Ensure RegisteredLogger is registred in all required instances

src/Serilog.Extensions.Hosting/SerilogHostBuilderExtensions.cs

@@ -84,6 +84,9 @@ public static IHostBuilder UseSerilog(
                 {
                     // This won't (and shouldn't) take ownership of the logger. 
                     collection.AddSingleton(logger);
+
+                    // Still need to use RegisteredLogger as it is used by ConfigureDiagnosticContext.
+                    collection.AddSingleton(new RegisteredLogger(logger));
                 }
                 bool useRegisteredLogger = logger != null;
                 ConfigureDiagnosticContext(collection, useRegisteredLogger);

test/Serilog.Extensions.Hosting.Tests/SerilogHostBuilderExtensionsTests.cs

@@ -0,0 +1,109 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Xunit;
+
+namespace Serilog.Extensions.Hosting.Tests
+{
+    public class SerilogHostBuilderExtensionsTests
+    {
+        [Fact]
+        public void ServicesAreRegisteredWhenCallingUseSerilog()
+        {
+            // Arrange
+            var collection = new ServiceCollection();
+            IHostBuilder builder = new FakeHostBuilder(collection);
+
+            // Act
+            builder.UseSerilog();
+
+            // Assert
+            IServiceProvider provider = collection.BuildServiceProvider();
+            provider.GetRequiredService<ILoggerFactory>();
+            provider.GetRequiredService<IDiagnosticContext>();
+        }
+
+        [Fact]
+        public void ServicesAreRegisteredWhenCallingUseSerilogWithLogger()
+        {
+            // Arrange
+            var collection = new ServiceCollection();
+            IHostBuilder builder = new FakeHostBuilder(collection);
+            ILogger logger = new LoggerConfiguration().CreateLogger();
+
+            // Act
+            builder.UseSerilog(logger);
+
+            // Assert
+            IServiceProvider provider = collection.BuildServiceProvider();
+            provider.GetRequiredService<ILogger>();
+            provider.GetRequiredService<ILoggerFactory>();
+            provider.GetRequiredService<IDiagnosticContext>();
+        }
+
+        [Fact]
+        public void ServicesAreRegisteredWhenCallingUseSerilogWithConfigureDelegate()
+        {
+            // Arrange
+            var collection = new ServiceCollection();
+            IHostBuilder builder = new FakeHostBuilder(collection);
+
+            // Act
+            builder.UseSerilog((_, _) => { });
+
+            // Assert
+            IServiceProvider provider = collection.BuildServiceProvider();
+            provider.GetRequiredService<ILogger>();
+            provider.GetRequiredService<ILoggerFactory>();
+            provider.GetRequiredService<IDiagnosticContext>();
+        }
+
+        private class FakeHostBuilder : IHostBuilder
+        {
+            private readonly IServiceCollection _collection;
+
+            public FakeHostBuilder(IServiceCollection collection) => _collection = collection;
+
+            public IHostBuilder ConfigureHostConfiguration(Action<IConfigurationBuilder> configureDelegate)
+            {
+                throw new NotImplementedException();
+            }
+
+            public IHostBuilder ConfigureAppConfiguration(Action<HostBuilderContext, IConfigurationBuilder> configureDelegate)
+            {
+                throw new NotImplementedException();
+            }
+
+            public IHostBuilder ConfigureServices(Action<HostBuilderContext, IServiceCollection> configureDelegate)
+            {
+                configureDelegate(null, _collection);
+                return this;
+            }
+
+            public IHostBuilder UseServiceProviderFactory<TContainerBuilder>(IServiceProviderFactory<TContainerBuilder> factory)
+            {
+                throw new NotImplementedException();
+            }
+
+            public IHostBuilder UseServiceProviderFactory<TContainerBuilder>(Func<HostBuilderContext, IServiceProviderFactory<TContainerBuilder>> factory)
+            {
+                throw new NotImplementedException();
+            }
+
+            public IHostBuilder ConfigureContainer<TContainerBuilder>(Action<HostBuilderContext, TContainerBuilder> configureDelegate)
+            {
+                throw new NotImplementedException();
+            }
+
+            public IHost Build()
+            {
+                throw new NotImplementedException();
+            }
+
+            public IDictionary<object, object> Properties { get; }
+        }
+    }
+}