Dependency injection into controllers in ASP.NET Core

By Shadi Alnamrouti and Rick Anderson

ASP.NET Core MVC controllers request dependencies explicitly via constructors. ASP.NET Core has built-in support for dependency injection (DI). DI makes apps easier to test and maintain.

View or download sample code (how to download)

Constructor injection

Services are added as a constructor parameter, and the runtime resolves the service from the service container. Services are typically defined using interfaces. For example, consider an app that requires the current time. The following interface exposes the IDateTime service:

public interface IDateTime
{
    DateTime Now { get; }
}

The following code implements the IDateTime interface:

public class SystemDateTime : IDateTime
{
    public DateTime Now
    {
        get { return DateTime.Now; }
    }
}

Add the service to the service container:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<IDateTime, SystemDateTime>();

    services.AddControllersWithViews();
}

For more information on AddSingleton, see DI service lifetimes.

The following code displays a greeting to the user based on the time of day:

public class HomeController : Controller
{
    private readonly IDateTime _dateTime;

    public HomeController(IDateTime dateTime)
    {
        _dateTime = dateTime;
    }

    public IActionResult Index()
    {
        var serverTime = _dateTime.Now;
        if (serverTime.Hour < 12)
        {
            ViewData["Message"] = "It's morning here - Good Morning!";
        }
        else if (serverTime.Hour < 17)
        {
            ViewData["Message"] = "It's afternoon here - Good Afternoon!";
        }
        else
        {
            ViewData["Message"] = "It's evening here - Good Evening!";
        }
        return View();
    }

Run the app and a message is displayed based on the time.

Action injection with FromServices

The FromServicesAttribute enables injecting a service directly into an action method without using constructor injection:

public IActionResult About([FromServices] IDateTime dateTime)
{
    return Content( $"Current server time: {dateTime.Now}");
}

Action injection with FromKeyedServices

The following code shows how to access keyed services from the DI container by using the [FromKeyedServices] attribute:

using Microsoft.AspNetCore.Mvc;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddKeyedSingleton<ICache, BigCache>("big");
builder.Services.AddKeyedSingleton<ICache, SmallCache>("small");
builder.Services.AddControllers();

var app = builder.Build();

app.MapControllers();

app.Run();

public interface ICache
{
    object Get(string key);
}
public class BigCache : ICache
{
    public object Get(string key) => $"Resolving {key} from big cache.";
}

public class SmallCache : ICache
{
    public object Get(string key) => $"Resolving {key} from small cache.";
}

[ApiController]
[Route("/cache")]
public class CustomServicesApiController : Controller
{
    [HttpGet("big")]
    public ActionResult<object> GetBigCache([FromKeyedServices("big")] ICache cache)
    {
        return cache.Get("data-mvc");
    }

    [HttpGet("small")]
    public ActionResult<object> GetSmallCache([FromKeyedServices("small")] ICache cache)
    {
        return cache.Get("data-mvc");
    }
}

Access settings from a controller

Accessing app or configuration settings from within a controller is a common pattern. The options pattern described in Options pattern in ASP.NET Core is the preferred approach to manage settings. Generally, don't directly inject IConfiguration into a controller.

Create a class that represents the options. For example:

public class SampleWebSettings
{
    public string Title { get; set; }
    public int Updates { get; set; }
}

Add the configuration class to the services collection:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<IDateTime, SystemDateTime>();
    services.Configure<SampleWebSettings>(Configuration);

    services.AddControllersWithViews();
}

Configure the app to read the settings from a JSON-formatted file:

public class Program
{
    public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureAppConfiguration((hostingContext, config) =>
            {
                config.AddJsonFile("samplewebsettings.json",
                    optional: false,
                    reloadOnChange: true);
            })
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });
}

The following code requests the IOptions<SampleWebSettings> settings from the service container and uses them in the Index method:

public class SettingsController : Controller
{
    private readonly SampleWebSettings _settings;

    public SettingsController(IOptions<SampleWebSettings> settingsOptions)
    {
        _settings = settingsOptions.Value;
    }

    public IActionResult Index()
    {
        ViewData["Title"] = _settings.Title;
        ViewData["Updates"] = _settings.Updates;
        return View();
    }
}

Additional resources

• See Test controller logic in ASP.NET Core to learn how to make code easier to test by explicitly requesting dependencies in controllers.
• Keyed service dependency injection container support
• Replace the default dependency injection container with a third party implementation.