How to Resolve Instance Inside ConfigureServices in ASP.NET Core

You can build a service provider using the BuildServiceProvider() method on the IServiceCollection:

public void ConfigureService(IServiceCollection services)
{
    // Configure the services
    services.AddTransient<IFooService, FooServiceImpl>();
    services.Configure<AppSettings>(configuration.GetSection(nameof(AppSettings)));

    // Build an intermediate service provider
    var sp = services.BuildServiceProvider();

    // Resolve the services from the service provider
    var fooService = sp.GetService<IFooService>();
    var options = sp.GetService<IOptions<AppSettings>>();
}

You need the Microsoft.Extensions.DependencyInjection package for this.

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In the case where you just need to bind some options in ConfigureServices, you can also use the Bind method:

var appSettings = new AppSettings();
configuration.GetSection(nameof(AppSettings)).Bind(appSettings);

This functionality is available through the Microsoft.Extensions.Configuration.Binder package.

The best way for instantiating classes that are dependent on other services is to use the AddXXX overload that provides you with the IServiceProvider. This way you do not need to instantiate an intermediate service provider.

The following samples show how you can use this overload in AddSingleton/AddTransient methods.

services.AddSingleton(serviceProvider =>
{
    var options = serviceProvider.GetService<IOptions<AppSettings>>();
    var foo = new Foo(options);
    return foo ;
});


services.AddTransient(serviceProvider =>
{
    var options = serviceProvider.GetService<IOptions<AppSettings>>();
    var bar = new Bar(options);
    return bar;
});

The simplest and most correct way to achieve this, in all versions of ASP.NET Core, is to implement the IConfigureOptions<TOptions> interface. While this has been around since .NET Core 1.0, it seems that few people know about how it makes things Just Work™.

As an example, you want to add a custom model validator that has a dependency on one of your application's other services. Initially it seems impossible - there's no way to resolve IMyServiceDependency because you have no access to an IServiceProvider:

public class MyModelValidatorProvider : IModelValidatorProvider
{
    public MyModelValidatorProvider(IMyServiceDependency dependency)
    {
        ...
    }
}

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers(options =>
    {
        options.ModelValidatorProviders.Add(new MyModelValidatorProvider(??????));
    });
}

But the "magic" of IConfigureOptions<TOptions> makes it so easy:

public class ConfigureMvcOptions : IConfigureOptions<MvcOptions>
{
    private IMyServiceDependency _dependency;

    public MyMvcOptions(IMyServiceDependency dependency)
        => _dependency = dependency;

    public void Configure(MvcOptions options)
        => options.ModelValidatorProviders.Add(new MyModelValidatorProvider(_dependency));
}

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

    ...

    // or scoped, or transient, as necessary for your service
    services.AddSingleton<IConfigureOptions<MvcOptions>, ConfigureMvcOptions>();
}

Essentially, any setup you would have done in the Add***(***Options) delegates in ConfigureServices is now moved to your IConfigureOptions<TOptions> class's Configure method. Then you register the options in the same way you'd register any other service, and away you go!

For more detail, as well as information on how this works behind the scenes, I refer you to the always-excellent Andrew Lock.