injection.