Adding warning log when no actions detected

src/Mvc/Mvc.Core/src/Infrastructure/DefaultActionDescriptorCollectionProvider.cs

@@ -7,14 +7,16 @@
 using System.Diagnostics;
 using System.Linq;
 using Microsoft.AspNetCore.Mvc.Abstractions;
+using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Primitives;
 
 namespace Microsoft.AspNetCore.Mvc.Infrastructure;
 
-internal class DefaultActionDescriptorCollectionProvider : ActionDescriptorCollectionProvider
+internal partial class DefaultActionDescriptorCollectionProvider : ActionDescriptorCollectionProvider
 {
     private readonly IActionDescriptorProvider[] _actionDescriptorProviders;
     private readonly IActionDescriptorChangeProvider[] _actionDescriptorChangeProviders;
+    private readonly ILogger _logger;
 
     // The lock is used to protect WRITES to the following (do not need to protect reads once initialized).
     private readonly object _lock;
@@ -25,7 +27,8 @@ internal class DefaultActionDescriptorCollectionProvider : ActionDescriptorColle
 
     public DefaultActionDescriptorCollectionProvider(
         IEnumerable<IActionDescriptorProvider> actionDescriptorProviders,
-        IEnumerable<IActionDescriptorChangeProvider> actionDescriptorChangeProviders)
+        IEnumerable<IActionDescriptorChangeProvider> actionDescriptorChangeProviders,
+        ILogger<DefaultActionDescriptorCollectionProvider> logger)
     {
         _actionDescriptorProviders = actionDescriptorProviders
             .OrderBy(p => p.Order)
@@ -35,6 +38,8 @@ public DefaultActionDescriptorCollectionProvider(
 
         _lock = new object();
 
+        _logger = logger;
+
         // IMPORTANT: this needs to be the last thing we do in the constructor. Change notifications can happen immediately!
         ChangeToken.OnChange(
             GetCompositeChangeToken,
@@ -124,6 +129,13 @@ private void UpdateCollection()
                 _actionDescriptorProviders[i].OnProvidersExecuted(context);
             }
 
+            if (context.Results.Count == 0)
+            {
+                // Emit a log message if after all providers still no action
+                // descriptors detected in the context.
+                Log.NoActionDescriptors(_logger);
+            }
+
             // The sequence for an update is important because we don't want anyone to obtain
             // the new change token but the old action descriptor collection.
             // 1. Obtain the old cancellation token source (don't trigger it yet)
@@ -156,4 +168,14 @@ private void UpdateCollection()
             oldCancellationTokenSource?.Cancel();
         }
     }
+
+    public static partial class Log
+    {
+        [LoggerMessage(
+            EventId = 1,
+            EventName = "NoActionDescriptors",
+            Level = LogLevel.Information,
+            Message = "No action descriptors found. This may indicate an incorrectly configured application or missing application parts. To learn more, visit https://aka.ms/aspnet/mvc/app-parts")]
+        public static partial void NoActionDescriptors(ILogger logger);
+    }
 }

src/Mvc/Mvc.Core/src/MvcCoreLoggerExtensions.cs

@@ -23,7 +23,7 @@
 
 namespace Microsoft.AspNetCore.Mvc;
 
-internal static class MvcCoreLoggerExtensions
+internal static partial class MvcCoreLoggerExtensions
 {
     public const string ActionFilter = "Action Filter";
     private static readonly string[] _noFilters = new[] { "None" };

src/Mvc/Mvc.Core/test/ActionConstraints/ActionConstraintCacheTest.cs

@@ -6,6 +6,7 @@
 using Microsoft.AspNetCore.Mvc.Controllers;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
 
 namespace Microsoft.AspNetCore.Mvc.ActionConstraints;
 
@@ -157,7 +158,8 @@ private static ActionConstraintCache CreateCache(params IActionConstraintProvide
     {
         var descriptorProvider = new DefaultActionDescriptorCollectionProvider(
             Enumerable.Empty<IActionDescriptorProvider>(),
-            Enumerable.Empty<IActionDescriptorChangeProvider>());
+            Enumerable.Empty<IActionDescriptorChangeProvider>(),
+            NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
         return new ActionConstraintCache(descriptorProvider, providers);
     }
 }

src/Mvc/Mvc.Core/test/Infrastructure/ActionSelectorTest.cs

@@ -975,7 +975,8 @@ private ControllerActionDescriptor InvokeActionSelector(RouteContext context)
         var actionDescriptorProvider = GetActionDescriptorProvider();
         var actionDescriptorCollectionProvider = new DefaultActionDescriptorCollectionProvider(
             new[] { actionDescriptorProvider },
-            Enumerable.Empty<IActionDescriptorChangeProvider>());
+            Enumerable.Empty<IActionDescriptorChangeProvider>(),
+            NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
 
         var actionConstraintProviders = new[]
         {
@@ -1134,7 +1135,8 @@ private static ActionConstraintCache GetActionConstraintCache(IActionConstraintP
     {
         var descriptorProvider = new DefaultActionDescriptorCollectionProvider(
             Enumerable.Empty<IActionDescriptorProvider>(),
-            Enumerable.Empty<IActionDescriptorChangeProvider>());
+            Enumerable.Empty<IActionDescriptorChangeProvider>(),
+            NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
         return new ActionConstraintCache(descriptorProvider, actionConstraintProviders.AsEnumerable() ?? new List<IActionConstraintProvider>());
     }
 

src/Mvc/Mvc.Core/test/Infrastructure/DefaultActionDescriptorCollectionProviderTest.cs

@@ -1,7 +1,8 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using Microsoft.AspNetCore.Mvc.Abstractions;
+using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Primitives;
 using Moq;
 
@@ -22,7 +23,8 @@ public void ActionDescriptors_ReadsDescriptorsFromActionDescriptorProviders()
 
         var actionDescriptorCollectionProvider = new DefaultActionDescriptorCollectionProvider(
             new[] { actionDescriptorProvider1, actionDescriptorProvider2 },
-            Enumerable.Empty<IActionDescriptorChangeProvider>());
+            Enumerable.Empty<IActionDescriptorChangeProvider>(),
+            NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
 
         // Act
         var collection = actionDescriptorCollectionProvider.ActionDescriptors;
@@ -44,7 +46,8 @@ public void ActionDescriptors_CachesValuesByDefault()
 
         var actionDescriptorCollectionProvider = new DefaultActionDescriptorCollectionProvider(
             new[] { actionDescriptorProvider },
-            Enumerable.Empty<IActionDescriptorChangeProvider>());
+            Enumerable.Empty<IActionDescriptorChangeProvider>(),
+            NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
 
         // Act - 1
         var collection1 = actionDescriptorCollectionProvider.ActionDescriptors;
@@ -93,7 +96,8 @@ public void ActionDescriptors_UpdatesAndResubscribes_WhenChangeTokenTriggers()
         var changeProvider = new TestChangeProvider();
         var actionDescriptorCollectionProvider = new DefaultActionDescriptorCollectionProvider(
             new[] { actionDescriptorProvider.Object },
-            new[] { changeProvider });
+            new[] { changeProvider },
+            NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
 
         // Act - 1
         var changeToken1 = actionDescriptorCollectionProvider.GetChangeToken();

src/Mvc/Mvc.Core/test/Routing/ActionEndpointDataSourceBaseTest.cs

@@ -6,6 +6,7 @@
 using Microsoft.AspNetCore.Routing;
 using Microsoft.AspNetCore.Routing.Patterns;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Primitives;
 using Moq;
 
@@ -129,7 +130,8 @@ private protected ActionEndpointDataSourceBase CreateDataSource(IActionDescripto
         {
             actions = new DefaultActionDescriptorCollectionProvider(
                 Array.Empty<IActionDescriptorProvider>(),
-                Array.Empty<IActionDescriptorChangeProvider>());
+                Array.Empty<IActionDescriptorChangeProvider>(),
+                NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
         }
 
         var services = new ServiceCollection();

src/Mvc/Mvc.Core/test/Routing/KnownRouteValueConstraintTests.cs

@@ -9,6 +9,7 @@
 using Microsoft.AspNetCore.Routing;
 using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
 using Moq;
 
 namespace Microsoft.AspNetCore.Mvc.Routing;
@@ -285,7 +286,8 @@ private static IActionDescriptorCollectionProvider CreateActionDescriptorCollect
 
         var descriptorCollectionProvider = new DefaultActionDescriptorCollectionProvider(
             new[] { actionProvider.Object },
-            Enumerable.Empty<IActionDescriptorChangeProvider>());
+            Enumerable.Empty<IActionDescriptorChangeProvider>(),
+            NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
         return descriptorCollectionProvider;
     }
 

src/Mvc/Mvc/test/Routing/ActionConstraintMatcherPolicyTest.cs

@@ -7,6 +7,7 @@
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 using Microsoft.AspNetCore.Routing;
 using Microsoft.AspNetCore.Routing.Matching;
+using Microsoft.Extensions.Logging.Abstractions;
 using Moq;
 
 namespace Microsoft.AspNetCore.Mvc.Routing;
@@ -449,7 +450,8 @@ private ActionConstraintMatcherPolicy CreateSelector(ActionDescriptor[] actions)
 
         var actionDescriptorCollectionProvider = new DefaultActionDescriptorCollectionProvider(
             new IActionDescriptorProvider[] { actionDescriptorProvider.Object, },
-            Enumerable.Empty<IActionDescriptorChangeProvider>());
+            Enumerable.Empty<IActionDescriptorChangeProvider>(),
+            NullLogger<DefaultActionDescriptorCollectionProvider>.Instance);
 
         var cache = new ActionConstraintCache(actionDescriptorCollectionProvider, new[]
         {