How to get an instance of IServiceProvider in .NET Core?

As goaty mentioned it's enough to create new ServiceCollection. Here's example class which can be used to access DI container in .NET Core:

public static class ServiceProviderFactory
{
    public static IServiceProvider ServiceProvider { get; }

    static ServiceProviderFactory()
    {
        HostingEnvironment env = new HostingEnvironment();
        env.ContentRootPath = Directory.GetCurrentDirectory();
        env.EnvironmentName = "Development";

        Startup startup = new Startup(env);
        ServiceCollection sc = new ServiceCollection();
        startup.ConfigureServices(sc);
        ServiceProvider = sc.BuildServiceProvider();
    }
}

Startup class is taken from tested project so the service registrations don't need to be repeated.

Then in test class simply use:

var foo = ServiceProviderFactory.ServiceProvider.GetServices(typeof(IFoo));

This is the default implementation of IServiceCollection from Microsoft: https://github.com/aspnet/DependencyInjection/blob/master/src/DI/ServiceCollection.cs

Looking at the code then you should be able to get an IServiceCollection simply by calling:

var serviceCollection = new Microsoft.Extensions.DependencyInjection.ServiceCollection();

Hope that helps :)

To get access to existing DI of ASP.NET Core application e.g. in some controller, you should just resolve it in a constructor. Example with some manager and workers:

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

   services.AddSingleton<IFooManager, FooManager>();
   services.AddTransient<IFooWorker, FooWorker>();
}

Manually resolve workers for manager:

public class FooManager: IFooManager
{
    private readonly IServiceProvider _serviceProvider;

    public FooManager(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public void Start()
    {
        var w1 = _serviceProvider.GetRequiredService<IFooWorker>();  // new instance of FooWorker
        var w2 = _serviceProvider.GetRequiredService<IFooWorker>();  // new instance of FooWorker
    }
}

Here is an updated approach:

var host = Host.CreateDefaultBuilder().ConfigureWebHostDefaults(builder =>
{
    builder.ConfigureAppConfiguration((hostingContext, config) =>
    {
        var env = hostingContext.HostingEnvironment;
        env.ContentRootPath = Directory.GetCurrentDirectory();
        env.EnvironmentName = "Development";
    });

    builder.UseStartup<Startup>();
}).Build();

Example usage:

host.Services.GetService<IFoo>();

First you need to install the Microsoft.Extensions.DependencyInjection NuGet package. (docs, API, API)

Then you create a new ServiceCollection and method chain it with the BuildServiceProvider method. In between that you can also register any service providers.

var serviceProvider = new ServiceCollection()
    .AddSingleton<IFooService, FooService>()
    .BuildServiceProvider();