[c#] Resolving instances with ASP.NET Core DI from within ConfigureServices

How do I manually resolve a type using the ASP.NET Core MVC built-in dependency injection framework?

Setting up the container is easy enough:

public void ConfigureServices(IServiceCollection services)
{
    // ...

    services.AddTransient<ISomeService, SomeConcreteService>();
}

But how can I resolve ISomeService without performing injection? For example, I want to do this:

ISomeService service = services.Resolve<ISomeService>();

There are no such methods in IServiceCollection.

You can inject dependencies in attributes like AuthorizeAttribute in this way

var someservice = (ISomeService)context.HttpContext.RequestServices.GetService(typeof(ISomeService));

If you just need to resolve one dependency for the purpose of passing it to the constructor of another dependency you are registering, you can do this.

Let's say you had a service that took in a string and an ISomeService.

public class AnotherService : IAnotherService
{
    public AnotherService(ISomeService someService, string serviceUrl)
    {
        ...
    }
}

When you go to register this inside Startup.cs, you'll need to do this:

services.AddScoped<IAnotherService>(ctx => 
      new AnotherService(ctx.GetService<ISomeService>(), "https://someservice.com/")
);

I know this is an old question but I'm astonished that a rather obvious and disgusting hack isn't here.

You can exploit the ability to define your own ctor function to grab necessary values out of your services as you define them... obviously this would be ran every time the service was requested unless you explicitly remove/clear and re-add the definition of this service within the first construction of the exploiting ctor.

This method has the advantage of not requiring you to build the service tree, or use it, during the configuration of the service. You are still defining how services will be configured.

public void ConfigureServices(IServiceCollection services)
{
    //Prey this doesn't get GC'd or promote to a static class var
    string? somevalue = null;

    services.AddSingleton<IServiceINeedToUse, ServiceINeedToUse>(scope => {
         //create service you need
         var service = new ServiceINeedToUse(scope.GetService<IDependantService>())
         //get the values you need
         somevalue = somevalue ?? service.MyDirtyHack();
         //return the instance
         return service;
    });
    services.AddTransient<IOtherService, OtherService>(scope => {
         //Explicitly ensuring the ctor function above is called, and also showcasing why this is an anti-pattern.
         scope.GetService<IServiceINeedToUse>();
         //TODO: Clean up both the IServiceINeedToUse and IOtherService configuration here, then somehow rebuild the service tree.
         //Wow!
         return new OtherService(somevalue);
    });
}

The way to fix this pattern would be to give OtherService an explicit dependency on IServiceINeedToUse, rather than either implicitly depending on it or its method's return value... or resolving that dependency explicitly in some other fashion.

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();

    services.AddDbContext<ConfigurationRepository>(options =>
        options.UseSqlServer(Configuration.GetConnectionString("SqlConnectionString")));

    services.AddScoped<IConfigurationBL, ConfigurationBL>();
    services.AddScoped<IConfigurationRepository, ConfigurationRepository>();
}

If you generate an application with a template you are going to have something like this on the Startup class:

public void ConfigureServices(IServiceCollection services)
{
    // Add framework services.
    services.AddApplicationInsightsTelemetry(Configuration);

    services.AddMvc();
}

You can then add dependencies there, for example:

services.AddTransient<ITestService, TestService>();

If you want to access ITestService on your controller you can add IServiceProvider on the constructor and it will be injected:

public HomeController(IServiceProvider serviceProvider)

Then you can resolve the service you added:

var service = serviceProvider.GetService<ITestService>();

Note that to use the generic version you have to include the namespace with the extensions:

using Microsoft.Extensions.DependencyInjection;

ITestService.cs

public interface ITestService
{
    int GenerateRandom();
}

TestService.cs

public class TestService : ITestService
{
    public int GenerateRandom()
    {
        return 4;
    }
}

Startup.cs (ConfigureServices)

public void ConfigureServices(IServiceCollection services)
{
    services.AddApplicationInsightsTelemetry(Configuration);
    services.AddMvc();

    services.AddTransient<ITestService, TestService>();
}

HomeController.cs

using Microsoft.Extensions.DependencyInjection;

namespace Core.Controllers
{
    public class HomeController : Controller
    {
        public HomeController(IServiceProvider serviceProvider)
        {
            var service = serviceProvider.GetService<ITestService>();
            int rnd = service.GenerateRandom();
        }

Manually resolving instances involves using the IServiceProvider interface:

Resolving Dependency in Startup.ConfigureServices

public void ConfigureServices(IServiceCollection services)
{
    services.AddTransient<IMyService, MyService>();

    var serviceProvider = services.BuildServiceProvider();
    var service = serviceProvider.GetService<IMyService>();
}

Resolving Dependencies in Startup.Configure

public void Configure(
    IApplicationBuilder application,
    IServiceProvider serviceProvider)
{
    // By type.
    var service1 = (MyService)serviceProvider.GetService(typeof(MyService));

    // Using extension method.
    var service2 = serviceProvider.GetService<MyService>();

    // ...
}

Resolving Dependencies in Startup.Configure in ASP.NET Core 3

public void Configure(
    IApplicationBuilder application,
    IWebHostEnvironment webHostEnvironment)
{
    application.ApplicationServices.GetService<MyService>();
}

Using Runtime Injected Services

Some types can be injected as method parameters:

public class Startup
{
    public Startup(
        IHostingEnvironment hostingEnvironment,
        ILoggerFactory loggerFactory)
    {
    }

    public void ConfigureServices(
        IServiceCollection services)
    {
    }

    public void Configure(
        IApplicationBuilder application,
        IHostingEnvironment hostingEnvironment,
        IServiceProvider serviceProvider,
        ILoggerFactory loggerfactory,
        IApplicationLifetime applicationLifetime)
    {
    }
}

Resolving Dependencies in Controller Actions

[HttpGet("/some-action")]
public string SomeAction([FromServices] IMyService myService) => "Hello";