diff --git a/src/DefaultBuilder/src/GenericHostBuilderExtensions.cs b/src/DefaultBuilder/src/GenericHostBuilderExtensions.cs
index 37ca82e8e5..201fb26fe0 100644
--- a/src/DefaultBuilder/src/GenericHostBuilderExtensions.cs
+++ b/src/DefaultBuilder/src/GenericHostBuilderExtensions.cs
@@ -25,6 +25,11 @@ namespace Microsoft.Extensions.Hosting
         /// <returns>The <see cref="IHostBuilder"/> for chaining.</returns>
         public static IHostBuilder ConfigureWebHostDefaults(this IHostBuilder builder, Action<IWebHostBuilder> configure)
         {
+            if (configure is null)
+            {
+                throw new ArgumentNullException(nameof(configure));
+            }
+
             return builder.ConfigureWebHost(webHostBuilder =>
             {
                 WebHost.ConfigureWebDefaults(webHostBuilder);
diff --git a/src/DefaultBuilder/src/Microsoft.AspNetCore.csproj b/src/DefaultBuilder/src/Microsoft.AspNetCore.csproj
index 58513c6253..2c66a8407d 100644
--- a/src/DefaultBuilder/src/Microsoft.AspNetCore.csproj
+++ b/src/DefaultBuilder/src/Microsoft.AspNetCore.csproj
@@ -7,6 +7,7 @@
     <Description>Microsoft.AspNetCore</Description>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/DefaultBuilder/src/WebHost.cs b/src/DefaultBuilder/src/WebHost.cs
index 2d02a8a437..ff79107d32 100644
--- a/src/DefaultBuilder/src/WebHost.cs
+++ b/src/DefaultBuilder/src/WebHost.cs
@@ -40,9 +40,9 @@ namespace Microsoft.AspNetCore
         /// <param name="url">The URL the hosted application will listen on.</param>
         /// <param name="app">A delegate that handles requests to the application.</param>
         /// <returns>A started <see cref="IWebHost"/> that hosts the application.</returns>
-        public static IWebHost Start(string url, RequestDelegate app)
+        public static IWebHost Start(string? url, RequestDelegate app)
         {
-            var startupAssemblyName = app.GetMethodInfo().DeclaringType.GetTypeInfo().Assembly.GetName().Name;
+            var startupAssemblyName = app.GetMethodInfo().DeclaringType!.GetTypeInfo().Assembly.GetName().Name;
             return StartWith(url: url, configureServices: null, app: appBuilder => appBuilder.Run(app), applicationName: startupAssemblyName);
         }
 
@@ -62,9 +62,9 @@ namespace Microsoft.AspNetCore
         /// <param name="url">The URL the hosted application will listen on.</param>
         /// <param name="routeBuilder">A delegate that configures the router for handling requests to the application.</param>
         /// <returns>A started <see cref="IWebHost"/> that hosts the application.</returns>
-        public static IWebHost Start(string url, Action<IRouteBuilder> routeBuilder)
+        public static IWebHost Start(string? url, Action<IRouteBuilder> routeBuilder)
         {
-            var startupAssemblyName = routeBuilder.GetMethodInfo().DeclaringType.GetTypeInfo().Assembly.GetName().Name;
+            var startupAssemblyName = routeBuilder.GetMethodInfo().DeclaringType!.GetTypeInfo().Assembly.GetName().Name;
             return StartWith(url, services => services.AddRouting(), appBuilder => appBuilder.UseRouter(routeBuilder), applicationName: startupAssemblyName);
         }
 
@@ -84,10 +84,10 @@ namespace Microsoft.AspNetCore
         /// <param name="url">The URL the hosted application will listen on.</param>
         /// <param name="app">The delegate that configures the <see cref="IApplicationBuilder"/>.</param>
         /// <returns>A started <see cref="IWebHost"/> that hosts the application.</returns>
-        public static IWebHost StartWith(string url, Action<IApplicationBuilder> app) =>
+        public static IWebHost StartWith(string? url, Action<IApplicationBuilder> app) =>
             StartWith(url: url, configureServices: null, app: app, applicationName: null);
 
-        private static IWebHost StartWith(string url, Action<IServiceCollection> configureServices, Action<IApplicationBuilder> app, string applicationName)
+        private static IWebHost StartWith(string? url, Action<IServiceCollection>? configureServices, Action<IApplicationBuilder> app, string? applicationName)
         {
             var builder = CreateDefaultBuilder();
 
@@ -153,7 +153,7 @@ namespace Microsoft.AspNetCore
         /// </remarks>
         /// <param name="args">The command line args.</param>
         /// <returns>The initialized <see cref="IWebHostBuilder"/>.</returns>
-        public static IWebHostBuilder CreateDefaultBuilder(string[] args)
+        public static IWebHostBuilder CreateDefaultBuilder(string[]? args)
         {
             var builder = new WebHostBuilder();
 
diff --git a/src/Hosting/Hosting/src/Builder/ApplicationBuilderFactory.cs b/src/Hosting/Hosting/src/Builder/ApplicationBuilderFactory.cs
index e9c410370c..01482dc91b 100644
--- a/src/Hosting/Hosting/src/Builder/ApplicationBuilderFactory.cs
+++ b/src/Hosting/Hosting/src/Builder/ApplicationBuilderFactory.cs
@@ -1,6 +1,8 @@
 // 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.
 
+#nullable enable
+
 using System;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http.Features;
diff --git a/src/Hosting/Hosting/src/Http/DefaultHttpContextFactory.cs b/src/Hosting/Hosting/src/Http/DefaultHttpContextFactory.cs
index 2637a8b3a6..9686aacb45 100644
--- a/src/Hosting/Hosting/src/Http/DefaultHttpContextFactory.cs
+++ b/src/Hosting/Hosting/src/Http/DefaultHttpContextFactory.cs
@@ -1,6 +1,8 @@
 // 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.
 
+#nullable enable
+
 using System;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
@@ -12,7 +14,7 @@ namespace Microsoft.AspNetCore.Http
 {
     public class DefaultHttpContextFactory : IHttpContextFactory
     {
-        private readonly IHttpContextAccessor _httpContextAccessor;
+        private readonly IHttpContextAccessor? _httpContextAccessor;
         private readonly FormOptions _formOptions;
         private readonly IServiceScopeFactory _serviceScopeFactory;
 
@@ -26,7 +28,7 @@ namespace Microsoft.AspNetCore.Http
             _serviceScopeFactory = serviceProvider.GetRequiredService<IServiceScopeFactory>();
         }
 
-        internal IHttpContextAccessor HttpContextAccessor => _httpContextAccessor;
+        internal IHttpContextAccessor? HttpContextAccessor => _httpContextAccessor;
 
         public HttpContext Create(IFeatureCollection featureCollection)
         {
diff --git a/src/Hosting/Hosting/src/Microsoft.AspNetCore.Hosting.csproj b/src/Hosting/Hosting/src/Microsoft.AspNetCore.Hosting.csproj
index c0e3e77284..de44d964bb 100644
--- a/src/Hosting/Hosting/src/Microsoft.AspNetCore.Hosting.csproj
+++ b/src/Hosting/Hosting/src/Microsoft.AspNetCore.Hosting.csproj
@@ -8,6 +8,7 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore;hosting</PackageTags>
     <IsPackable>false</IsPackable>
+    <LangVersion>9.0</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Hosting/Hosting/src/Server/Features/ServerAddressesFeature.cs b/src/Hosting/Hosting/src/Server/Features/ServerAddressesFeature.cs
index 098ec8cdb0..3b4c161f60 100644
--- a/src/Hosting/Hosting/src/Server/Features/ServerAddressesFeature.cs
+++ b/src/Hosting/Hosting/src/Server/Features/ServerAddressesFeature.cs
@@ -1,6 +1,8 @@
 // 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.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace Microsoft.AspNetCore.Hosting.Server.Features
diff --git a/src/Hosting/Hosting/src/Startup/DelegateStartup.cs b/src/Hosting/Hosting/src/Startup/DelegateStartup.cs
index d354ad946e..0c1bcb5c04 100644
--- a/src/Hosting/Hosting/src/Startup/DelegateStartup.cs
+++ b/src/Hosting/Hosting/src/Startup/DelegateStartup.cs
@@ -1,6 +1,8 @@
 // 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.
 
+#nullable enable
+
 using System;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
@@ -18,4 +20,4 @@ namespace Microsoft.AspNetCore.Hosting
 
         public override void Configure(IApplicationBuilder app) => _configureApp(app);
     }
-}
\ No newline at end of file
+}
diff --git a/src/Hosting/Hosting/src/StaticWebAssets/StaticWebAssetsLoader.cs b/src/Hosting/Hosting/src/StaticWebAssets/StaticWebAssetsLoader.cs
index 60d0f8181d..6b9ee360d4 100644
--- a/src/Hosting/Hosting/src/StaticWebAssets/StaticWebAssetsLoader.cs
+++ b/src/Hosting/Hosting/src/StaticWebAssets/StaticWebAssetsLoader.cs
@@ -2,7 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 #nullable enable
-using System.Collections.Generic;
+
 using System.IO;
 using System.Linq;
 using System.Reflection;
diff --git a/src/Hosting/Hosting/src/WebHostBuilder.cs b/src/Hosting/Hosting/src/WebHostBuilder.cs
index 68568ffb27..a4b9373234 100644
--- a/src/Hosting/Hosting/src/WebHostBuilder.cs
+++ b/src/Hosting/Hosting/src/WebHostBuilder.cs
@@ -1,9 +1,12 @@
 // 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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Linq;
 using System.Reflection;
@@ -24,13 +27,13 @@ namespace Microsoft.AspNetCore.Hosting
     public class WebHostBuilder : IWebHostBuilder
     {
         private readonly HostingEnvironment _hostingEnvironment;
-        private Action<WebHostBuilderContext, IServiceCollection> _configureServices;
+        private readonly IConfiguration _config;
+        private readonly WebHostBuilderContext _context;
 
-        private IConfiguration _config;
-        private WebHostOptions _options;
-        private WebHostBuilderContext _context;
+        private WebHostOptions? _options;
         private bool _webHostBuilt;
-        private Action<WebHostBuilderContext, IConfigurationBuilder> _configureAppConfigurationBuilder;
+        private Action<WebHostBuilderContext, IServiceCollection>? _configureServices;
+        private Action<WebHostBuilderContext, IConfigurationBuilder>? _configureAppConfigurationBuilder;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="WebHostBuilder"/> class.
@@ -78,7 +81,7 @@ namespace Microsoft.AspNetCore.Hosting
         /// <param name="key">The key of the setting to add or replace.</param>
         /// <param name="value">The value of the setting to add or replace.</param>
         /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
-        public IWebHostBuilder UseSetting(string key, string value)
+        public IWebHostBuilder UseSetting(string key, string? value)
         {
             _config[key] = value;
             return this;
@@ -212,7 +215,8 @@ namespace Microsoft.AspNetCore.Hosting
             }
         }
 
-        private IServiceCollection BuildCommonServices(out AggregateException hostingStartupErrors)
+        [MemberNotNull(nameof(_options))]
+        private IServiceCollection BuildCommonServices(out AggregateException? hostingStartupErrors)
         {
             hostingStartupErrors = null;
 
@@ -231,7 +235,7 @@ namespace Microsoft.AspNetCore.Hosting
 
                         foreach (var attribute in assembly.GetCustomAttributes<HostingStartupAttribute>())
                         {
-                            var hostingStartup = (IHostingStartup)Activator.CreateInstance(attribute.HostingStartupType);
+                            var hostingStartup = (IHostingStartup)Activator.CreateInstance(attribute.HostingStartupType)!;
                             hostingStartup.Configure(this);
                         }
                     }
@@ -330,8 +334,8 @@ namespace Microsoft.AspNetCore.Hosting
             // NOTE: This code overrides original services lifetime. Instances would always be singleton in
             // application container.
             var listener = hostingServiceProvider.GetService<DiagnosticListener>();
-            services.Replace(ServiceDescriptor.Singleton(typeof(DiagnosticListener), listener));
-            services.Replace(ServiceDescriptor.Singleton(typeof(DiagnosticSource), listener));
+            services.Replace(ServiceDescriptor.Singleton(typeof(DiagnosticListener), listener!));
+            services.Replace(ServiceDescriptor.Singleton(typeof(DiagnosticSource), listener!));
         }
 
         private string ResolveContentRootPath(string contentRootPath, string basePath)
diff --git a/src/Hosting/Hosting/src/WebHostBuilderExtensions.cs b/src/Hosting/Hosting/src/WebHostBuilderExtensions.cs
index 20151ebe9e..252d5c5d43 100644
--- a/src/Hosting/Hosting/src/WebHostBuilderExtensions.cs
+++ b/src/Hosting/Hosting/src/WebHostBuilderExtensions.cs
@@ -1,6 +1,8 @@
 // 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.
 
+#nullable enable
+
 using System;
 using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
@@ -25,7 +27,7 @@ namespace Microsoft.AspNetCore.Hosting
         /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
         public static IWebHostBuilder Configure(this IWebHostBuilder hostBuilder, Action<IApplicationBuilder> configureApp)
         {
-            return hostBuilder.Configure((_, app) => configureApp(app), configureApp.GetMethodInfo().DeclaringType.GetTypeInfo().Assembly.GetName().Name);
+            return hostBuilder.Configure((_, app) => configureApp(app), configureApp.GetMethodInfo().DeclaringType!.GetTypeInfo().Assembly.GetName().Name!);
         }
 
         /// <summary>
@@ -36,7 +38,7 @@ namespace Microsoft.AspNetCore.Hosting
         /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
         public static IWebHostBuilder Configure(this IWebHostBuilder hostBuilder, Action<WebHostBuilderContext, IApplicationBuilder> configureApp)
         {
-            return hostBuilder.Configure(configureApp, configureApp.GetMethodInfo().DeclaringType.GetTypeInfo().Assembly.GetName().Name);
+            return hostBuilder.Configure(configureApp, configureApp.GetMethodInfo().DeclaringType!.GetTypeInfo().Assembly.GetName().Name!);
         }
 
         private static IWebHostBuilder Configure(this IWebHostBuilder hostBuilder, Action<WebHostBuilderContext, IApplicationBuilder> configureApp, string startupAssemblyName)
@@ -77,7 +79,7 @@ namespace Microsoft.AspNetCore.Hosting
                 throw new ArgumentNullException(nameof(startupFactory));
             }
 
-            var startupAssemblyName = startupFactory.GetMethodInfo().DeclaringType.GetTypeInfo().Assembly.GetName().Name;
+            var startupAssemblyName = startupFactory.GetMethodInfo().DeclaringType!.GetTypeInfo().Assembly.GetName().Name;
 
             hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName);
 
diff --git a/src/Hosting/Hosting/src/WebHostExtensions.cs b/src/Hosting/Hosting/src/WebHostExtensions.cs
index fe77b13443..f3197e15d3 100644
--- a/src/Hosting/Hosting/src/WebHostExtensions.cs
+++ b/src/Hosting/Hosting/src/WebHostExtensions.cs
@@ -1,6 +1,8 @@
 // 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.
 
+#nullable enable
+
 using System;
 using System.Threading;
 using System.Threading.Tasks;
@@ -103,7 +105,7 @@ namespace Microsoft.AspNetCore.Hosting
             }
         }
 
-        private static async Task RunAsync(this IWebHost host, CancellationToken token, string startupMessage)
+        private static async Task RunAsync(this IWebHost host, CancellationToken token, string? startupMessage)
         {
             try
             {
@@ -114,8 +116,8 @@ namespace Microsoft.AspNetCore.Hosting
 
                 if (!options.SuppressStatusMessages)
                 {
-                    Console.WriteLine($"Hosting environment: {hostingEnvironment.EnvironmentName}");
-                    Console.WriteLine($"Content root path: {hostingEnvironment.ContentRootPath}");
+                    Console.WriteLine($"Hosting environment: {hostingEnvironment?.EnvironmentName}");
+                    Console.WriteLine($"Content root path: {hostingEnvironment?.ContentRootPath}");
 
 
                     var serverAddresses = host.ServerFeatures.Get<IServerAddressesFeature>()?.Addresses;
@@ -150,18 +152,18 @@ namespace Microsoft.AspNetCore.Hosting
 
         private static async Task WaitForTokenShutdownAsync(this IWebHost host, CancellationToken token)
         {
-            var applicationLifetime = host.Services.GetService<IHostApplicationLifetime>();
+            var applicationLifetime = host.Services.GetRequiredService<IHostApplicationLifetime>();
 
             token.Register(state =>
             {
-                ((IHostApplicationLifetime)state).StopApplication();
+                ((IHostApplicationLifetime)state!).StopApplication();
             },
             applicationLifetime);
 
             var waitForStop = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
             applicationLifetime.ApplicationStopping.Register(obj =>
             {
-                var tcs = (TaskCompletionSource)obj;
+                var tcs = (TaskCompletionSource)obj!;
                 tcs.TrySetResult();
             }, waitForStop);
 
diff --git a/src/Hosting/Server.Abstractions/src/IHostContextContainer.cs b/src/Hosting/Server.Abstractions/src/IHostContextContainer.cs
index 603a323912..f413ad451a 100644
--- a/src/Hosting/Server.Abstractions/src/IHostContextContainer.cs
+++ b/src/Hosting/Server.Abstractions/src/IHostContextContainer.cs
@@ -8,7 +8,7 @@ namespace Microsoft.AspNetCore.Hosting.Server.Abstractions
     /// its <typeparamref name="TContext"/> between requests.
     /// </summary>
     /// <typeparam name="TContext">The <see cref="IHttpApplication{TContext}"/> Host context</typeparam>
-    public interface IHostContextContainer<TContext>
+    public interface IHostContextContainer<TContext> where TContext : notnull
     {
         TContext HostContext { get; set; }
     }
diff --git a/src/Hosting/Server.Abstractions/src/IHttpApplication.cs b/src/Hosting/Server.Abstractions/src/IHttpApplication.cs
index cc9d173da6..ea1b660bac 100644
--- a/src/Hosting/Server.Abstractions/src/IHttpApplication.cs
+++ b/src/Hosting/Server.Abstractions/src/IHttpApplication.cs
@@ -11,7 +11,7 @@ namespace Microsoft.AspNetCore.Hosting.Server
     /// Represents an application.
     /// </summary>
     /// <typeparam name="TContext">The context associated with the application.</typeparam>
-    public interface IHttpApplication<TContext>
+    public interface IHttpApplication<TContext> where TContext : notnull
     {
         /// <summary>
         /// Create a TContext given a collection of HTTP features.
diff --git a/src/Hosting/Server.Abstractions/src/IServer.cs b/src/Hosting/Server.Abstractions/src/IServer.cs
index b04e5a0d47..78d0f5a32e 100644
--- a/src/Hosting/Server.Abstractions/src/IServer.cs
+++ b/src/Hosting/Server.Abstractions/src/IServer.cs
@@ -24,7 +24,7 @@ namespace Microsoft.AspNetCore.Hosting.Server
         /// <param name="application">An instance of <see cref="IHttpApplication{TContext}"/>.</param>
         /// <typeparam name="TContext">The context associated with the application.</typeparam>
         /// <param name="cancellationToken">Indicates if the server startup should be aborted.</param>
-        Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken);
+        Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) where TContext : notnull;
 
         /// <summary>
         /// Stop processing requests and shut down the server, gracefully if possible.
diff --git a/src/Hosting/Server.Abstractions/src/IServerIntegratedAuth.cs b/src/Hosting/Server.Abstractions/src/IServerIntegratedAuth.cs
index d099656ac4..218b461b0a 100644
--- a/src/Hosting/Server.Abstractions/src/IServerIntegratedAuth.cs
+++ b/src/Hosting/Server.Abstractions/src/IServerIntegratedAuth.cs
@@ -16,6 +16,6 @@ namespace Microsoft.AspNetCore.Hosting.Server
         /// <summary>
         /// The name of the authentication scheme for the server authentication handler.
         /// </summary>
-        string AuthenticationScheme { get; }
+        string? AuthenticationScheme { get; }
     }
 }
diff --git a/src/Hosting/Server.Abstractions/src/Microsoft.AspNetCore.Hosting.Server.Abstractions.csproj b/src/Hosting/Server.Abstractions/src/Microsoft.AspNetCore.Hosting.Server.Abstractions.csproj
index 4e6c350a51..758bf3bff0 100644
--- a/src/Hosting/Server.Abstractions/src/Microsoft.AspNetCore.Hosting.Server.Abstractions.csproj
+++ b/src/Hosting/Server.Abstractions/src/Microsoft.AspNetCore.Hosting.Server.Abstractions.csproj
@@ -8,6 +8,7 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <PackageTags>aspnetcore;hosting</PackageTags>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Hosting/Server.Abstractions/src/ServerIntegratedAuth.cs b/src/Hosting/Server.Abstractions/src/ServerIntegratedAuth.cs
index 93d9029913..0ab301cfaf 100644
--- a/src/Hosting/Server.Abstractions/src/ServerIntegratedAuth.cs
+++ b/src/Hosting/Server.Abstractions/src/ServerIntegratedAuth.cs
@@ -16,6 +16,6 @@ namespace Microsoft.AspNetCore.Hosting.Server
         /// <summary>
         /// The name of the authentication scheme for the server authentication handler.
         /// </summary>
-        public string AuthenticationScheme { get; set; }
+        public string? AuthenticationScheme { get; set; }
     }
 }
diff --git a/src/Hosting/build.cmd b/src/Hosting/build.cmd
new file mode 100644
index 0000000000..2406296662
--- /dev/null
+++ b/src/Hosting/build.cmd
@@ -0,0 +1,3 @@
+@ECHO OFF
+SET RepoRoot=%~dp0..\..
+%RepoRoot%\build.cmd -projects %~dp0**\*.*proj %*