How to resolve a DI class in a class library with .NET Core?

Question

I understand the basics of DI in .NET Core, but I'm having trouble figuring out how to use it with multiple projects. Imagine I'm setting up a database context in the Startup class of ASP.NET Core:

public void ConfigureServices(IServiceCollection services)
{
  services.AddDbContext<GalleryDb>();
}

I know how to access that context in an API controller:

public class AlbumController : Microsoft.AspNetCore.Mvc.Controller
{
  private GalleryDb _ctx;

  public AlbumController(GalleryDb ctx)
  {
    _ctx = ctx;
  }
}

But what does one do when there are many layers and functions between the API controller and the data access class? Eventually the code reaches my repository class, which is the one that actually requires the context. It looks like this:

public class AlbumRepository
{
  private GalleryDb _ctx;
  public AlbumRepository(GalleryDb ctx)
  {
    _ctx = ctx;
  }

  public void Save(AlbumEntity entity)
  {
    // Use _ctx to persist to DB.
  }
}

I understand that I could pass the context from the API entry point all the way down, but that seems like an anti-pattern because it means passing it as a parameter through multiple classes and functions that have no interest in it.

Instead, I'd like to do something like this at the point where I invoke the repository class:

public void Save(AlbumEntity album)
{
  var ctx = DependencyResolver.GetInstance<GalleryDb>();
  var repo = new AlbumRepository(ctx);
  repo.Save(album);
}

I believe some DI frameworks have something like this, but I'm trying to figure out how to do it with native .NET Core 2.0. Is this possible? What is the best practice? I found one thread (ASP.NET Core DependencyResolver) talk about using IServiceProvider but the implication was that this was not a desirable solution.

I'm hoping whatever the solution is, I can extend it to apply to other DI classes like ASP.NET Identity's RoleManager and SignInManager.

Answer 1

The key breakthrough chris-pratt helped me understand is that the only way this works is to use DI through all the layers. For example, down in the data layer I get a DB context through DI:

public class AlbumRepository
{
  private GalleryDb _ctx;
  public AlbumRepository(GalleryDb ctx)
  {
    _ctx = ctx;
  }
}

In the business layer I use DI to get a reference to the data layer:

public class Album
{
  private AlbumRepository _repo;
  public Album(AlbumRepository repo)
  {
    _repo = repo;
  }
}

Then, in the web layer, I use DI to get a reference to the business layer class:

[Route("api/[controller]")]
public class AlbumController : Microsoft.AspNetCore.Mvc.Controller
{
  private Album _album;
  public AlbumController (Album album)
  {
    _album = album;
  }
}

By using DI through every layer, the DI system is able to construct all the necessary classes at the point where they are needed.

This requirement has a profound impact on the architecture of an application, and I now realize that my initial hope to tweak an existing, non-DI app to start using DI for the DB context is a major undertaking.