Added support for generic host based IWebHostBuilder (#1580)

- This adds an implementation of IWebHostBuilder as a facade over the IHostBuilder.
This removes the 2 container issue by executing the Startup.ConfigureServies and Startup.ConfigureContainer inline as part of building the IHostBuilder.
- The implementation is highly compatible implementation since it exposes the same IWebHostBuilder interface.
Existing extensions mostly work.
- There are some caveats with this approach.
    - Injecting services into Startup is not extremely constrained to the
    services availble on HostBuilderContext. This includes the IHostingEnvironment
    and the IConfiguration.
    - IStartup is broken when using this pattern because it isn't composable.
    - The IStartupConfigureServicesFilter and IStartupConfigureContainer The before
    and after filters added in 2.1 are also broken  because there's a single container (it could maybe be fixed by downcasting and doing something specific on the GenericHostBuilder instance).
    - Calling into IWebHostBuilder.Build will throw a NotSupportedException since
    this implementation is just a facade over the IHostBuilder.

samples/GenericWebHost/Program.cs

@@ -1,10 +1,9 @@
-using System;
+using System.Threading.Tasks;
-using System.Net;
-using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.Hosting;
+using Microsoft.AspNetCore.Hosting;
 
 namespace GenericWebHost
 {
@@ -19,22 +18,20 @@ namespace GenericWebHost
                     config.AddJsonFile("appsettings.json", optional: true);
                     config.AddCommandLine(args);
                 })
-                .ConfigureServices((hostContext, services) =>
-                {
-                })
                 .UseFakeServer()
-                .ConfigureWebHost((hostContext, app) =>
+                .ConfigureWebHost(builder =>
+                {
+                    builder.Configure(app =>
                     {
                         app.Run(async (context) =>
                         {
                             await context.Response.WriteAsync("Hello World!");
                         });
+                    });
                 })
                 .UseConsoleLifetime()
                 .Build();
 
-            var s = host.Services;
-
             await host.RunAsync();
         }
     }

samples/GenericWebHost/WebHostExtensions.cs

@@ -1,43 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Diagnostics;
-using System.Text;
-using Microsoft.AspNetCore.Builder;
-using Microsoft.AspNetCore.Hosting;
-using Microsoft.AspNetCore.Hosting.Internal;
-using Microsoft.AspNetCore.Http;
-using Microsoft.Extensions.DependencyInjection;
-using Microsoft.Extensions.Hosting;
-using Microsoft.Extensions.ObjectPool;
-
-namespace GenericWebHost
-{
-    public static class WebHostExtensions
-    {
-        public static IHostBuilder ConfigureWebHost(this IHostBuilder builder, Action<HostBuilderContext, IApplicationBuilder> configureApp)
-        {
-            return builder.ConfigureServices((bulderContext, services) =>
-            {
-                services.Configure<WebHostServiceOptions>(options =>
-                {
-                    options.ConfigureApp = configureApp;
-                });
-                services.AddHostedService<WebHostService>();
-                
-                var listener = new DiagnosticListener("Microsoft.AspNetCore");
-                services.AddSingleton<DiagnosticListener>(listener);
-                services.AddSingleton<DiagnosticSource>(listener);
-                
-                services.AddTransient<IHttpContextFactory, HttpContextFactory>();
-                services.AddScoped<IMiddlewareFactory, MiddlewareFactory>();
-
-                // Conjure up a RequestServices
-                services.AddTransient<IStartupFilter, AutoRequestServicesStartupFilter>();
-                services.AddTransient<IServiceProviderFactory<IServiceCollection>, DefaultServiceProviderFactory>();
-
-                // Ensure object pooling is available everywhere.
-                services.AddSingleton<ObjectPoolProvider, DefaultObjectPoolProvider>();
-            });
-        }
-    }
-}

samples/GenericWebHost/WebHostService.cs

@@ -1,62 +0,0 @@
-using System;
-using System.Diagnostics;
-using System.Threading;
-using System.Threading.Tasks;
-using Microsoft.AspNetCore.Builder.Internal;
-using Microsoft.AspNetCore.Hosting.Internal;
-using Microsoft.AspNetCore.Hosting.Server;
-using Microsoft.AspNetCore.Hosting.Server.Features;
-using Microsoft.AspNetCore.Http;
-using Microsoft.Extensions.DependencyInjection;
-using Microsoft.Extensions.Hosting;
-using Microsoft.Extensions.Logging;
-using Microsoft.Extensions.Options;
-
-namespace GenericWebHost
-{
-    internal class WebHostService : IHostedService
-    {
-        public WebHostService(IOptions<WebHostServiceOptions> options, IServiceProvider services, HostBuilderContext hostBuilderContext, IServer server,
-            ILogger<WebHostService> logger, DiagnosticListener diagnosticListener, IHttpContextFactory httpContextFactory)
-        {
-            Options = options?.Value ?? throw new System.ArgumentNullException(nameof(options));
-
-            if (Options.ConfigureApp == null)
-            {
-                throw new ArgumentException(nameof(Options.ConfigureApp));
-            }
-
-            Services = services ?? throw new ArgumentNullException(nameof(services));
-            HostBuilderContext = hostBuilderContext ?? throw new ArgumentNullException(nameof(hostBuilderContext));
-            Server = server ?? throw new ArgumentNullException(nameof(server));
-            Logger = logger ?? throw new ArgumentNullException(nameof(logger));
-            DiagnosticListener = diagnosticListener ?? throw new ArgumentNullException(nameof(diagnosticListener));
-            HttpContextFactory = httpContextFactory ?? throw new ArgumentNullException(nameof(httpContextFactory));
-        }
-
-        public WebHostServiceOptions Options { get; }
-        public IServiceProvider Services { get; }
-        public HostBuilderContext HostBuilderContext { get; }
-        public IServer Server { get; }
-        public ILogger<WebHostService> Logger { get; }
-        public DiagnosticListener DiagnosticListener { get; }
-        public IHttpContextFactory HttpContextFactory { get; }
-
-        public Task StartAsync(CancellationToken cancellationToken)
-        {
-            Server.Features.Get<IServerAddressesFeature>()?.Addresses.Add("http://localhost:5000");
-
-            var builder = new ApplicationBuilder(Services, Server.Features);
-            Options.ConfigureApp(HostBuilderContext, builder);
-            var app = builder.Build();
-
-            var httpApp = new HostingApplication(app, Logger, DiagnosticListener, HttpContextFactory);
-            return Server.StartAsync(httpApp, cancellationToken);
-        }
-
-        public Task StopAsync(CancellationToken cancellationToken)
-        {
-            return Server.StopAsync(cancellationToken);
-        }
-    }
-}

samples/GenericWebHost/WebHostServiceOptions.cs

@@ -1,11 +0,0 @@
-using System;
-using Microsoft.AspNetCore.Builder;
-using Microsoft.Extensions.Hosting;
-
-namespace GenericWebHost
-{
-    public class WebHostServiceOptions
-    {
-        public Action<HostBuilderContext, IApplicationBuilder> ConfigureApp { get; internal set; }
-    }
-}

src/Microsoft.AspNetCore.Hosting/GenericHost/GenericWebHostApplicationLifetime.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Threading;
+
+namespace Microsoft.AspNetCore.Hosting.Internal
+{
+    internal class GenericWebHostApplicationLifetime : IApplicationLifetime
+    {
+        private readonly Microsoft.Extensions.Hosting.IApplicationLifetime _applicationLifetime;
+        public GenericWebHostApplicationLifetime(Microsoft.Extensions.Hosting.IApplicationLifetime applicationLifetime)
+        {
+            _applicationLifetime = applicationLifetime;
+        }
+
+        public CancellationToken ApplicationStarted => _applicationLifetime.ApplicationStarted;
+
+        public CancellationToken ApplicationStopping => _applicationLifetime.ApplicationStopping;
+
+        public CancellationToken ApplicationStopped => _applicationLifetime.ApplicationStopped;
+
+        public void StopApplication() => _applicationLifetime.StopApplication();
+    }
+}

src/Microsoft.AspNetCore.Hosting/GenericHost/GenericWebHostBuilder.cs

@@ -0,0 +1,376 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Reflection;
+using System.Runtime.ExceptionServices;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting.Builder;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.ObjectPool;
+
+namespace Microsoft.AspNetCore.Hosting.Internal
+{
+    internal class GenericWebHostBuilder : IWebHostBuilder, ISupportsStartup, ISupportsUseDefaultServiceProvider
+    {
+        private readonly IHostBuilder _builder;
+        private readonly IConfiguration _config;
+        private readonly object _startupKey = new object();
+
+        private AggregateException _hostingStartupErrors;
+        private HostingStartupWebHostBuilder _hostingStartupWebHostBuilder;
+
+        public GenericWebHostBuilder(IHostBuilder builder)
+        {
+            _builder = builder;
+
+            _config = new ConfigurationBuilder()
+                .AddEnvironmentVariables(prefix: "ASPNETCORE_")
+                .Build();
+
+            _builder.ConfigureHostConfiguration(config =>
+            {
+                config.AddConfiguration(_config);
+
+                // We do this super early but still late enough that we can process the configuration
+                // wired up by calls to UseSetting
+                ExecuteHostingStartups();
+            });
+
+            // IHostingStartup needs to be executed before any direct methods on the builder
+            // so register these callbacks first
+            _builder.ConfigureAppConfiguration((context, configurationBuilder) =>
+            {
+                if (_hostingStartupWebHostBuilder != null)
+                {
+                    var webhostContext = GetWebHostBuilderContext(context);
+                    _hostingStartupWebHostBuilder.ConfigureAppConfiguration(webhostContext, configurationBuilder);
+                }
+            });
+
+            _builder.ConfigureServices((context, services) =>
+            {
+                if (_hostingStartupWebHostBuilder != null)
+                {
+                    var webhostContext = GetWebHostBuilderContext(context);
+                    _hostingStartupWebHostBuilder.ConfigureServices(webhostContext, services);
+                }
+            });
+
+            _builder.ConfigureServices((context, services) =>
+            {
+                var webhostContext = GetWebHostBuilderContext(context);
+                var webHostOptions = (WebHostOptions)context.Properties[typeof(WebHostOptions)];
+
+                // Add the IHostingEnvironment and IApplicationLifetime from Microsoft.AspNetCore.Hosting
+                services.AddSingleton(webhostContext.HostingEnvironment);
+                services.AddSingleton<IApplicationLifetime, GenericWebHostApplicationLifetime>();
+
+                services.Configure<GenericWebHostServiceOptions>(options =>
+                {
+                    // Set the options
+                    options.WebHostOptions = webHostOptions;
+                    // Store and forward any startup errors
+                    options.HostingStartupExceptions = _hostingStartupErrors;
+                });
+
+                services.AddHostedService<GenericWebHostService>();
+
+                // REVIEW: This is bad since we don't own this type. Anybody could add one of these and it would mess things up
+                // We need to flow this differently
+                var listener = new DiagnosticListener("Microsoft.AspNetCore");
+                services.TryAddSingleton<DiagnosticListener>(listener);
+                services.TryAddSingleton<DiagnosticSource>(listener);
+
+                services.TryAddSingleton<IHttpContextFactory, HttpContextFactory>();
+                services.TryAddScoped<IMiddlewareFactory, MiddlewareFactory>();
+                services.TryAddSingleton<IApplicationBuilderFactory, ApplicationBuilderFactory>();
+
+                // Conjure up a RequestServices
+                services.TryAddTransient<IStartupFilter, AutoRequestServicesStartupFilter>();
+
+                // Ensure object pooling is available everywhere.
+                services.TryAddSingleton<ObjectPoolProvider, DefaultObjectPoolProvider>();
+
+                // Support UseStartup(assemblyName)
+                if (!string.IsNullOrEmpty(webHostOptions.StartupAssembly))
+                {
+                    try
+                    {
+                        var startupType = StartupLoader.FindStartupType(webHostOptions.StartupAssembly, webhostContext.HostingEnvironment.EnvironmentName);
+                        UseStartup(startupType, context, services);
+                    }
+                    catch (Exception ex) when (webHostOptions.CaptureStartupErrors)
+                    {
+                        var capture = ExceptionDispatchInfo.Capture(ex);
+
+                        services.Configure<GenericWebHostServiceOptions>(options =>
+                        {
+                            options.ConfigureApplication = app =>
+                            {
+                                // Throw if there was any errors initializing startup
+                                capture.Throw();
+                            };
+                        });
+                    }
+                }
+            });
+        }
+
+        private void ExecuteHostingStartups()
+        {
+            var webHostOptions = new WebHostOptions(_config, Assembly.GetEntryAssembly()?.GetName().Name);
+
+            if (webHostOptions.PreventHostingStartup)
+            {
+                return;
+            }
+
+            var exceptions = new List<Exception>();
+            _hostingStartupWebHostBuilder = new HostingStartupWebHostBuilder(this);
+
+            // Execute the hosting startup assemblies
+            foreach (var assemblyName in webHostOptions.GetFinalHostingStartupAssemblies().Distinct(StringComparer.OrdinalIgnoreCase))
+            {
+                try
+                {
+                    var assembly = Assembly.Load(new AssemblyName(assemblyName));
+
+                    foreach (var attribute in assembly.GetCustomAttributes<HostingStartupAttribute>())
+                    {
+                        var hostingStartup = (IHostingStartup)Activator.CreateInstance(attribute.HostingStartupType);
+                        hostingStartup.Configure(_hostingStartupWebHostBuilder);
+                    }
+                }
+                catch (Exception ex)
+                {
+                    // Capture any errors that happen during startup
+                    exceptions.Add(new InvalidOperationException($"Startup assembly {assemblyName} failed to execute. See the inner exception for more details.", ex));
+                }
+            }
+
+            if (exceptions.Count > 0)
+            {
+                _hostingStartupErrors = new AggregateException(exceptions);
+            }
+        }
+
+        public IWebHost Build()
+        {
+            throw new NotSupportedException($"Building this implementation of {nameof(IWebHostBuilder)} is not supported.");
+        }
+
+        public IWebHostBuilder ConfigureAppConfiguration(Action<WebHostBuilderContext, IConfigurationBuilder> configureDelegate)
+        {
+            _builder.ConfigureAppConfiguration((context, builder) =>
+            {
+                var webhostBuilderContext = GetWebHostBuilderContext(context);
+                configureDelegate(webhostBuilderContext, builder);
+            });
+
+            return this;
+        }
+
+        public IWebHostBuilder ConfigureServices(Action<IServiceCollection> configureServices)
+        {
+            return ConfigureServices((context, services) => configureServices(services));
+        }
+
+        public IWebHostBuilder ConfigureServices(Action<WebHostBuilderContext, IServiceCollection> configureServices)
+        {
+            _builder.ConfigureServices((context, builder) =>
+            {
+                var webhostBuilderContext = GetWebHostBuilderContext(context);
+                configureServices(webhostBuilderContext, builder);
+            });
+
+            return this;
+        }
+
+        public IWebHostBuilder UseDefaultServiceProvider(Action<WebHostBuilderContext, ServiceProviderOptions> configure)
+        {
+            // REVIEW: This is a hack to change the builder with the HostBuilderContext in scope,
+            // we're not actually using configuration here
+            _builder.ConfigureAppConfiguration((context, _) =>
+            {
+                var webHostBuilderContext = GetWebHostBuilderContext(context);
+                var options = new ServiceProviderOptions();
+                configure(webHostBuilderContext, options);
+
+                // This is only fine because this runs last
+                _builder.UseServiceProviderFactory(new DefaultServiceProviderFactory(options));
+            });
+
+            return this;
+        }
+
+        public IWebHostBuilder UseStartup(Type startupType)
+        {
+            _builder.ConfigureServices((context, services) =>
+            {
+                UseStartup(startupType, context, services);
+            });
+
+            return this;
+        }
+
+        private void UseStartup(Type startupType, HostBuilderContext context, IServiceCollection services)
+        {
+            var webHostBuilderContext = GetWebHostBuilderContext(context);
+            var webHostOptions = (WebHostOptions)context.Properties[typeof(WebHostOptions)];
+
+            ExceptionDispatchInfo startupError = null;
+            object instance = null;
+            ConfigureBuilder configureBuilder = null;
+
+            try
+            {
+                // We cannot support methods that return IServiceProvider as that is terminal and we need ConfigureServices to compose
+                if (typeof(IStartup).IsAssignableFrom(startupType))
+                {
+                    throw new NotSupportedException($"{typeof(IStartup)} isn't supported");
+                }
+
+                instance = ActivatorUtilities.CreateInstance(new HostServiceProvider(webHostBuilderContext), startupType);
+                context.Properties[_startupKey] = instance;
+
+                // Startup.ConfigureServices
+                var configureServicesBuilder = StartupLoader.FindConfigureServicesDelegate(startupType, context.HostingEnvironment.EnvironmentName);
+                var configureServices = configureServicesBuilder.Build(instance);
+
+                configureServices(services);
+
+                // REVIEW: We're doing this in the callback so that we have access to the hosting environment
+                // Startup.ConfigureContainer
+                var configureContainerBuilder = StartupLoader.FindConfigureContainerDelegate(startupType, context.HostingEnvironment.EnvironmentName);
+                if (configureContainerBuilder.MethodInfo != null)
+                {
+                    var containerType = configureContainerBuilder.GetContainerType();
+                    // Store the builder in the property bag
+                    _builder.Properties[typeof(ConfigureContainerBuilder)] = configureContainerBuilder;
+
+                    var actionType = typeof(Action<,>).MakeGenericType(typeof(HostBuilderContext), containerType);
+
+                    // Get the private ConfigureContainer method on this type then close over the container type
+                    var configureCallback = GetType().GetMethod(nameof(ConfigureContainer), BindingFlags.NonPublic | BindingFlags.Instance)
+                                                     .MakeGenericMethod(containerType)
+                                                     .CreateDelegate(actionType, this);
+
+                    // _builder.ConfigureContainer<T>(ConfigureContainer);
+                    typeof(IHostBuilder).GetMethods().First(m => m.Name == nameof(IHostBuilder.ConfigureContainer))
+                        .MakeGenericMethod(containerType)
+                        .Invoke(_builder, new object[] { configureCallback });
+                }
+
+                // Resolve Configure after calling ConfigureServices and ConfigureContainer
+                configureBuilder = StartupLoader.FindConfigureDelegate(startupType, context.HostingEnvironment.EnvironmentName);
+            }
+            catch (Exception ex) when (webHostOptions.CaptureStartupErrors)
+            {
+                startupError = ExceptionDispatchInfo.Capture(ex);
+            }
+
+            // Startup.Configure
+            services.Configure<GenericWebHostServiceOptions>(options =>
+            {
+                options.ConfigureApplication = app =>
+                {
+                    // Throw if there was any errors initializing startup
+                    startupError?.Throw();
+
+                    // Execute Startup.Configure
+                    if (instance != null && configureBuilder != null)
+                    {
+                        configureBuilder.Build(instance)(app);
+                    }
+                };
+            });
+        }
+
+        private void ConfigureContainer<TContainer>(HostBuilderContext context, TContainer container)
+        {
+            var instance = context.Properties[_startupKey];
+            var builder = (ConfigureContainerBuilder)context.Properties[typeof(ConfigureContainerBuilder)];
+            builder.Build(instance)(container);
+        }
+
+        public IWebHostBuilder Configure(Action<IApplicationBuilder> configure)
+        {
+            _builder.ConfigureServices((context, services) =>
+            {
+                services.Configure<GenericWebHostServiceOptions>(options =>
+                {
+                    options.ConfigureApplication = configure;
+                });
+            });
+
+            return this;
+        }
+
+        private WebHostBuilderContext GetWebHostBuilderContext(HostBuilderContext context)
+        {
+            if (!context.Properties.TryGetValue(typeof(WebHostBuilderContext), out var contextVal))
+            {
+                var options = new WebHostOptions(context.Configuration, Assembly.GetEntryAssembly()?.GetName().Name);
+                var hostingEnvironment = new HostingEnvironment();
+                hostingEnvironment.Initialize(context.HostingEnvironment.ContentRootPath, options);
+
+                var webHostBuilderContext = new WebHostBuilderContext
+                {
+                    Configuration = context.Configuration,
+                    HostingEnvironment = hostingEnvironment
+                };
+                context.Properties[typeof(WebHostBuilderContext)] = webHostBuilderContext;
+                context.Properties[typeof(WebHostOptions)] = options;
+                return webHostBuilderContext;
+            }
+
+            return (WebHostBuilderContext)contextVal;
+        }
+
+        public string GetSetting(string key)
+        {
+            return _config[key];
+        }
+
+        public IWebHostBuilder UseSetting(string key, string value)
+        {
+            _config[key] = value;
+            return this;
+        }
+
+        // This exists just so that we can use ActivatorUtilities.CreateInstance on the Startup class
+        private class HostServiceProvider : IServiceProvider
+        {
+            private readonly WebHostBuilderContext _context;
+
+            public HostServiceProvider(WebHostBuilderContext context)
+            {
+                _context = context;
+            }
+
+            public object GetService(Type serviceType)
+            {
+                // The implementation of the HostingEnvironment supports both interfaces
+                if (serviceType == typeof(Microsoft.AspNetCore.Hosting.IHostingEnvironment) || serviceType == typeof(IHostingEnvironment))
+                {
+                    return _context.HostingEnvironment;
+                }
+
+                if (serviceType == typeof(IConfiguration))
+                {
+                    return _context.Configuration;
+                }
+
+                return null;
+            }
+        }
+    }
+}

src/Microsoft.AspNetCore.Hosting/GenericHost/GenericWebHostServiceOptions.cs

@@ -0,0 +1,17 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Builder;
+
+namespace Microsoft.AspNetCore.Hosting.Internal
+{
+    internal class GenericWebHostServiceOptions
+    {
+        public Action<IApplicationBuilder> ConfigureApplication { get; set; }
+
+        public WebHostOptions WebHostOptions { get; set; }
+
+        public AggregateException HostingStartupExceptions { get; set; }
+    }
+}

src/Microsoft.AspNetCore.Hosting/GenericHost/GenericWebHostedService.cs

@@ -0,0 +1,201 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting.Builder;
+using Microsoft.AspNetCore.Hosting.Server;
+using Microsoft.AspNetCore.Hosting.Server.Features;
+using Microsoft.AspNetCore.Hosting.Views;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using Microsoft.Extensions.StackTrace.Sources;
+
+namespace Microsoft.AspNetCore.Hosting.Internal
+{
+    internal class GenericWebHostService : IHostedService
+    {
+        public GenericWebHostService(IOptions<GenericWebHostServiceOptions> options,
+                                     IServer server,
+                                     ILogger<GenericWebHostService> logger,
+                                     DiagnosticListener diagnosticListener,
+                                     IHttpContextFactory httpContextFactory,
+                                     IApplicationBuilderFactory applicationBuilderFactory,
+                                     IEnumerable<IStartupFilter> startupFilters,
+                                     IConfiguration configuration,
+                                     IHostingEnvironment hostingEnvironment)
+        {
+            Options = options.Value;
+            Server = server;
+            Logger = logger;
+            DiagnosticListener = diagnosticListener;
+            HttpContextFactory = httpContextFactory;
+            ApplicationBuilderFactory = applicationBuilderFactory;
+            StartupFilters = startupFilters;
+            Configuration = configuration;
+            HostingEnvironment = hostingEnvironment;
+        }
+
+        public GenericWebHostServiceOptions Options { get; }
+        public IServer Server { get; }
+        public ILogger<GenericWebHostService> Logger { get; }
+        public DiagnosticListener DiagnosticListener { get; }
+        public IHttpContextFactory HttpContextFactory { get; }
+        public IApplicationBuilderFactory ApplicationBuilderFactory { get; }
+        public IEnumerable<IStartupFilter> StartupFilters { get; }
+        public IConfiguration Configuration { get; }
+        public IHostingEnvironment HostingEnvironment { get; }
+
+        public async Task StartAsync(CancellationToken cancellationToken)
+        {
+            HostingEventSource.Log.HostStart();
+
+            var serverAddressesFeature = Server.Features?.Get<IServerAddressesFeature>();
+            var addresses = serverAddressesFeature?.Addresses;
+            if (addresses != null && !addresses.IsReadOnly && addresses.Count == 0)
+            {
+                var urls = Configuration[WebHostDefaults.ServerUrlsKey];
+                if (!string.IsNullOrEmpty(urls))
+                {
+                    serverAddressesFeature.PreferHostingUrls = WebHostUtilities.ParseBool(Configuration, WebHostDefaults.PreferHostingUrlsKey);
+
+                    foreach (var value in urls.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries))
+                    {
+                        addresses.Add(value);
+                    }
+                }
+            }
+
+            RequestDelegate application = null;
+
+            try
+            {
+                Action<IApplicationBuilder> configure = Options.ConfigureApplication;
+
+                if (configure == null)
+                {
+                    throw new InvalidOperationException($"No application configured. Please specify an application via IWebHostBuilder.UseStartup, IWebHostBuilder.Configure, or specifying the startup assembly via {nameof(WebHostDefaults.StartupAssemblyKey)} in the web host configuration.");
+                }
+
+                var builder = ApplicationBuilderFactory.CreateBuilder(Server.Features);
+
+                foreach (var filter in StartupFilters.Reverse())
+                {
+                    configure = filter.Configure(configure);
+                }
+
+                configure(builder);
+
+                // Build the request pipeline
+                application = builder.Build();
+            }
+            catch (Exception ex)
+            {
+                Logger.ApplicationError(ex);
+
+                if (!Options.WebHostOptions.CaptureStartupErrors)
+                {
+                    throw;
+                }
+
+                application = BuildErrorPageApplication(ex);
+            }
+
+            var httpApplication = new HostingApplication(application, Logger, DiagnosticListener, HttpContextFactory);
+
+            await Server.StartAsync(httpApplication, cancellationToken);
+
+            if (addresses != null)
+            {
+                foreach (var address in addresses)
+                {
+                    Logger.LogInformation("Now listening on: {address}", address);
+                }
+            }
+
+            if (Logger.IsEnabled(LogLevel.Debug))
+            {
+                foreach (var assembly in Options.WebHostOptions.GetFinalHostingStartupAssemblies())
+                {
+                    Logger.LogDebug("Loaded hosting startup assembly {assemblyName}", assembly);
+                }
+            }
+
+            if (Options.HostingStartupExceptions != null)
+            {
+                foreach (var exception in Options.HostingStartupExceptions.InnerExceptions)
+                {
+                    Logger.HostingStartupAssemblyError(exception);
+                }
+            }
+        }
+
+        private RequestDelegate BuildErrorPageApplication(Exception exception)
+        {
+            if (exception is TargetInvocationException tae)
+            {
+                exception = tae.InnerException;
+            }
+
+            var showDetailedErrors = HostingEnvironment.IsDevelopment() || Options.WebHostOptions.DetailedErrors;
+
+            var model = new ErrorPageModel
+            {
+                RuntimeDisplayName = RuntimeInformation.FrameworkDescription
+            };
+            var systemRuntimeAssembly = typeof(System.ComponentModel.DefaultValueAttribute).GetTypeInfo().Assembly;
+            var assemblyVersion = new AssemblyName(systemRuntimeAssembly.FullName).Version.ToString();
+            var clrVersion = assemblyVersion;
+            model.RuntimeArchitecture = RuntimeInformation.ProcessArchitecture.ToString();
+            var currentAssembly = typeof(ErrorPage).GetTypeInfo().Assembly;
+            model.CurrentAssemblyVesion = currentAssembly
+                .GetCustomAttribute<AssemblyInformationalVersionAttribute>()
+                .InformationalVersion;
+            model.ClrVersion = clrVersion;
+            model.OperatingSystemDescription = RuntimeInformation.OSDescription;
+
+            if (showDetailedErrors)
+            {
+                var exceptionDetailProvider = new ExceptionDetailsProvider(
+                    HostingEnvironment.ContentRootFileProvider,
+                    sourceCodeLineCount: 6);
+
+                model.ErrorDetails = exceptionDetailProvider.GetDetails(exception);
+            }
+            else
+            {
+                model.ErrorDetails = new ExceptionDetails[0];
+            }
+
+            var errorPage = new ErrorPage(model);
+            return context =>
+            {
+                context.Response.StatusCode = 500;
+                context.Response.Headers["Cache-Control"] = "no-cache";
+                return errorPage.ExecuteAsync(context);
+            };
+        }
+
+        public async Task StopAsync(CancellationToken cancellationToken)
+        {
+            try
+            {
+                await Server.StopAsync(cancellationToken);
+            }
+            finally
+            {
+                HostingEventSource.Log.HostStop();
+            }
+        }
+    }
+}

src/Microsoft.AspNetCore.Hosting/GenericHost/HostingStartupWebHostBuilder.cs

@@ -0,0 +1,79 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Hosting.Internal
+{
+    // We use this type to capture calls to the IWebHostBuilder so the we can properly order calls to 
+    // to GenericHostWebHostBuilder.
+    internal class HostingStartupWebHostBuilder : IWebHostBuilder, ISupportsStartup, ISupportsUseDefaultServiceProvider
+    {
+        private readonly GenericWebHostBuilder _builder;
+        private Action<WebHostBuilderContext, IConfigurationBuilder> _configureConfiguration;
+        private Action<WebHostBuilderContext, IServiceCollection> _configureServices;
+
+        public HostingStartupWebHostBuilder(GenericWebHostBuilder builder)
+        {
+            _builder = builder;
+        }
+
+        public IWebHost Build()
+        {
+            throw new NotSupportedException($"Building this implementation of {nameof(IWebHostBuilder)} is not supported.");
+        }
+
+        public IWebHostBuilder ConfigureAppConfiguration(Action<WebHostBuilderContext, IConfigurationBuilder> configureDelegate)
+        {
+            _configureConfiguration += configureDelegate;
+            return this;
+        }
+
+        public IWebHostBuilder ConfigureServices(Action<IServiceCollection> configureServices)
+        {
+            return ConfigureServices((context, services) => configureServices(services));
+        }
+
+        public IWebHostBuilder ConfigureServices(Action<WebHostBuilderContext, IServiceCollection> configureServices)
+        {
+            _configureServices += configureServices;
+            return this;
+        }
+
+        public string GetSetting(string key) => _builder.GetSetting(key);
+
+        public IWebHostBuilder UseSetting(string key, string value)
+        {
+            _builder.UseSetting(key, value);
+            return this;
+        }
+
+        public void ConfigureServices(WebHostBuilderContext context, IServiceCollection services)
+        {
+            _configureServices?.Invoke(context, services);
+        }
+
+        public void ConfigureAppConfiguration(WebHostBuilderContext context, IConfigurationBuilder builder)
+        {
+            _configureConfiguration?.Invoke(context, builder);
+        }
+
+        public IWebHostBuilder UseDefaultServiceProvider(Action<WebHostBuilderContext, ServiceProviderOptions> configure)
+        {
+            return _builder.UseDefaultServiceProvider(configure);
+        }
+
+        public IWebHostBuilder Configure(Action<IApplicationBuilder> configure)
+        {
+            return _builder.Configure(configure);
+        }
+
+        public IWebHostBuilder UseStartup(Type startupType)
+        {
+            return _builder.UseStartup(startupType);
+        }
+    }
+}

src/Microsoft.AspNetCore.Hosting/GenericHost/ISupportsStartup.cs

@@ -0,0 +1,14 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Builder;
+
+namespace Microsoft.AspNetCore.Hosting.Internal
+{
+    internal interface ISupportsStartup
+    {
+        IWebHostBuilder Configure(Action<IApplicationBuilder> configure);
+        IWebHostBuilder UseStartup(Type startupType);
+    }
+}

src/Microsoft.AspNetCore.Hosting/GenericHost/ISupportsUseDefaultServiceProvider.cs

@@ -0,0 +1,13 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Hosting.Internal
+{
+    internal interface ISupportsUseDefaultServiceProvider
+    {
+        IWebHostBuilder UseDefaultServiceProvider(Action<WebHostBuilderContext, ServiceProviderOptions> configure);
+    }
+}

src/Microsoft.AspNetCore.Hosting/GenericHostWebHostBuilderExtensions.cs

@@ -0,0 +1,16 @@
+using System;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Hosting.Internal;
+
+namespace Microsoft.Extensions.Hosting
+{
+    public static class GenericHostWebHostBuilderExtensions
+    {
+        public static IHostBuilder ConfigureWebHost(this IHostBuilder builder, Action<IWebHostBuilder> configure)
+        {
+            var webhostBuilder = new GenericWebHostBuilder(builder);
+            configure(webhostBuilder);
+            return builder;
+        }
+    }
+}

src/Microsoft.AspNetCore.Hosting/Internal/ConfigureContainerBuilder.cs

@@ -15,7 +15,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
 
         public MethodInfo MethodInfo { get; }
 
-        public Func<Action<object>, Action<object>> ConfigureContainerFilters { get; set; }
+        public Func<Action<object>, Action<object>> ConfigureContainerFilters { get; set; } = f => f;
 
         public Action<object> Build(object instance) => container => Invoke(instance, container);
 

src/Microsoft.AspNetCore.Hosting/Internal/ConfigureServicesBuilder.cs

@@ -17,7 +17,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
 
         public MethodInfo MethodInfo { get; }
 
-        public Func<Func<IServiceCollection, IServiceProvider>, Func<IServiceCollection, IServiceProvider>> StartupServiceFilters { get; set; }
+        public Func<Func<IServiceCollection, IServiceProvider>, Func<IServiceCollection, IServiceProvider>> StartupServiceFilters { get; set; } = f => f;
 
         public Func<IServiceCollection, IServiceProvider> Build(object instance) => services => Invoke(instance, services);
 

src/Microsoft.AspNetCore.Hosting/Internal/StartupLoader.cs

@@ -267,19 +267,19 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             return type;
         }
 
-        private static ConfigureBuilder FindConfigureDelegate(Type startupType, string environmentName)
+        internal static ConfigureBuilder FindConfigureDelegate(Type startupType, string environmentName)
         {
             var configureMethod = FindMethod(startupType, "Configure{0}", environmentName, typeof(void), required: true);
             return new ConfigureBuilder(configureMethod);
         }
 
-        private static ConfigureContainerBuilder FindConfigureContainerDelegate(Type startupType, string environmentName)
+        internal static ConfigureContainerBuilder FindConfigureContainerDelegate(Type startupType, string environmentName)
         {
             var configureMethod = FindMethod(startupType, "Configure{0}Container", environmentName, typeof(void), required: false);
             return new ConfigureContainerBuilder(configureMethod);
         }
 
-        private static ConfigureServicesBuilder FindConfigureServicesDelegate(Type startupType, string environmentName)
+        internal static ConfigureServicesBuilder FindConfigureServicesDelegate(Type startupType, string environmentName)
         {
             var servicesMethod = FindMethod(startupType, "Configure{0}Services", environmentName, typeof(IServiceProvider), required: false)
                 ?? FindMethod(startupType, "Configure{0}Services", environmentName, typeof(void), required: false);

src/Microsoft.AspNetCore.Hosting/Internal/WebHost.cs

@@ -192,7 +192,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
 
             if (_startup == null)
             {
-                throw new InvalidOperationException($"No startup configured. Please specify startup via WebHostBuilder.UseStartup, WebHostBuilder.Configure, injecting {nameof(IStartup)} or specifying the startup assembly via {nameof(WebHostDefaults.StartupAssemblyKey)} in the web host configuration.");
+                throw new InvalidOperationException($"No application configured. Please specify startup via IWebHostBuilder.UseStartup, IWebHostBuilder.Configure, injecting {nameof(IStartup)} or specifying the startup assembly via {nameof(WebHostDefaults.StartupAssemblyKey)} in the web host configuration.");
             }
         }
 

src/Microsoft.AspNetCore.Hosting/WebHostBuilderExtensions.cs

@@ -29,9 +29,15 @@ namespace Microsoft.AspNetCore.Hosting
 
             var startupAssemblyName = configureApp.GetMethodInfo().DeclaringType.GetTypeInfo().Assembly.GetName().Name;
 
-            return hostBuilder
+            hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName);
-                .UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName)
+
-                .ConfigureServices(services =>
+            // Light up the ISupportsStartup implementation
+            if (hostBuilder is ISupportsStartup supportsStartup)
+            {
+                return supportsStartup.Configure(configureApp);
+            }
+
+            return hostBuilder.ConfigureServices(services =>
             {
                 services.AddSingleton<IStartup>(sp =>
                 {
@@ -51,8 +57,15 @@ namespace Microsoft.AspNetCore.Hosting
         {
             var startupAssemblyName = startupType.GetTypeInfo().Assembly.GetName().Name;
 
+            hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName);
+
+            // Light up the GenericWebHostBuilder implementation
+            if (hostBuilder is ISupportsStartup supportsStartup)
+            {
+                return supportsStartup.UseStartup(startupType);
+            }
+
             return hostBuilder
-                .UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName)
                 .ConfigureServices(services =>
                 {
                     if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo()))
@@ -100,6 +113,12 @@ namespace Microsoft.AspNetCore.Hosting
         /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
         public static IWebHostBuilder UseDefaultServiceProvider(this IWebHostBuilder hostBuilder, Action<WebHostBuilderContext, ServiceProviderOptions> configure)
         {
+            // Light up the GenericWebHostBuilder implementation
+            if (hostBuilder is ISupportsUseDefaultServiceProvider supportsDefaultServiceProvider)
+            {
+                return supportsDefaultServiceProvider.UseDefaultServiceProvider(configure);
+            }
+
             return hostBuilder.ConfigureServices((context, services) =>
             {
                 var options = new ServiceProviderOptions();

test/Microsoft.AspNetCore.Hosting.Tests/Fakes/GenericWebHost.cs

@@ -0,0 +1,35 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Hosting.Server;
+using Microsoft.AspNetCore.Http.Features;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+
+namespace Microsoft.AspNetCore.Hosting.Tests.Fakes
+{
+    internal class GenericWebHost : IWebHost
+    {
+        private readonly IHost _host;
+
+        public GenericWebHost(IHost host)
+        {
+            _host = host;
+        }
+
+        public IFeatureCollection ServerFeatures => Services.GetRequiredService<IServer>().Features;
+
+        public IServiceProvider Services => _host.Services;
+
+        public void Dispose() => _host.Dispose();
+
+        public void Start() => _host.Start();
+
+        public Task StartAsync(CancellationToken cancellationToken = default) => _host.StartAsync(cancellationToken);
+
+        public Task StopAsync(CancellationToken cancellationToken = default) => _host.StopAsync(cancellationToken);
+    }
+}

test/Microsoft.AspNetCore.Hosting.Tests/Fakes/GenericWebHostBuilderWrapper.cs

@@ -0,0 +1,77 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting.Internal;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+
+namespace Microsoft.AspNetCore.Hosting.Tests.Fakes
+{
+    public class GenericWebHostBuilderWrapper : IWebHostBuilder, ISupportsStartup, ISupportsUseDefaultServiceProvider
+    {
+        private readonly GenericWebHostBuilder _builder;
+        private readonly HostBuilder _hostBuilder;
+
+        internal GenericWebHostBuilderWrapper(HostBuilder hostBuilder)
+        {
+            _builder = new GenericWebHostBuilder(hostBuilder);
+            _hostBuilder = hostBuilder;
+        }
+
+        // This is the only one that doesn't pass through
+        public IWebHost Build()
+        {
+            return new GenericWebHost(_hostBuilder.Build());
+        }
+
+        public IWebHostBuilder Configure(Action<IApplicationBuilder> configure)
+        {
+            _builder.Configure(configure);
+            return this;
+        }
+
+        public IWebHostBuilder ConfigureAppConfiguration(Action<WebHostBuilderContext, IConfigurationBuilder> configureDelegate)
+        {
+            _builder.ConfigureAppConfiguration(configureDelegate);
+            return this;
+        }
+
+        public IWebHostBuilder ConfigureServices(Action<IServiceCollection> configureServices)
+        {
+            _builder.ConfigureServices(configureServices);
+            return this;
+        }
+
+        public IWebHostBuilder ConfigureServices(Action<WebHostBuilderContext, IServiceCollection> configureServices)
+        {
+            _builder.ConfigureServices(configureServices);
+            return this;
+        }
+
+        public string GetSetting(string key)
+        {
+            return _builder.GetSetting(key);
+        }
+
+        public IWebHostBuilder UseDefaultServiceProvider(Action<WebHostBuilderContext, ServiceProviderOptions> configure)
+        {
+            _builder.UseDefaultServiceProvider(configure);
+            return this;
+        }
+
+        public IWebHostBuilder UseSetting(string key, string value)
+        {
+            _builder.UseSetting(key, value);
+            return this;
+        }
+
+        public IWebHostBuilder UseStartup(Type startupType)
+        {
+            _builder.UseStartup(startupType);
+            return this;
+        }
+    }
+}

test/Microsoft.AspNetCore.Hosting.Tests/Fakes/StartupNoServicesNoInterface.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Hosting.Fakes
+{
+    public class StartupNoServicesNoInterface
+    {
+        public void ConfigureServices(IServiceCollection services)
+        {
+
+        }
+
+        public void Configure(IApplicationBuilder app)
+        {
+
+        }
+    }
+}

test/Microsoft.AspNetCore.Hosting.Tests/Microsoft.AspNetCore.Hosting.Tests.csproj

@@ -10,6 +10,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\..\src\Microsoft.AspNetCore.Hosting\Microsoft.AspNetCore.Hosting.csproj" />
+    <ProjectReference Include="..\..\src\Microsoft.Extensions.Hosting\Microsoft.Extensions.Hosting.csproj" />
     <ProjectReference Include="..\TestAssets\TestStartupAssembly1\TestStartupAssembly1.csproj" />
   </ItemGroup>