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I have a strange occurrence happening in a brand new .net core 2 web app. This is the standard web api from the template built into Visual Studio. VS 2017, all the jazz. Here's the the entire startup.cs:

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Routing;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using VerySimpleAPI.Helpers;
namespace VerySimpleAPI
    public class Startup
        public Startup(IConfiguration configuration)
            Configuration = configuration;
        public IConfiguration Configuration { get; }
        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
            services.AddMvc();
            services.AddHostedService<MainLoop>();
        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
            if (env.IsDevelopment())
                app.UseDeveloperExceptionPage();
            app.UseMvc(routes => {
                routes.MapRoute(
                    name: "Default",
                    template: "{controller}/{action}/{id?}",
                    defaults: new { controller = "Home", action = "Index"}
MainLoop implements IHostedService and IDisposable.
The services.AddHostedService<MainLoop>(); does not resolve, producing the error "'IServiceCollection' does not contain a definition for 'AddHostedService' and no extension method..." all that jazz. I checked out the Microsoft.Extensions.DependencyInjection source, and I clearly see the definition for public static IServiceCollection AddHostedService<THostedService>(this IServiceCollection services)
Without the hosted service reference, the project compiles just fine. Is there something I'm missing?
                AddHostedService is part of Microsoft.Extensions.Hosting.Abstractions. While it is defined in the Microsoft.Extensions.DependencyInjection namespace, it belongs to  Microsoft.Extensions.Hosting.Abstractions in the ServiceCollectionHostedServiceExtensions class
– Nkosi
                Jul 9, 2018 at 15:18
                @Nkosi adding using Microsoft.Extensions.Hosting; and using Microsoft.Extensions.Hosting.Abstractions; does nothing. I can clearly see the the definition for AddHostedService here: github.com/aspnet/Hosting/blob/master/src/…
– Nomenator
                Jul 9, 2018 at 15:23
AddHostedService is part of Microsoft.Extensions.Hosting.Abstractions. 
While it is defined in the Microsoft.Extensions.DependencyInjection namespace, it belongs to  Microsoft.Extensions.Hosting.Abstractions in the ServiceCollectionHostedServiceExtensions class
using Microsoft.Extensions.DependencyInjection.Extensions;
using Microsoft.Extensions.Hosting;
namespace Microsoft.Extensions.DependencyInjection
    public static class ServiceCollectionHostedServiceExtensions
        /// <summary>
        /// Add an <see cref="IHostedService"/> registration for the given type.
        /// </summary>
        /// <typeparam name="THostedService">An <see cref="IHostedService"/> to register.</typeparam>
        /// <param name="services">The <see cref="IServiceCollection"/> to register with.</param>
        /// <returns>The original <see cref="IServiceCollection"/>.</returns>
        public static IServiceCollection AddHostedService<THostedService>(this IServiceCollection services)
            where THostedService : class, IHostedService
            services.TryAddEnumerable(ServiceDescriptor.Singleton<IHostedService, THostedService>());
            return services;
And ensure that the relevant packages are installed and referenced to have access to the extension method
                Good find, I'm glad you showed the source, I could of sworn hosted services were added as singletons +1
– johnny 5
                Jul 9, 2018 at 15:26
                This is the source I originally turned to, when encountered that problem. using Microsoft.Extensions.Hosting; does nothing. There's something else missing.
– Nomenator
                Jul 9, 2018 at 15:34
                @Nkosi Good question. Apparently, Microsoft.Extensions.Hosting and Microsoft.Extensions.Hosting.Abstractions were not installed as nuget packages. I assumed they were available, since using Microsoft.Extensions.Hosting did not complain about not having a definition. Apparently, now we can use packages which are not installed. I had to drop using Microsoft.Extensions.Hosting because `'IHostingEnvironment' is an ambiguous reference between 'Microsoft.AspNetCore.Hosting.IHostingEnvironment' and 'Microsoft.Extensions.Hosting.IHostingEnvironment``
– Nomenator
                Jul 9, 2018 at 15:51
                @Nomenator, Have you resolved the issue? I have the same problem. I am on core 2.0. Interesting to note, ServiceCollectionHostedServiceExtensions.AddHostedService is applied to .NET Platform Extensions 3.1 3.0 2.2 2.1 only.  It seems like it is not available in  2.0.
– armyllc
                Feb 6, 2020 at 17:40
        
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