ASP.net core: replace DI services after Startup.

WebHostBuilderExtensions.cs

using System;
using System.Linq;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.DependencyInjection;

namespace Tests
{

    public static class WebHostBuilderExtensions
    {
        private class DeferredServiceConfigurator : IStartup
        {
            private IStartup OriginalStartup { get; }

            private IServiceProvider ServiceProvider { get; }

            private Action<IServiceCollection> ConfigureAction { get; }

            public DeferredServiceConfigurator(IStartup originalStartup, IServiceProvider serviceProvider, Action<IServiceCollection> configureAction)
            {
                this.OriginalStartup = originalStartup;
                this.ServiceProvider = serviceProvider;
                this.ConfigureAction = configureAction;
            }

            public void Configure(IApplicationBuilder app)
            {
                this.OriginalStartup?.Configure(app);
            }

            public IServiceProvider ConfigureServices(IServiceCollection services)
            {
                this.OriginalStartup?.ConfigureServices(services);
                this.ConfigureAction(services);
                return services.BuildServiceProvider();
            }
        }

        public static IWebHostBuilder ConfigureServicesDeferred(this IWebHostBuilder builder, Action<IServiceCollection> configureAction)
        {
            return builder.ConfigureServices(services =>
            {
                var originalStartupDescription = services.LastOrDefault(s => s.ServiceType == typeof(IStartup));
                services.Remove(originalStartupDescription);
                services.AddSingleton<IStartup>(serviceProvider =>
                {
                    var startup = (IStartup)(originalStartupDescription.ImplementationInstance
                        ?? originalStartupDescription.ImplementationFactory?.Invoke(serviceProvider)
                        ?? ActivatorUtilities.CreateInstance(serviceProvider, originalStartupDescription.ImplementationType));
                    return new DeferredServiceConfigurator(startup, serviceProvider, configureAction);
                });
            });
        }
    }
}