Improved the AddCustomTagHelperActivation.

Since MVC contains many tag helpers out of the box, custom activation
should not blindly redirect the creation of tag helpers to the
application container, since the application container will not be able
to build up a framework tag helper (with its dependencies). A predicate
argument has been added to the AddCustomTagHelperActivation to allow
control which tag helpers should be built-up by the application
container, and which should be pulled in from the built-in DI system.

src/MissingDIExtensions/AspNetCoreMvcExtensions.cs

@@ -1,10 +1,13 @@
 using System;
 using System.Linq;
+using System.Reflection;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.ApplicationParts;
 using Microsoft.AspNetCore.Mvc.Controllers;
+using Microsoft.AspNetCore.Mvc.Internal;
 using Microsoft.AspNetCore.Mvc.Razor;
+using Microsoft.AspNetCore.Mvc.Razor.Internal;
 using Microsoft.AspNetCore.Mvc.Rendering;
 using Microsoft.AspNetCore.Mvc.ViewComponents;
 using Microsoft.AspNetCore.Razor.TagHelpers;
@@ -31,12 +34,24 @@ public static void AddCustomViewComponentActivation(this IServiceCollection serv
             services.AddSingleton<IViewComponentActivator>(new DelegatingViewComponentActivator(activator));
         }
 
-        public static void AddCustomTagHelperActivation(this IServiceCollection services, Func<Type, object> activator)
+        public static void AddCustomTagHelperActivation(this IServiceCollection services, Func<Type, object> activator,
+            Predicate<Type> applicationTypeSelector = null)
         {
             if (services == null) throw new ArgumentNullException(nameof(services));
             if (activator == null) throw new ArgumentNullException(nameof(activator));
 
-            services.AddSingleton<ITagHelperActivator>(new DelegatingTagHelperActivator(activator));
+            // There are tag helpers OOTB in MVC. Letting the application container try to create them will fail
+            // because of the dependencies these tag helpers have. This means that OOTB tag helpers need to remain
+            // created by the framework's DefaultTagHelperActivator, hence the selector predicate.
+            applicationTypeSelector =
+                applicationTypeSelector ?? (type => !type.GetTypeInfo().Namespace.StartsWith("Microsoft"));
+
+            services.AddSingleton<ITagHelperActivator>(provider =>
+                new DelegatingTagHelperActivator(
+                    customCreatorSelector: applicationTypeSelector,
+                    customTagHelperCreator: activator,
+                    defaultTagHelperActivator: 
+                        new DefaultTagHelperActivator(provider.GetRequiredService<ITypeActivatorCache>())));
         }
 
         public static Type[] GetControllerTypes(this IApplicationBuilder builder)
@@ -104,16 +119,25 @@ public void Release(ViewComponentContext context, object viewComponent) =>
 
     internal sealed class DelegatingTagHelperActivator : ITagHelperActivator
     {
-        private readonly Func<Type, object> tagHelperCreator;
+        private readonly Predicate<Type> customCreatorSelector;
+        private readonly Func<Type, object> customTagHelperCreator;
+        private readonly ITagHelperActivator defaultTagHelperActivator;
 
-        public DelegatingTagHelperActivator(Func<Type, object> tagHelperCreator)
+        public DelegatingTagHelperActivator(Predicate<Type> customCreatorSelector, Func<Type, object> customTagHelperCreator,
+            ITagHelperActivator defaultTagHelperActivator)
         {
-            if (tagHelperCreator == null) throw new ArgumentNullException(nameof(tagHelperCreator));
+            if (customCreatorSelector == null) throw new ArgumentNullException(nameof(customCreatorSelector));
+            if (customTagHelperCreator == null) throw new ArgumentNullException(nameof(customTagHelperCreator));
+            if (defaultTagHelperActivator == null) throw new ArgumentNullException(nameof(defaultTagHelperActivator));
 
-            this.tagHelperCreator = tagHelperCreator;
+            this.customCreatorSelector = customCreatorSelector;
+            this.customTagHelperCreator = customTagHelperCreator;
+            this.defaultTagHelperActivator = defaultTagHelperActivator;
         }
 
         public TTagHelper Create<TTagHelper>(ViewContext context) where TTagHelper : ITagHelper =>
-            (TTagHelper)this.tagHelperCreator(typeof(TTagHelper));
+            this.customCreatorSelector(typeof(TTagHelper))
+                ? (TTagHelper)this.customTagHelperCreator(typeof(TTagHelper))
+                : defaultTagHelperActivator.Create<TTagHelper>(context);
     }
-}
+}

src/SampleApplication.SimpleInjector/Startup.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Reflection;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Http;
@@ -28,7 +29,7 @@ public Startup(IHostingEnvironment env)
 
         public IConfigurationRoot Configuration { get; }
 
-        // This method gets called by the runtime. Use this method to add services to the container.
+            // This method gets called by the runtime. Use this method to add services to the container.
         public void ConfigureServices(IServiceCollection services)
         {
             // Add framework services.
@@ -39,6 +40,7 @@ public void ConfigureServices(IServiceCollection services)
             services.AddRequestScopingMiddleware(container.BeginExecutionContextScope);
             services.AddCustomControllerActivation(container.GetInstance);
             services.AddCustomViewComponentActivation(container.GetInstance);
+            services.AddCustomTagHelperActivation(this.container.GetInstance, IsApplicationType);
         }
 
         // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
@@ -79,6 +81,8 @@ private void RegisterApplicationComponents(IApplicationBuilder app, ILoggerFacto
 
             container.Verify();
         }
+
+        private static bool IsApplicationType(Type type) => type.GetTypeInfo().Namespace.StartsWith("SampleApplication");
     }
 
     public interface IUserService { }