Allow HostFactoryResolver to listen on arbitrary events

src/libraries/Microsoft.Extensions.HostFactoryResolver/Microsoft.Extensions.HostFactoryResolver.slnx

@@ -1279,6 +1279,14 @@
       <Build Solution="Checked|x64" Project="false" />
       <Build Solution="Checked|x86" Project="false" />
     </Project>
+    <Project Path="tests/ArbitraryDiagnosticEventPatternTestSite/ArbitraryDiagnosticEventPatternTestSite.csproj">
+      <BuildType Solution="Checked|*" Project="Debug" />
+      <Build Solution="*|arm" Project="false" />
+      <Build Solution="*|arm64" Project="false" />
+      <Build Solution="Checked|Any CPU" Project="false" />
+      <Build Solution="Checked|x64" Project="false" />
+      <Build Solution="Checked|x86" Project="false" />
+    </Project>
     <Project Path="tests/BuildWebHostInvalidSignature/BuildWebHostInvalidSignature.csproj">
       <BuildType Solution="Checked|*" Project="Debug" />
       <Build Solution="*|arm" Project="false" />

src/libraries/Microsoft.Extensions.HostFactoryResolver/src/HostFactoryResolver.cs

@@ -63,7 +63,8 @@ private static TimeSpan SetupDefaultTimeout()
                                                                  TimeSpan? waitTimeout = null,
                                                                  bool stopApplication = true,
                                                                  Action<object>? configureHostBuilder = null,
-                                                                 Action<Exception?>? entrypointCompleted = null)
+                                                                 Action<Exception?>? entrypointCompleted = null,
+                                                                 IDictionary<string, Action<object?>>? arbitraryActions = null)
         {
             if (assembly.EntryPoint is null)
             {
@@ -89,7 +90,7 @@ private static TimeSpan SetupDefaultTimeout()
                 return null;
             }
 
-            return args => new HostingListener(args, assembly.EntryPoint, waitTimeout ?? s_defaultWaitTimeout, stopApplication, configureHostBuilder, entrypointCompleted).CreateHost();
+            return args => new HostingListener(args, assembly.EntryPoint, waitTimeout ?? s_defaultWaitTimeout, stopApplication, configureHostBuilder, entrypointCompleted, arbitraryActions).CreateHost();
         }
 
         private static Func<string[], T>? ResolveFactory<T>(Assembly assembly, string name)
@@ -204,16 +205,25 @@ private sealed class HostingListener : IObserver<DiagnosticListener>, IObserver<
             private IDisposable? _disposable;
             private readonly Action<object>? _configure;
             private readonly Action<Exception?>? _entrypointCompleted;
+            private readonly IDictionary<string, Action<object?>>? _arbitraryActions;
             private static readonly AsyncLocal<HostingListener> _currentListener = new();
 
-            public HostingListener(string[] args, MethodInfo entryPoint, TimeSpan waitTimeout, bool stopApplication, Action<object>? configure, Action<Exception?>? entrypointCompleted)
+            public HostingListener(
+                string[] args,
+                MethodInfo entryPoint,
+                TimeSpan waitTimeout,
+                bool stopApplication,
+                Action<object>? configure,
+                Action<Exception?>? entrypointCompleted,
+                IDictionary<string, Action<object?>>? arbitraryActions)
             {
                 _args = args;
                 _entryPoint = entryPoint;
                 _waitTimeout = waitTimeout;
                 _stopApplication = stopApplication;
                 _configure = configure;
                 _entrypointCompleted = entrypointCompleted;
+                _arbitraryActions = arbitraryActions;
             }
 
             public object CreateHost()
@@ -332,8 +342,7 @@ public void OnNext(KeyValuePair<string, object?> value)
                 {
                     _configure?.Invoke(value.Value!);
                 }
-
-                if (value.Key == "HostBuilt")
+                else if (value.Key == "HostBuilt")
                 {
                     _hostTcs.TrySetResult(value.Value!);
 
@@ -343,6 +352,10 @@ public void OnNext(KeyValuePair<string, object?> value)
                         ThrowHostAborted();
                     }
                 }
+                else if (_arbitraryActions?.TryGetValue(value.Key, out var arbitraryAction) == true)
+                {
+                    arbitraryAction.Invoke(value.Value);
+                }
             }
 
             // HostFactoryResolver is used by tools that explicitly don't want to reference Microsoft.Extensions.Hosting assemblies.

src/libraries/Microsoft.Extensions.HostFactoryResolver/tests/ArbitraryDiagnosticEventPatternTestSite/ArbitraryDiagnosticEventPatternTestSite.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFrameworks>$(NetCoreAppCurrent);$(NetFrameworkCurrent)</TargetFrameworks>
+    <EnableDefaultItems>true</EnableDefaultItems>
+    <OutputType>Exe</OutputType>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\MockHostTypes\MockHostTypes.csproj" />
+  </ItemGroup>
+
+</Project>

src/libraries/Microsoft.Extensions.HostFactoryResolver/tests/ArbitraryDiagnosticEventPatternTestSite/Program.cs

@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Diagnostics;
+using Microsoft.Extensions.Hosting;
+
+namespace ArbitraryDiagnosticEventPatternTestSite
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            using var listener = new DiagnosticListener("Microsoft.Extensions.Hosting");
+            listener.Write("CustomEvent", 42);
+
+            new HostBuilder().Build().Dispose();
+        }
+    }
+}

src/libraries/Microsoft.Extensions.HostFactoryResolver/tests/HostFactoryResolverTests.cs

@@ -4,6 +4,7 @@
 using Microsoft.Extensions.Configuration;
 using MockHostTypes;
 using System;
+using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 using System.Threading;
@@ -163,6 +164,57 @@ void ConfigureHostBuilder(object hostBuilder)
             Assert.True(called);
         }
 
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
+        [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(ArbitraryDiagnosticEventPatternTestSite.Program))]
+        [InlineData(true)]
+        [InlineData(false)]
+        public void ArbitraryActionsCustomEventCallbackIsCalled(bool stopApplication)
+        {
+            using var callbackCalled = new ManualResetEventSlim(false);
+            object? callbackValue = null;
+            void CustomCallback(object? value)
+            {
+                callbackValue = value;
+                callbackCalled.Set();
+            }
+
+            var arbitraryActions = new Dictionary<string, Action<object?>>
+            {
+                ["CustomEvent"] = CustomCallback
+            };
+
+            var factory = HostFactoryResolver.ResolveHostFactory(
+                typeof(ArbitraryDiagnosticEventPatternTestSite.Program).Assembly,
+                waitTimeout: s_WaitTimeout,
+                stopApplication: stopApplication,
+                arbitraryActions: arbitraryActions);
+
+            Assert.NotNull(factory);
+            using var host = Assert.IsAssignableFrom<IHost>(factory(Array.Empty<string>()));
+            Assert.True(callbackCalled.Wait(s_WaitTimeout));
+            Assert.Equal(42, callbackValue);
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
+        [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(ArbitraryDiagnosticEventPatternTestSite.Program))]
+        public void ArbitraryActionsExceptionIsPropagated()
+        {
+            var arbitraryActions = new Dictionary<string, Action<object?>>
+            {
+                ["CustomEvent"] = _ => throw new InvalidOperationException("arbitrary action failed")
+            };
+
+            var factory = HostFactoryResolver.ResolveHostFactory(
+                typeof(ArbitraryDiagnosticEventPatternTestSite.Program).Assembly,
+                waitTimeout: s_WaitTimeout,
+                stopApplication: true,
+                arbitraryActions: arbitraryActions);
+
+            Assert.NotNull(factory);
+            var exception = Assert.Throws<InvalidOperationException>(() => factory(Array.Empty<string>()));
+            Assert.Equal("arbitrary action failed", exception.Message);
+        }
+
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
         [DynamicDependency(DynamicallyAccessedMemberTypes.All, typeof(NoSpecialEntryPointPattern.Program))]
         public void NoSpecialEntryPointPatternBuildsThenThrowsCallsEntryPointCompletedCallback()

src/libraries/Microsoft.Extensions.HostFactoryResolver/tests/Microsoft.Extensions.HostFactoryResolver.Tests.csproj

@@ -25,6 +25,7 @@
     <ProjectReference Include="CreateHostBuilderPatternTestSite\CreateHostBuilderPatternTestSite.csproj" />
     <ProjectReference Include="CreateWebHostBuilderInvalidSignature\CreateWebHostBuilderInvalidSignature.csproj" />
     <ProjectReference Include="CreateWebHostBuilderPatternTestSite\CreateWebHostBuilderPatternTestSite.csproj" />
+    <ProjectReference Include="ArbitraryDiagnosticEventPatternTestSite\ArbitraryDiagnosticEventPatternTestSite.csproj" />
     <ProjectReference Include="NoSpecialEntryPointPattern\NoSpecialEntryPointPattern.csproj" />
     <ProjectReference Include="NoSpecialEntryPointPatternBuildsThenThrows\NoSpecialEntryPointPatternBuildsThenThrows.csproj" />
     <ProjectReference Include="NoSpecialEntryPointPatternThrows\NoSpecialEntryPointPatternThrows.csproj" />