ASP.Net Core MVC Dependency Injection not working

Question

I am trying to inject a interface into to my HomeController and I am getting this error:

InvalidOperationException: Unable to resolve service for type

Answer 1

Try injecting it as an IConfigurationRoot instead of IConfiguration:

 public HomeController(IConfigurationRoot configuration
    , IEmailSender mailService)
{
    _mailService = mailService;
    _to = configuration["emailAddress.Support"];
}

In this case, the line

services.AddSingleton(provider => Configuration);

is equivalent to

services.AddSingleton<IConfigurationRoot>(provider => Configuration);

because the Configuration property on the class is declared as such, and injection will be done by matching whatever type it was registered as. We can replicate this pretty easily, which might make it clearer:

public interface IParent { }

public interface IChild : IParent { }

public class ConfigurationTester : IChild { }

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

    IChild example = new ConfigurationTester();
    services.AddSingleton(provider => example);
}

public class HomeController : Controller
{
    public HomeController(IParent configuration)
    {
        // this will blow up
    }
}

However

As stephen.vakil mentioned in the comments, it would be better to load your configuration file into a class, and then inject that class into controllers as needed. That would look something like this:

services.Configure<AppSettings>(Configuration.GetSection("AppSettings"));

You can grab these configurations with the IOptions interface:

public HomeController(IOptions<AppSettings> appSettings)

Answer 2

When moving a project from .Net Core 1.x to 2.0, change all IConfigurationRoot to IConfiguration

Answer 3

In Core 2.0 it's recommended to use IConfiguration instead of IConfigurationRoot

public Startup(IConfiguration configuration)
{
    Configuration = configuration;
}

public IConfiguration Configuration { get; }

from https://docs.microsoft.com/en-us/aspnet/core/migration/1x-to-2x/#add-configuration-providers