This change introduces a new service `IStartup` that can be registered in the hosting container to override any startup logic. `UseStartup` overloads have been changed to detect `IStartup` and directly put it in the container, or to wrapping it with a `ConventionBasedStartup` implementation to preserve the existing behavior.

- Remove IStartupLoader and add `IStartup` instead that matches the signature hosting cares about
- Moved `UseStartup` to extension methods
- Move existing logic into `ConventionBasedStartup` class

global.json

@@ -1,3 +1,3 @@
 {
     "projects": ["src"]
-}
+}

samples/SampleStartups/StartupBlockingOnStart.cs

@@ -8,16 +8,17 @@ using Microsoft.Extensions.DependencyInjection;
 
 namespace SampleStartups
 {
-    public class StartupBlockingOnStart
+    public class StartupBlockingOnStart : StartupBase
     {
-        // This method gets called by the runtime. Use this method to add services to the container.
+        public override IServiceProvider ConfigureServices(IServiceCollection services)
-        // For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=398940
-        public void ConfigureServices(IServiceCollection services)
         {
+            // This method gets called by the runtime. Use this method to add services to the container.
+            // For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=398940
+            return base.ConfigureServices(services);
         }
 
         // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
-        public void Configure(IApplicationBuilder app)
+        public override void Configure(IApplicationBuilder app)
         {
             app.Run(async (context) =>
             {

samples/SampleStartups/StartupConfigureAddresses.cs

@@ -1,22 +1,15 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
-using Microsoft.Extensions.DependencyInjection;
 
 // Note that this sample will not run. It is only here to illustrate usage patterns.
 
 namespace SampleStartups
 {
-    public class StartupConfigureAddresses
+    public class StartupConfigureAddresses : StartupBase
     {
-        // This method gets called by the runtime. Use this method to add services to the container.
-        // For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=398940
-        public void ConfigureServices(IServiceCollection services)
-        {
-        }
-
         // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
-        public void Configure(IApplicationBuilder app)
+        public override void Configure(IApplicationBuilder app)
         {
             app.Run(async (context) =>
             {

samples/SampleStartups/StartupExternallyControlled.cs

@@ -3,25 +3,18 @@ using System.Collections.Generic;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
-using Microsoft.Extensions.DependencyInjection;
 
 // Note that this sample will not run. It is only here to illustrate usage patterns.
 
 namespace SampleStartups
 {
-    public class StartupExternallyControlled
+    public class StartupExternallyControlled : StartupBase
     {
         private IWebHost _host;
         private readonly List<string> _urls = new List<string>();
-
+        
-        // This method gets called by the runtime. Use this method to add services to the container.
-        // For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=398940
-        public void ConfigureServices(IServiceCollection services)
-        {
-        }
-
         // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
-        public void Configure(IApplicationBuilder app)
+        public override void Configure(IApplicationBuilder app)
         {
             app.Run(async (context) =>
             {

samples/SampleStartups/StartupHelloWorld.cs

@@ -1,22 +1,15 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
-using Microsoft.Extensions.DependencyInjection;
 
 // Note that this sample will not run. It is only here to illustrate usage patterns.
 
 namespace SampleStartups
 {
-    public class StartupHelloWorld
+    public class StartupHelloWorld : StartupBase
     {
-        // This method gets called by the runtime. Use this method to add services to the container.
-        // For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=398940
-        public void ConfigureServices(IServiceCollection services)
-        {
-        }
-
         // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
-        public void Configure(IApplicationBuilder app)
+        public override void Configure(IApplicationBuilder app)
         {
             app.Run(async (context) =>
             {

src/Microsoft.AspNetCore.Hosting.Abstractions/IStartup.cs

@@ -0,0 +1,16 @@
+// 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.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Hosting
+{
+    public interface IStartup
+    {
+        IServiceProvider ConfigureServices(IServiceCollection services);
+
+        void Configure(IApplicationBuilder app);
+    }
+}

src/Microsoft.AspNetCore.Hosting.Abstractions/IWebHostBuilder.cs

@@ -2,8 +2,6 @@
 // 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.Server;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
 
@@ -26,13 +24,6 @@ namespace Microsoft.AspNetCore.Hosting
         /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
         IWebHostBuilder UseLoggerFactory(ILoggerFactory loggerFactory);
 
-        /// <summary>
-        /// Specify the startup type to be used by the web host. 
-        /// </summary>
-        /// <param name="startupType">The <see cref="Type"/> to be used.</param>
-        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
-        IWebHostBuilder UseStartup(Type startupType);
-
         /// <summary>
         /// Specify the delegate that is used to configure the services of the web application.
         /// </summary>
@@ -40,13 +31,6 @@ namespace Microsoft.AspNetCore.Hosting
         /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
         IWebHostBuilder ConfigureServices(Action<IServiceCollection> configureServices);
 
-        /// <summary>
-        /// Specify the startup method to be used to configure the web application. 
-        /// </summary>
-        /// <param name="configureApplication">The delegate that configures the <see cref="IApplicationBuilder"/>.</param>
-        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
-        IWebHostBuilder Configure(Action<IApplicationBuilder> configureApplication);
-
         /// <summary>
         /// Adds a delegate for configuring the provided <see cref="ILoggerFactory"/>. This may be called multiple times.
         /// </summary>

src/Microsoft.AspNetCore.Hosting.Abstractions/WebHostDefaults.cs

@@ -5,7 +5,9 @@ namespace Microsoft.AspNetCore.Hosting
 {
     public static class WebHostDefaults
     {
-        public static readonly string ApplicationKey = "application";
+        public static readonly string ApplicationKey = "applicationName";
+        public static readonly string StartupAssemblyKey = "startupAssembly";
+        
         public static readonly string DetailedErrorsKey = "detailedErrors";
         public static readonly string EnvironmentKey = "environment";
         public static readonly string ServerKey = "server";

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

@@ -3,7 +3,6 @@
 
 using System;
 using Microsoft.AspNetCore.Builder;
-using Microsoft.AspNetCore.Hosting.Startup;
 
 namespace Microsoft.AspNetCore.Hosting.Internal
 {

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

@@ -6,7 +6,7 @@ using System.Reflection;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
 
-namespace Microsoft.AspNetCore.Hosting.Startup
+namespace Microsoft.AspNetCore.Hosting.Internal
 {
     public class ConfigureBuilder
     {
@@ -51,4 +51,4 @@ namespace Microsoft.AspNetCore.Hosting.Startup
             MethodInfo.Invoke(instance, parameters);
         }
     }
-}
+}

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

@@ -6,7 +6,7 @@ using System.Linq;
 using System.Reflection;
 using Microsoft.Extensions.DependencyInjection;
 
-namespace Microsoft.AspNetCore.Hosting.Startup
+namespace Microsoft.AspNetCore.Hosting.Internal
 {
     public class ConfigureServicesBuilder
     {

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

@@ -1,4 +1,7 @@
-using System;
+// 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.Linq;
 using System.Reflection;
 using Microsoft.AspNetCore.Hosting.Server;

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

@@ -2,45 +2,31 @@
 // 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.Globalization;
 using System.Linq;
 using System.Reflection;
 using Microsoft.Extensions.DependencyInjection;
 
-namespace Microsoft.AspNetCore.Hosting.Startup
+namespace Microsoft.AspNetCore.Hosting.Internal
 {
-    public class StartupLoader : IStartupLoader
+    public class StartupLoader
     {
-        private readonly IServiceProvider _services;
+        public static StartupMethods LoadMethods(IServiceProvider services, Type startupType, string environmentName)
-        private readonly IHostingEnvironment _hostingEnv;
-
-        public StartupLoader(IServiceProvider services, IHostingEnvironment hostingEnv)
         {
-            _services = services;
-            _hostingEnv = hostingEnv;
-        }
-
-        public StartupMethods LoadMethods(
-            Type startupType,
-            IList<string> diagnosticMessages)
-        {
-            var environmentName = _hostingEnv.EnvironmentName;
             var configureMethod = FindConfigureDelegate(startupType, environmentName);
             var servicesMethod = FindConfigureServicesDelegate(startupType, environmentName);
 
             object instance = null;
             if (!configureMethod.MethodInfo.IsStatic || (servicesMethod != null && !servicesMethod.MethodInfo.IsStatic))
             {
-                instance = ActivatorUtilities.GetServiceOrCreateInstance(_services, startupType);
+                instance = ActivatorUtilities.GetServiceOrCreateInstance(services, startupType);
             }
 
             return new StartupMethods(configureMethod.Build(instance), servicesMethod?.Build(instance));
         }
 
-        public Type FindStartupType(string startupAssemblyName, IList<string> diagnosticMessages)
+        public static Type FindStartupType(string startupAssemblyName, string environmentName)
         {
-            var environmentName = _hostingEnv.EnvironmentName;
             if (string.IsNullOrEmpty(startupAssemblyName))
             {
                 throw new ArgumentException(

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

@@ -5,7 +5,7 @@ using System;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
 
-namespace Microsoft.AspNetCore.Hosting.Startup
+namespace Microsoft.AspNetCore.Hosting.Internal
 {
     public class StartupMethods
     {

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

@@ -6,10 +6,10 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 using System.Threading;
+using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting.Builder;
 using Microsoft.AspNetCore.Hosting.Server;
 using Microsoft.AspNetCore.Hosting.Server.Features;
-using Microsoft.AspNetCore.Hosting.Startup;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.Extensions.Configuration;
@@ -22,7 +22,9 @@ namespace Microsoft.AspNetCore.Hosting.Internal
     public class WebHost : IWebHost
     {
         private readonly IServiceCollection _applicationServiceCollection;
-        private readonly IStartupLoader _startupLoader;
+        private IStartup _startup;
+
+        private readonly IServiceProvider _hostingServiceProvider;
         private readonly ApplicationLifetime _applicationLifetime;
         private readonly WebHostOptions _options;
         private readonly IConfiguration _config;
@@ -34,16 +36,11 @@ namespace Microsoft.AspNetCore.Hosting.Internal
         // Used for testing only
         internal WebHostOptions Options => _options;
 
-        // Only one of these should be set
-        internal string StartupAssemblyName { get; set; }
-        internal StartupMethods Startup { get; set; }
-        internal Type StartupType { get; set; }
-
         private IServer Server { get; set; }
 
         public WebHost(
             IServiceCollection appServices,
-            IStartupLoader startupLoader,
+            IServiceProvider hostingServiceProvider,
             WebHostOptions options,
             IConfiguration config)
         {
@@ -52,9 +49,9 @@ namespace Microsoft.AspNetCore.Hosting.Internal
                 throw new ArgumentNullException(nameof(appServices));
             }
 
-            if (startupLoader == null)
+            if (hostingServiceProvider == null)
             {
-                throw new ArgumentNullException(nameof(startupLoader));
+                throw new ArgumentNullException(nameof(hostingServiceProvider));
             }
 
             if (config == null)
@@ -65,7 +62,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             _config = config;
             _options = options;
             _applicationServiceCollection = appServices;
-            _startupLoader = startupLoader;
+            _hostingServiceProvider = hostingServiceProvider;
             _applicationLifetime = new ApplicationLifetime();
             _applicationServiceCollection.AddSingleton<IApplicationLifetime>(_applicationLifetime);
         }
@@ -116,37 +113,18 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             if (_applicationServices == null)
             {
                 EnsureStartup();
-                _applicationServices = Startup.ConfigureServicesDelegate(_applicationServiceCollection);
+                _applicationServices = _startup.ConfigureServices(_applicationServiceCollection);
             }
         }
 
         private void EnsureStartup()
         {
-            if (Startup != null)
+            if (_startup != null)
             {
                 return;
             }
 
-            if (StartupType == null)
+            _startup = _hostingServiceProvider.GetRequiredService<IStartup>();
-            {
-                var diagnosticTypeMessages = new List<string>();
-                StartupType = _startupLoader.FindStartupType(StartupAssemblyName, diagnosticTypeMessages);
-                if (StartupType == null)
-                {
-                    throw new ArgumentException(
-                        diagnosticTypeMessages.Aggregate("Failed to find a startup type for the web application.", (a, b) => a + "\r\n" + b),
-                        StartupAssemblyName);
-                }
-            }
-
-            var diagnosticMessages = new List<string>();
-            Startup = _startupLoader.LoadMethods(StartupType, diagnosticMessages);
-            if (Startup == null)
-            {
-                throw new ArgumentException(
-                    diagnosticMessages.Aggregate("Failed to find a startup entry point for the web application.", (a, b) => a + "\r\n" + b),
-                    StartupAssemblyName);
-            }
         }
 
         private RequestDelegate BuildApplication()
@@ -161,7 +139,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
                 builder.ApplicationServices = _applicationServices;
 
                 var startupFilters = _applicationServices.GetService<IEnumerable<IStartupFilter>>();
-                var configure = Startup.ConfigureDelegate;
+                Action<IApplicationBuilder> configure = _startup.Configure;
                 foreach (var filter in startupFilters.Reverse())
                 {
                     configure = filter.Configure(configure);

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

@@ -19,7 +19,8 @@ namespace Microsoft.AspNetCore.Hosting.Internal
                 throw new ArgumentNullException(nameof(configuration));
             }
 
-            Application = configuration[WebHostDefaults.ApplicationKey];
+            ApplicationName = configuration[WebHostDefaults.ApplicationKey];
+            StartupAssembly = configuration[WebHostDefaults.StartupAssemblyKey];
             DetailedErrors = ParseBool(configuration, WebHostDefaults.DetailedErrorsKey);
             CaptureStartupErrors = ParseBool(configuration, WebHostDefaults.CaptureStartupErrorsKey);
             Environment = configuration[WebHostDefaults.EnvironmentKey];
@@ -28,7 +29,7 @@ namespace Microsoft.AspNetCore.Hosting.Internal
             ContentRootPath = configuration[WebHostDefaults.ContentRootKey];
         }
 
-        public string Application { get; set; }
+        public string ApplicationName { get; set; }
 
         public bool DetailedErrors { get; set; }
 
@@ -37,6 +38,8 @@ namespace Microsoft.AspNetCore.Hosting.Internal
         public string Environment { get; set; }
 
         public string ServerAssembly { get; set; }
+        
+        public string StartupAssembly { get; set; }
 
         public string WebRoot { get; set; }
 

src/Microsoft.AspNetCore.Hosting/Startup/ConventionBasedStartup.cs

@@ -0,0 +1,31 @@
+// 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;
+using Microsoft.AspNetCore.Hosting.Internal;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNet.Hosting
+{
+    public class ConventionBasedStartup : IStartup
+    {
+        private readonly StartupMethods _methods;
+
+        public ConventionBasedStartup(StartupMethods methods)
+        {
+            _methods = methods;
+        }
+        
+        public void Configure(IApplicationBuilder app)
+        {
+            _methods.ConfigureDelegate(app);
+        }
+
+        public IServiceProvider ConfigureServices(IServiceCollection services)
+        {
+            return _methods.ConfigureServicesDelegate(services);
+        }
+    }
+}

src/Microsoft.AspNetCore.Hosting/Startup/DelegateStartup.cs

@@ -0,0 +1,21 @@
+// 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;
+
+namespace Microsoft.AspNet.Hosting
+{
+    public class DelegateStartup : StartupBase
+    {
+        private Action<IApplicationBuilder> _configureApp;
+        
+        public DelegateStartup(Action<IApplicationBuilder> configureApp)
+        {
+            _configureApp = configureApp;
+        }
+
+        public override void Configure(IApplicationBuilder app) => _configureApp(app);
+    }
+}

src/Microsoft.AspNetCore.Hosting/Startup/IStartupLoader.cs

@@ -1,15 +0,0 @@
-// 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;
-
-namespace Microsoft.AspNetCore.Hosting.Startup
-{
-    public interface IStartupLoader
-    {
-        Type FindStartupType(string startupAssemblyName, IList<string> diagnosticMessages);
-
-        StartupMethods LoadMethods(Type startupType, IList<string> diagnosticMessages);
-    }
-}

src/Microsoft.AspNetCore.Hosting/Startup/StartupBase.cs

@@ -0,0 +1,19 @@
+// 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.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Hosting
+{
+    public abstract class StartupBase : IStartup
+    {
+        public abstract void Configure(IApplicationBuilder app);
+
+        public virtual IServiceProvider ConfigureServices(IServiceCollection services)
+        {
+            return services.BuildServiceProvider();
+        }
+    }
+}

src/Microsoft.AspNetCore.Hosting/Startup/StartupExceptionPage.cs

@@ -15,7 +15,7 @@ using System.Text.Encodings.Web;
 using Microsoft.Extensions.Internal;
 using Microsoft.Extensions.PlatformAbstractions;
 
-namespace Microsoft.AspNetCore.Hosting.Startup
+namespace Microsoft.AspNetCore.Hosting
 {
     internal static class StartupExceptionPage
     {

src/Microsoft.AspNetCore.Hosting/WebHostBuilder.cs

@@ -5,13 +5,12 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
-using System.Linq;
 using System.Reflection;
-using Microsoft.AspNetCore.Builder;
+using System.Runtime.ExceptionServices;
+using Microsoft.AspNet.Hosting;
 using Microsoft.AspNetCore.Hosting.Builder;
 using Microsoft.AspNetCore.Hosting.Internal;
 using Microsoft.AspNetCore.Hosting.Server;
-using Microsoft.AspNetCore.Hosting.Startup;
 using Microsoft.AspNetCore.Http;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
@@ -34,10 +33,6 @@ namespace Microsoft.AspNetCore.Hosting
         private ILoggerFactory _loggerFactory;
         private WebHostOptions _options;
 
-        // Only one of these should be set
-        private StartupMethods _startup;
-        private Type _startupType;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="WebHostBuilder"/> class.
         /// </summary>
@@ -86,22 +81,6 @@ namespace Microsoft.AspNetCore.Hosting
             return this;
         }
 
-        /// <summary>
-        /// Specify the startup type to be used by the web host.
-        /// </summary>
-        /// <param name="startupType">The <see cref="Type"/> to be used.</param>
-        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
-        public IWebHostBuilder UseStartup(Type startupType)
-        {
-            if (startupType == null)
-            {
-                throw new ArgumentNullException(nameof(startupType));
-            }
-
-            _startupType = startupType;
-            return this;
-        }
-
         /// <summary>
         /// Adds a delegate for configuring additional services for the host or web application. This may be called
         /// multiple times.
@@ -119,22 +98,6 @@ namespace Microsoft.AspNetCore.Hosting
             return this;
         }
 
-        /// <summary>
-        /// Specify the startup method to be used to configure the web application.
-        /// </summary>
-        /// <param name="configureApp">The delegate that configures the <see cref="IApplicationBuilder"/>.</param>
-        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
-        public IWebHostBuilder Configure(Action<IApplicationBuilder> configureApp)
-        {
-            if (configureApp == null)
-            {
-                throw new ArgumentNullException(nameof(configureApp));
-            }
-
-            _startup = new StartupMethods(configureApp);
-            return this;
-        }
-
         /// <summary>
         /// Adds a delegate for configuring the provided <see cref="ILoggerFactory"/>. This may be called multiple times.
         /// </summary>
@@ -159,14 +122,7 @@ namespace Microsoft.AspNetCore.Hosting
             var hostingServices = BuildHostingServices();
             var hostingContainer = hostingServices.BuildServiceProvider();
 
-            var startupLoader = hostingContainer.GetRequiredService<IStartupLoader>();
+            var host = new WebHost(hostingServices, hostingContainer, _options, _config);
-
-            var host = new WebHost(hostingServices, startupLoader, _options, _config);
-
-            // Only one of these should be set, but they are used in priority
-            host.Startup = _startup;
-            host.StartupType = _startupType;
-            host.StartupAssemblyName = _options.Application;
 
             host.Initialize();
 
@@ -180,7 +136,7 @@ namespace Microsoft.AspNetCore.Hosting
             var defaultPlatformServices = PlatformServices.Default;
             var appEnvironment = defaultPlatformServices.Application;
             var contentRootPath = ResolveContentRootPath(_options.ContentRootPath, appEnvironment.ApplicationBasePath);
-            var applicationName = ResolveApplicationName() ?? appEnvironment.ApplicationName;
+            var applicationName = _options.ApplicationName ?? appEnvironment.ApplicationName;
 
             // Initialize the hosting environment
             _hostingEnvironment.Initialize(applicationName, contentRootPath, _options);
@@ -204,7 +160,6 @@ namespace Microsoft.AspNetCore.Hosting
             //This is required to add ILogger of T.
             services.AddLogging();
 
-            services.AddTransient<IStartupLoader, StartupLoader>();
             services.AddTransient<IApplicationBuilderFactory, ApplicationBuilderFactory>();
             services.AddTransient<IHttpContextFactory, HttpContextFactory>();
             services.AddOptions();
@@ -225,8 +180,51 @@ namespace Microsoft.AspNetCore.Hosting
             if (!string.IsNullOrEmpty(_options.ServerAssembly))
             {
                 // Add the server
-                var serverType = ServerLoader.ResolveServerType(_options.ServerAssembly);
+                try
-                services.AddSingleton(typeof(IServer), serverType);
+                {
+                    var serverType = ServerLoader.ResolveServerType(_options.ServerAssembly);
+                    services.AddSingleton(typeof(IServer), serverType);
+                }
+                catch (Exception ex)
+                {
+                    var capture = ExceptionDispatchInfo.Capture(ex);
+                    services.AddSingleton<IServer>(_ =>
+                    {
+                        capture.Throw();
+                        return null;
+                    });
+                }
+            }
+
+            if (!string.IsNullOrEmpty(_options.StartupAssembly))
+            {
+                try
+                {
+                    var startupType = StartupLoader.FindStartupType(_options.StartupAssembly, _hostingEnvironment.EnvironmentName);
+                    
+                    if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo()))
+                    {
+                        services.AddSingleton(typeof(IStartup), startupType);
+                    }
+                    else
+                    {
+                        services.AddSingleton(typeof(IStartup), sp =>
+                        {
+                            var hostingEnvironment = sp.GetRequiredService<IHostingEnvironment>();
+                            var methods = StartupLoader.LoadMethods(sp, startupType, hostingEnvironment.EnvironmentName);
+                            return new ConventionBasedStartup(methods);
+                        });
+                    }
+                }
+                catch (Exception ex)
+                {
+                    var capture = ExceptionDispatchInfo.Capture(ex);
+                    services.AddSingleton<IStartup>(_ =>
+                    {
+                        capture.Throw();
+                        return null;
+                    });
+                }
             }
 
             foreach (var configureServices in _configureServicesDelegates)
@@ -249,22 +247,5 @@ namespace Microsoft.AspNetCore.Hosting
             }
             return Path.Combine(Path.GetFullPath(basePath), contentRootPath);
         }
-
-        private string ResolveApplicationName()
-        {
-            if (_startup != null)
-            {
-                return _startup.ConfigureDelegate.Target.GetType().GetTypeInfo().Assembly.GetName().Name;
-            }
-            if (_startupType != null)
-            {
-                return _startupType.GetTypeInfo().Assembly.GetName().Name;
-            }
-            if (!string.IsNullOrEmpty(_options.Application))
-            {
-                return _options.Application;
-            }
-            return null;
-        }
     }
 }

src/Microsoft.AspNetCore.Hosting/WebHostBuilderExtensions.cs

@@ -4,6 +4,8 @@
 using System;
 using System.Linq;
 using System.Reflection;
+using Microsoft.AspNet.Hosting;
+using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting.Internal;
 using Microsoft.AspNetCore.Hosting.Server;
 using Microsoft.Extensions.Configuration;
@@ -63,7 +65,58 @@ namespace Microsoft.AspNetCore.Hosting
         {
             return hostBuilder.UseSetting(WebHostDefaults.CaptureStartupErrorsKey, captureStartupErrors ? "true" : "false");
         }
-
+        
+        /// <summary>
+        /// Specify the startup method to be used to configure the web application.
+        /// </summary>
+        /// <param name="hostBuilder">The <see cref="IWebHostBuilder"/> to configure.</param>
+        /// <param name="configureApp">The delegate that configures the <see cref="IApplicationBuilder"/>.</param>
+        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
+        public static IWebHostBuilder Configure(this IWebHostBuilder hostBuilder, Action<IApplicationBuilder> configureApp)
+        {
+            if (configureApp == null)
+            {
+                throw new ArgumentNullException(nameof(configureApp));
+            }
+            
+            var startupAssemblyName = configureApp.Target.GetType().GetTypeInfo().Assembly.GetName().Name;
+            
+            return hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName)
+                              .ConfigureServices(services =>
+                              {
+                                  services.AddSingleton<IStartup>(new DelegateStartup(configureApp));
+                              });
+        }
+        
+        
+        /// <summary>
+        /// Specify the startup type to be used by the web host. 
+        /// </summary>
+        /// <param name="hostBuilder">The <see cref="IWebHostBuilder"/> to configure.</param>
+        /// <param name="startupType">The <see cref="Type"/> to be used.</param>
+        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
+        public static IWebHostBuilder UseStartup(this IWebHostBuilder hostBuilder, Type startupType)
+        {
+            var startupAssemblyName = startupType.GetTypeInfo().Assembly.GetName().Name;
+            
+            return hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName)
+                              .ConfigureServices(services =>
+                              {
+                                  if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo()))
+                                  {
+                                      services.AddSingleton(typeof(IStartup), startupType);
+                                  }
+                                  else
+                                  {
+                                      services.AddSingleton(typeof(IStartup), sp => 
+                                      {
+                                          var hostingEnvironment = sp.GetRequiredService<IHostingEnvironment>();
+                                          return new ConventionBasedStartup(StartupLoader.LoadMethods(sp, startupType, hostingEnvironment.EnvironmentName));
+                                      });
+                                  }
+                              });
+        }
+        
         /// <summary>
         /// Specify the startup type to be used by the web host.
         /// </summary>
@@ -88,7 +141,10 @@ namespace Microsoft.AspNetCore.Hosting
                 throw new ArgumentNullException(nameof(startupAssemblyName));
             }
 
-            return hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName);
+                
+            return hostBuilder
+                    .UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName)
+                    .UseSetting(WebHostDefaults.StartupAssemblyKey, startupAssemblyName);
         }
 
         /// <summary>

test/Microsoft.AspNetCore.Hosting.Tests/ConfigureBuilderTests.cs

@@ -5,10 +5,11 @@ using System;
 using System.Reflection;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Builder.Internal;
+using Microsoft.AspNetCore.Hosting.Internal;
 using Microsoft.Extensions.DependencyInjection;
 using Xunit;
 
-namespace Microsoft.AspNetCore.Hosting.Startup.Tests 
+namespace Microsoft.AspNetCore.Hosting.Tests 
 {
     public class ConfigureBuilderTests
     {
@@ -51,4 +52,4 @@ namespace Microsoft.AspNetCore.Hosting.Startup.Tests
             }
         }
     }
-}
+}

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

@@ -5,13 +5,13 @@ using Microsoft.AspNetCore.Builder;
 
 namespace Microsoft.AspNetCore.Hosting.Fakes
 {
-    public class StartupNoServices
+    public class StartupNoServices : Hosting.StartupBase
     {
         public StartupNoServices()
         {
         }
 
-        public void Configure(IApplicationBuilder builder)
+        public override void Configure(IApplicationBuilder builder)
         {
         }
     }

test/Microsoft.AspNetCore.Hosting.Tests/StartupManagerTests.cs

@@ -8,7 +8,6 @@ using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Builder.Internal;
 using Microsoft.AspNetCore.Hosting.Fakes;
 using Microsoft.AspNetCore.Hosting.Internal;
-using Microsoft.AspNetCore.Hosting.Startup;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Options;
 using Xunit;
@@ -26,11 +25,8 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             serviceCollection.AddSingleton<IFakeStartupCallback>(this);
             var services = serviceCollection.BuildServiceProvider();
 
-            var diagnosticMessages = new List<string>();
+            var type = StartupLoader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", "WithServices");
-            var hostingEnv = new HostingEnvironment { EnvironmentName = "WithServices" };
+            var startup = StartupLoader.LoadMethods(services, type, "WithServices");
-            var loader = new StartupLoader(services, hostingEnv);
-            var type = loader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", diagnosticMessages);
-            var startup = loader.LoadMethods(type, diagnosticMessages);
 
             var app = new ApplicationBuilder(services);
             app.ApplicationServices = startup.ConfigureServicesDelegate(serviceCollection);
@@ -52,11 +48,8 @@ namespace Microsoft.AspNetCore.Hosting.Tests
         {
             var services = new ServiceCollection().BuildServiceProvider();
 
-            var diagnosticMessages = new List<string>();
+            var type = StartupLoader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", environment);
-            var hostingEnv = new HostingEnvironment { EnvironmentName = environment };
+            var startup = StartupLoader.LoadMethods(services, type, environment);
-            var loader = new StartupLoader(services, hostingEnv);
-            var type = loader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", diagnosticMessages);
-            var startup = loader.LoadMethods(type, diagnosticMessages);
 
             var app = new ApplicationBuilder(services);
             app.ApplicationServices = startup.ConfigureServicesDelegate(new ServiceCollection());
@@ -74,13 +67,8 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             var serviceCollection = new ServiceCollection();
             serviceCollection.AddSingleton<IFakeStartupCallback>(this);
             var services = serviceCollection.BuildServiceProvider();
-
+            var type = StartupLoader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", "Boom");
-            var diagnosticMessages = new List<string>();
+            var ex = Assert.Throws<InvalidOperationException>(() => StartupLoader.LoadMethods(services, type, "Boom"));
-            var hostingEnv = new HostingEnvironment { EnvironmentName = "Boom" };
-            var loader = new StartupLoader(services, hostingEnv);
-            var type = loader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", diagnosticMessages);
-
-            var ex = Assert.Throws<InvalidOperationException>(() => loader.LoadMethods(type, diagnosticMessages));
             Assert.Equal("A public method named 'ConfigureBoom' or 'Configure' could not be found in the 'Microsoft.AspNetCore.Hosting.Fakes.StartupBoom' type.", ex.Message);
         }
 
@@ -91,12 +79,9 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             serviceCollection.AddSingleton<IFakeStartupCallback>(this);
             var services = serviceCollection.BuildServiceProvider();
 
-            var diagnosticMessages = new List<string>();
+            var type = StartupLoader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", "TwoConfigures");
-            var hostingEnv = new HostingEnvironment { EnvironmentName = "TwoConfigures" };
-            var loader = new StartupLoader(services, hostingEnv);
-            var type = loader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", diagnosticMessages);
 
-            var ex = Assert.Throws<InvalidOperationException>(() => loader.LoadMethods(type, diagnosticMessages));
+            var ex = Assert.Throws<InvalidOperationException>(() => StartupLoader.LoadMethods(services, type, "TwoConfigures"));
             Assert.Equal("Having multiple overloads of method 'Configure' is not supported.", ex.Message);
         }
         
@@ -108,11 +93,9 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             var services = serviceCollection.BuildServiceProvider();
 
             var diagnosticMessages = new List<string>();
-            var hostingEnv = new HostingEnvironment { EnvironmentName = "PrivateConfigure" };
+            var type = StartupLoader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", "PrivateConfigure");
-            var loader = new StartupLoader(services, hostingEnv);
-            var type = loader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", diagnosticMessages);
 
-            var ex = Assert.Throws<InvalidOperationException>(() => loader.LoadMethods(type, diagnosticMessages));
+            var ex = Assert.Throws<InvalidOperationException>(() => StartupLoader.LoadMethods(services, type, "PrivateConfigure"));
             Assert.Equal("A public method named 'ConfigurePrivateConfigure' or 'Configure' could not be found in the 'Microsoft.AspNetCore.Hosting.Fakes.StartupPrivateConfigure' type.", ex.Message);
         }
 
@@ -123,12 +106,9 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             serviceCollection.AddSingleton<IFakeStartupCallback>(this);
             var services = serviceCollection.BuildServiceProvider();
 
-            var diagnosticMessages = new List<string>();
+            var type = StartupLoader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", "TwoConfigureServices");
-            var hostingEnv = new HostingEnvironment { EnvironmentName = "TwoConfigureServices" };
-            var loader = new StartupLoader(services, hostingEnv);
-            var type = loader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", diagnosticMessages);
 
-            var ex = Assert.Throws<InvalidOperationException>(() => loader.LoadMethods(type, diagnosticMessages));
+            var ex = Assert.Throws<InvalidOperationException>(() => StartupLoader.LoadMethods(services, type, "TwoConfigureServices"));
             Assert.Equal("Having multiple overloads of method 'ConfigureServices' is not supported.", ex.Message);
         }
 
@@ -138,11 +118,8 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             var serviceCollection = new ServiceCollection();
             var services = serviceCollection.BuildServiceProvider();
 
-            var diagnosticMessages = new List<string>();
+            var type = StartupLoader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", "WithNullConfigureServices");
-            var hostingEnv = new HostingEnvironment { EnvironmentName = "WithNullConfigureServices" };
+            var startup = StartupLoader.LoadMethods(services, type, "WithNullConfigureServices");
-            var loader = new StartupLoader(services, hostingEnv);
-            var type = loader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", diagnosticMessages);
-            var startup = loader.LoadMethods(type, diagnosticMessages);
 
             var app = new ApplicationBuilder(services);
             app.ApplicationServices = startup.ConfigureServicesDelegate(new ServiceCollection());
@@ -157,11 +134,8 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             var serviceCollection = new ServiceCollection();
             var services = serviceCollection.BuildServiceProvider();
 
-            var diagnosticMessages = new List<string>();
+            var type = StartupLoader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", "WithConfigureServices");
-            var hostingEnv = new HostingEnvironment { EnvironmentName = "WithConfigureServices" };
+            var startup = StartupLoader.LoadMethods(services, type, "WithConfigureServices");
-            var loader = new StartupLoader(services, hostingEnv);
-            var type = loader.FindStartupType("Microsoft.AspNetCore.Hosting.Tests", diagnosticMessages);
-            var startup = loader.LoadMethods(type, diagnosticMessages);
 
             var app = new ApplicationBuilder(services);
             app.ApplicationServices = startup.ConfigureServicesDelegate(serviceCollection);
@@ -177,10 +151,8 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             var serviceCollection = new ServiceCollection();
             var services = serviceCollection.BuildServiceProvider();
 
-            var diagnosticMessages = new List<string>();
             var hostingEnv = new HostingEnvironment();
-            var loader = new StartupLoader(services, hostingEnv);
+            var startup = StartupLoader.LoadMethods(services, typeof(TestStartup), hostingEnv.EnvironmentName);
-            var startup = loader.LoadMethods(typeof(TestStartup), diagnosticMessages);
 
             var app = new ApplicationBuilder(services);
             app.ApplicationServices = startup.ConfigureServicesDelegate(serviceCollection);
@@ -196,10 +168,7 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             var serviceCollection = new ServiceCollection();
             var services = serviceCollection.BuildServiceProvider();
 
-            var diagnosticMessages = new List<string>();
+            var startup = StartupLoader.LoadMethods(services, typeof(TestStartup), "No");
-            var hostingEnv = new HostingEnvironment { EnvironmentName = "No" };
-            var loader = new StartupLoader(services, hostingEnv);
-            var startup = loader.LoadMethods(typeof(TestStartup), diagnosticMessages);
 
             var app = new ApplicationBuilder(services);
             app.ApplicationServices = startup.ConfigureServicesDelegate(serviceCollection);
@@ -245,4 +214,4 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             _configurationMethodCalledList.Add(instance);
         }
     }
-}
+}

test/Microsoft.AspNetCore.Hosting.Tests/WebHostBuilderTests.cs

@@ -28,7 +28,8 @@ namespace Microsoft.AspNetCore.Hosting
 
             var host = (WebHost)builder.UseStartup("MyStartupAssembly").Build();
 
-            Assert.Equal("MyStartupAssembly", host.StartupAssemblyName);
+            Assert.Equal("MyStartupAssembly", host.Options.ApplicationName);
+            Assert.Equal("MyStartupAssembly", host.Options.StartupAssembly);
         }
 
         [Fact]
@@ -454,7 +455,7 @@ namespace Microsoft.AspNetCore.Hosting
                 .Build();
 
             var hostingEnv = host.Services.GetService<IHostingEnvironment>();
-            Assert.Equal("Microsoft.AspNetCore.Hosting.Tests", hostingEnv.ApplicationName);
+            Assert.Equal("Microsoft.AspNetCore.Hosting.Tests.NonExistent", hostingEnv.ApplicationName);
             var appEnv = host.Services.GetService<IApplicationEnvironment>();
             Assert.Equal(PlatformServices.Default.Application.ApplicationName, appEnv.ApplicationName);
             Assert.Equal(PlatformServices.Default.Application.ApplicationBasePath, appEnv.ApplicationBasePath);
@@ -471,7 +472,7 @@ namespace Microsoft.AspNetCore.Hosting
                 .Build();
 
             var hostingEnv = host.Services.GetService<IHostingEnvironment>();
-            Assert.Equal("Microsoft.AspNetCore.Hosting.Tests", hostingEnv.ApplicationName);
+            Assert.Equal("Microsoft.AspNetCore.Hosting.Tests.NonExistent", hostingEnv.ApplicationName);
             var appEnv = host.Services.GetService<IApplicationEnvironment>();
             Assert.Equal(PlatformServices.Default.Application.ApplicationName, appEnv.ApplicationName);
             Assert.Equal(PlatformServices.Default.Application.ApplicationBasePath, appEnv.ApplicationBasePath);

test/Microsoft.AspNetCore.Hosting.Tests/WebHostConfigurationsTests.cs

@@ -25,7 +25,8 @@ namespace Microsoft.AspNetCore.Hosting.Tests
             {
                 { "webroot", "wwwroot"},
                 { "server", "Microsoft.AspNetCore.Server.Kestrel"},
-                { "application", "MyProjectReference"},
+                { "applicationName", "MyProjectReference"},
+                { "startupAssembly", "MyProjectReference" },
                 { "environment", "Development"},
                 { "detailederrors", "true"},
                 { "captureStartupErrors", "true" }
@@ -35,7 +36,8 @@ namespace Microsoft.AspNetCore.Hosting.Tests
 
             Assert.Equal("wwwroot", config.WebRoot);
             Assert.Equal("Microsoft.AspNetCore.Server.Kestrel", config.ServerAssembly);
-            Assert.Equal("MyProjectReference", config.Application);
+            Assert.Equal("MyProjectReference", config.ApplicationName);
+            Assert.Equal("MyProjectReference", config.StartupAssembly);
             Assert.Equal("Development", config.Environment);
             Assert.True(config.CaptureStartupErrors);
             Assert.True(config.DetailedErrors);

test/Microsoft.AspNetCore.Hosting.Tests/WebHostTests.cs

@@ -10,10 +10,8 @@ using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting.Fakes;
-using Microsoft.AspNetCore.Hosting.Internal;
 using Microsoft.AspNetCore.Hosting.Server;
 using Microsoft.AspNetCore.Hosting.Server.Features;
-using Microsoft.AspNetCore.Hosting.Startup;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.Extensions.Configuration;
@@ -228,7 +226,7 @@ namespace Microsoft.AspNetCore.Hosting
             var builder = CreateBuilder()
                 .ConfigureServices(services =>
                 {
-                    services.AddTransient<IStartupLoader, TestLoader>();
+                    services.AddTransient<IStartup, TestStartup>();
                 })
                 .UseServer(this)
                 .UseStartup("Microsoft.AspNetCore.Hosting.Tests");
@@ -496,14 +494,14 @@ namespace Microsoft.AspNetCore.Hosting
             }
         }
 
-        private class TestLoader : IStartupLoader
+        private class TestStartup : IStartup
         {
-            public Type FindStartupType(string startupAssemblyName, IList<string> diagnosticMessages)
+            public void Configure(IApplicationBuilder app)
             {
                 throw new NotImplementedException();
             }
 
-            public StartupMethods LoadMethods(Type startupType, IList<string> diagnosticMessages)
+            public IServiceProvider ConfigureServices(IServiceCollection services)
             {
                 throw new NotImplementedException();
             }
@@ -653,4 +651,4 @@ namespace Microsoft.AspNetCore.Hosting
             public string TraceIdentifier { get; set; }
         }
     }
-}
+}

test/Microsoft.AspNetCore.TestHost.Tests/TestServerTests.cs

@@ -628,4 +628,4 @@ namespace Microsoft.AspNetCore.TestHost
             }
         }
     }
-}
+}