Pass existing LoggerFactory into WebHostBuilder

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

@@ -5,6 +5,7 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting.Server;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
 
 namespace Microsoft.AspNetCore.Hosting
 {
@@ -18,6 +19,13 @@ public interface IWebHostBuilder
         /// </summary>
         IWebHost Build();
 
+        /// <summary>
+        /// Specify the <see cref="ILoggerFactory"/> to be used by the web host.
+        /// </summary>
+        /// <param name="loggerFactory">The <see cref="ILoggerFactory"/> to be used.</param>
+        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
+        IWebHostBuilder UseLoggerFactory(ILoggerFactory loggerFactory);
+
         /// <summary>
         /// Specify the <see cref="IServerFactory"/> to be used by the web host.
         /// </summary>
@@ -46,6 +54,13 @@ public interface IWebHostBuilder
         /// <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>
+        /// <param name="configureLogging">The delegate that configures the <see cref="ILoggerFactory"/>.</param>
+        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
+        IWebHostBuilder ConfigureLogging(Action<ILoggerFactory> configureLogging);
+
         /// <summary>
         /// Add or replace a setting in the configuration.
         /// </summary>

src/Microsoft.AspNetCore.Hosting.Abstractions/project.json

@@ -18,7 +18,8 @@
     "Microsoft.AspNetCore.Hosting.Server.Abstractions": "1.0.0-*",
     "Microsoft.Extensions.Configuration.Abstractions": "1.0.0-*",
     "Microsoft.Extensions.DependencyInjection.Abstractions": "1.0.0-*",
-    "Microsoft.Extensions.FileProviders.Abstractions": "1.0.0-*"
+    "Microsoft.Extensions.FileProviders.Abstractions": "1.0.0-*",
+    "Microsoft.Extensions.Logging.Abstractions": "1.0.0-*"
   },
   "frameworks": {
     "net451": {},

src/Microsoft.AspNetCore.Hosting/WebHostBuilder.cs

@@ -27,10 +27,11 @@ namespace Microsoft.AspNetCore.Hosting
     public class WebHostBuilder : IWebHostBuilder
     {
         private readonly IHostingEnvironment _hostingEnvironment;
-        private readonly ILoggerFactory _loggerFactory;
         private readonly List<Action<IServiceCollection>> _configureServicesDelegates;
+        private readonly List<Action<ILoggerFactory>> _configureLoggingDelegates;
 
         private IConfiguration _config = new ConfigurationBuilder().AddInMemoryCollection().Build();
+        private ILoggerFactory _loggerFactory;
         private WebHostOptions _options;
 
         // Only one of these should be set
@@ -40,11 +41,14 @@ public class WebHostBuilder : IWebHostBuilder
         // Only one of these should be set
         private IServerFactory _serverFactory;
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="WebHostBuilder"/> class.
+        /// </summary>
         public WebHostBuilder()
         {
             _hostingEnvironment = new HostingEnvironment();
-            _loggerFactory = new LoggerFactory();
             _configureServicesDelegates = new List<Action<IServiceCollection>>();
+            _configureLoggingDelegates = new List<Action<ILoggerFactory>>();
         }
 
         /// <summary>
@@ -69,6 +73,22 @@ public string GetSetting(string key)
             return _config[key];
         }
 
+        /// <summary>
+        /// Specify the <see cref="ILoggerFactory"/> to be used by the web host.
+        /// </summary>
+        /// <param name="loggerFactory">The <see cref="ILoggerFactory"/> to be used.</param>
+        /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
+        public IWebHostBuilder UseLoggerFactory(ILoggerFactory loggerFactory)
+        {
+            if (loggerFactory == null)
+            {
+                throw new ArgumentNullException(nameof(loggerFactory));
+            }
+
+            _loggerFactory = loggerFactory;
+            return this;
+        }
+
         /// <summary>
         /// Specify the <see cref="IServerFactory"/> to be used by the web host.
         /// </summary>
@@ -135,13 +155,18 @@ public IWebHostBuilder Configure(Action<IApplicationBuilder> configureApp)
         }
 
         /// <summary>
-        /// Configure the provided <see cref="ILoggerFactory"/> which will be available as a hosting service. 
+        /// Adds a delegate for configuring the provided <see cref="ILoggerFactory"/>. This may be called multiple times.
         /// </summary>
         /// <param name="configureLogging">The delegate that configures the <see cref="ILoggerFactory"/>.</param>
         /// <returns>The <see cref="IWebHostBuilder"/>.</returns>
         public IWebHostBuilder ConfigureLogging(Action<ILoggerFactory> configureLogging)
         {
-            configureLogging(_loggerFactory);
+            if (configureLogging == null)
+            {
+                throw new ArgumentNullException(nameof(configureLogging));
+            }
+
+            _configureLoggingDelegates.Add(configureLogging);
             return this;
         }
 
@@ -184,14 +209,25 @@ private IServiceCollection BuildHostingServices()
 
             var services = new ServiceCollection();
             services.AddSingleton(_hostingEnvironment);
+
+            if (_loggerFactory == null)
+            {
+                _loggerFactory = new LoggerFactory();
+            }
+
+            foreach (var configureLogging in _configureLoggingDelegates)
+            {
+                configureLogging(_loggerFactory);
+            }
+
             services.AddSingleton(_loggerFactory);
+            services.AddLogging();
 
             services.AddTransient<IStartupLoader, StartupLoader>();
 
             services.AddTransient<IServerLoader, ServerLoader>();
             services.AddTransient<IApplicationBuilderFactory, ApplicationBuilderFactory>();
             services.AddTransient<IHttpContextFactory, HttpContextFactory>();
-            services.AddLogging();
             services.AddOptions();
 
             var diagnosticSource = new DiagnosticListener("Microsoft.AspNetCore");

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

@@ -13,6 +13,7 @@
 using Microsoft.AspNetCore.Http.Internal;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.PlatformAbstractions;
 using System.Reflection;
 using Xunit;
@@ -124,6 +125,33 @@ public async Task StartupConfigureThrows_Fallback()
             }
         }
 
+        [Fact]
+        public void DefaultCreatesLoggerFactory()
+        {
+            var hostBuilder = new WebHostBuilder()
+                .UseServer(new TestServer())
+                .UseStartup<StartupNoServices>();
+
+            var host = (WebHost)hostBuilder.Build();
+
+            Assert.NotNull(host.Services.GetService<ILoggerFactory>());
+        }
+
+        [Fact]
+        public void UseLoggerFactoryHonored()
+        {
+            var loggerFactory = new LoggerFactory();
+
+            var hostBuilder = new WebHostBuilder()
+                .UseLoggerFactory(loggerFactory)
+                .UseServer(new TestServer())
+                .UseStartup<StartupNoServices>();
+
+            var host = (WebHost)hostBuilder.Build();
+
+            Assert.Same(loggerFactory, host.Services.GetService<ILoggerFactory>());
+        }
+
         [Fact]
         public void DefaultConfigurationCapturesStartupErrors()
         {