Add Host.CreateEmptyApplicationBuilder

Introduce a way to create an "empty" HostApplicationBuilder that doesn't bring any optional/unnecessary dependencies into the app's closure. This has the same functional behavior as DisableDefaults=true while allowing for the unused code to be trimmed.

Also add CreateApplicationBuilder(HostApplicationBuilderSettings settings) overload to make the Host factory methods complete.

Fix dotnet#81280

src/libraries/Microsoft.Extensions.Hosting/ref/Microsoft.Extensions.Hosting.cs

@@ -26,9 +26,11 @@ public ConsoleLifetimeOptions() { }
     public static partial class Host
     {
         public static Microsoft.Extensions.Hosting.HostApplicationBuilder CreateApplicationBuilder() { throw null; }
+        public static Microsoft.Extensions.Hosting.HostApplicationBuilder CreateApplicationBuilder(Microsoft.Extensions.Hosting.HostApplicationBuilderSettings? settings) { throw null; }
         public static Microsoft.Extensions.Hosting.HostApplicationBuilder CreateApplicationBuilder(string[]? args) { throw null; }
         public static Microsoft.Extensions.Hosting.IHostBuilder CreateDefaultBuilder() { throw null; }
         public static Microsoft.Extensions.Hosting.IHostBuilder CreateDefaultBuilder(string[]? args) { throw null; }
+        public static Microsoft.Extensions.Hosting.HostApplicationBuilder CreateEmptyApplicationBuilder(Microsoft.Extensions.Hosting.HostApplicationBuilderSettings? settings) { throw null; }
     }
     public sealed partial class HostApplicationBuilder
     {

src/libraries/Microsoft.Extensions.Hosting/src/Host.cs

@@ -64,15 +64,14 @@ public static IHostBuilder CreateDefaultBuilder(string[]? args)
         ///   <list type="bullet">
         ///     <item><description>set the <see cref="IHostEnvironment.ContentRootPath"/> to the result of <see cref="Directory.GetCurrentDirectory()"/></description></item>
         ///     <item><description>load host <see cref="IConfiguration"/> from "DOTNET_" prefixed environment variables</description></item>
-        ///     <item><description>load host <see cref="IConfiguration"/> from supplied command line args</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from 'appsettings.json' and 'appsettings.[<see cref="IHostEnvironment.EnvironmentName"/>].json'</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from User Secrets when <see cref="IHostEnvironment.EnvironmentName"/> is 'Development' using the entry assembly</description></item>
         ///     <item><description>load app <see cref="IConfiguration"/> from environment variables</description></item>
-        ///     <item><description>load app <see cref="IConfiguration"/> from supplied command line args</description></item>
         ///     <item><description>configure the <see cref="ILoggerFactory"/> to log to the console, debug, and event source output</description></item>
         ///     <item><description>enables scope validation on the dependency injection container when <see cref="IHostEnvironment.EnvironmentName"/> is 'Development'</description></item>
         ///   </list>
         /// </remarks>
+        /// <returns>The initialized <see cref="HostApplicationBuilder"/>.</returns>
         public static HostApplicationBuilder CreateApplicationBuilder() => new HostApplicationBuilder();
 
         /// <summary>
@@ -93,6 +92,20 @@ public static IHostBuilder CreateDefaultBuilder(string[]? args)
         ///   </list>
         /// </remarks>
         /// <param name="args">The command line args.</param>
+        /// <returns>The initialized <see cref="HostApplicationBuilder"/>.</returns>
         public static HostApplicationBuilder CreateApplicationBuilder(string[]? args) => new HostApplicationBuilder(args);
+
+        /// <inheritdoc cref="CreateApplicationBuilder()" />
+        /// <param name="settings">Controls the initial configuration and other settings for constructing the <see cref="HostApplicationBuilder"/>.</param>
+        public static HostApplicationBuilder CreateApplicationBuilder(HostApplicationBuilderSettings? settings)
+            => new HostApplicationBuilder(settings);
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="HostApplicationBuilder"/> class with no pre-configured defaults.
+        /// </summary>
+        /// <param name="settings">Controls the initial configuration and other settings for constructing the <see cref="HostApplicationBuilder"/>.</param>
+        /// <returns>The initialized <see cref="HostApplicationBuilder"/>.</returns>
+        public static HostApplicationBuilder CreateEmptyApplicationBuilder(HostApplicationBuilderSettings? settings)
+            => new HostApplicationBuilder(settings, empty: true);
     }
 }

src/libraries/Microsoft.Extensions.Hosting/src/HostApplicationBuilder.cs

@@ -20,6 +20,8 @@ public sealed class HostApplicationBuilder
     {
         private readonly HostBuilderContext _hostBuilderContext;
         private readonly ServiceCollection _serviceCollection = new();
+        private readonly IHostEnvironment _environment;
+        private readonly LoggingBuilder _logging;
 
         private Func<IServiceProvider> _createServiceProvider;
         private Action<object> _configureContainer = _ => { };
@@ -98,6 +100,47 @@ public HostApplicationBuilder(HostApplicationBuilderSettings? settings)
                 HostingHostBuilderExtensions.AddCommandLineConfig(Configuration, settings.Args);
             }
 
+            Initialize(settings, out _hostBuilderContext, out _environment, out _logging);
+
+            ServiceProviderOptions? serviceProviderOptions = null;
+            if (!settings.DisableDefaults)
+            {
+                HostingHostBuilderExtensions.ApplyDefaultAppConfiguration(_hostBuilderContext, Configuration, settings.Args);
+                HostingHostBuilderExtensions.AddDefaultServices(_hostBuilderContext, Services);
+                serviceProviderOptions = HostingHostBuilderExtensions.CreateDefaultServiceProviderOptions(_hostBuilderContext);
+            }
+
+            _createServiceProvider = () =>
+            {
+                // Call _configureContainer in case anyone adds callbacks via HostBuilderAdapter.ConfigureContainer<IServiceCollection>() during build.
+                // Otherwise, this no-ops.
+                _configureContainer(Services);
+                return serviceProviderOptions is null ? Services.BuildServiceProvider() : Services.BuildServiceProvider(serviceProviderOptions);
+            };
+        }
+
+        internal HostApplicationBuilder(HostApplicationBuilderSettings? settings, bool empty)
+        {
+            Debug.Assert(empty, "should only be called with empty: true");
+
+            settings ??= new HostApplicationBuilderSettings();
+            Configuration = settings.Configuration ?? new ConfigurationManager();
+
+            HostingHostBuilderExtensions.AddCommandLineConfig(Configuration, settings.Args);
+
+            Initialize(settings, out _hostBuilderContext, out _environment, out _logging);
+
+            _createServiceProvider = () =>
+            {
+                // Call _configureContainer in case anyone adds callbacks via HostBuilderAdapter.ConfigureContainer<IServiceCollection>() during build.
+                // Otherwise, this no-ops.
+                _configureContainer(Services);
+                return Services.BuildServiceProvider();
+            };
+        }
+
+        private void Initialize(HostApplicationBuilderSettings settings, out HostBuilderContext hostBuilderContext, out IHostEnvironment environment, out LoggingBuilder logging)
+        {
             // HostApplicationBuilderSettings override all other config sources.
             List<KeyValuePair<string, string?>>? optionList = null;
             if (settings.ApplicationName is not null)
@@ -124,46 +167,29 @@ public HostApplicationBuilder(HostApplicationBuilderSettings? settings)
 
             Configuration.SetFileProvider(physicalFileProvider);
 
-            _hostBuilderContext = new HostBuilderContext(new Dictionary<object, object>())
+            hostBuilderContext = new HostBuilderContext(new Dictionary<object, object>())
             {
                 HostingEnvironment = hostingEnvironment,
                 Configuration = Configuration,
             };
 
-            Environment = hostingEnvironment;
+            environment = hostingEnvironment;
 
             HostBuilder.PopulateServiceCollection(
                 Services,
-                _hostBuilderContext,
+                hostBuilderContext,
                 hostingEnvironment,
                 physicalFileProvider,
                 Configuration,
                 () => _appServices!);
 
-            Logging = new LoggingBuilder(Services);
-
-            ServiceProviderOptions? serviceProviderOptions = null;
-
-            if (!settings.DisableDefaults)
-            {
-                HostingHostBuilderExtensions.ApplyDefaultAppConfiguration(_hostBuilderContext, Configuration, settings.Args);
-                HostingHostBuilderExtensions.AddDefaultServices(_hostBuilderContext, Services);
-                serviceProviderOptions = HostingHostBuilderExtensions.CreateDefaultServiceProviderOptions(_hostBuilderContext);
-            }
-
-            _createServiceProvider = () =>
-            {
-                // Call _configureContainer in case anyone adds callbacks via HostBuilderAdapter.ConfigureContainer<IServiceCollection>() during build.
-                // Otherwise, this no-ops.
-                _configureContainer(Services);
-                return serviceProviderOptions is null ? Services.BuildServiceProvider() : Services.BuildServiceProvider(serviceProviderOptions);
-            };
+            logging = new LoggingBuilder(Services);
         }
 
         /// <summary>
         /// Provides information about the hosting environment an application is running in.
         /// </summary>
-        public IHostEnvironment Environment { get; }
+        public IHostEnvironment Environment => _environment;
 
         /// <summary>
         /// A collection of services for the application to compose. This is useful for adding user provided or framework provided services.
@@ -178,7 +204,7 @@ public HostApplicationBuilder(HostApplicationBuilderSettings? settings)
         /// <summary>
         /// A collection of logging providers for the application to compose. This is useful for adding new logging providers.
         /// </summary>
-        public ILoggingBuilder Logging { get; }
+        public ILoggingBuilder Logging => _logging;
 
         /// <summary>
         /// Registers a <see cref="IServiceProviderFactory{TContainerBuilder}" /> instance to be used to create the <see cref="IServiceProvider" />.