[Fixes #4014] Add overload to AddControllerAsServices that uses the default controller discovery logic.

* Added ControllerFeature and ControllerFeatureProvider to perform controller discovery.
* Changed controller discovery to use application parts.
* Changed ControllerActionDescriptorProvider to make use of Application parts.
* Simplified AddControllerAsServices to not accept any parameter and perform
  controller discovery through the ApplicationPartManager in the IMvcBuilder
  and IMvcCoreBuilder. Assemblies should be added to the ApplicationPartManager
  in order to discover controllers in them.

Author: javiercn

src/Microsoft.AspNetCore.Mvc.Core/ApplicationParts/AssemblyPart.cs

@@ -10,7 +10,7 @@ namespace Microsoft.AspNetCore.Mvc.ApplicationParts
     /// <summary>
     /// An <see cref="ApplicationPart"/> backed by an <see cref="Assembly"/>.
     /// </summary>
-    public class AssemblyPart : ApplicationPart
+    public class AssemblyPart : ApplicationPart, IApplicationPartTypeProvider
     {
         /// <summary>
         /// Initalizes a new <see cref="AssemblyPart"/> instance.
@@ -35,5 +35,8 @@ public AssemblyPart(Assembly assembly)
         /// Gets the name of the <see cref="ApplicationPart"/>.
         /// </summary>
         public override string Name => Assembly.GetName().Name;
+
+        /// <inheritdoc />
+        public IEnumerable<TypeInfo> Types => Assembly.DefinedTypes;
     }
 }

src/Microsoft.AspNetCore.Mvc.Core/ApplicationParts/IApplicationPartTypeProvider.cs

@@ -0,0 +1,19 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+using System.Reflection;
+
+namespace Microsoft.AspNetCore.Mvc.ApplicationParts
+{
+    /// <summary>
+    /// Exposes a set of types from an <see cref="ApplicationPart"/>.
+    /// </summary>
+    public interface IApplicationPartTypeProvider
+    {
+        /// <summary>
+        /// Gets the list of available types in the <see cref="ApplicationPart"/>.
+        /// </summary>
+        IEnumerable<TypeInfo> Types { get; }
+    }
+}

src/Microsoft.AspNetCore.Mvc.Core/Controllers/ControllerFeature.cs

@@ -0,0 +1,24 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System.Collections.Generic;
+using System.Reflection;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace Microsoft.AspNetCore.Mvc.Controllers
+{
+    /// <summary>
+    /// The list of controllers types in an MVC application. The <see cref="ControllerFeature"/> can be populated
+    /// using the <see cref="ApplicationPartManager"/> that is available during startup at <see cref="IMvcBuilder.PartManager"/>
+    /// and <see cref="IMvcCoreBuilder.PartManager"/> or at a later stage by requiring the <see cref="ApplicationPartManager"/>
+    /// as a dependency in a component.
+    /// </summary>
+    public class ControllerFeature
+    {
+        /// <summary>
+        /// Gets the list of controller types in an MVC application.
+        /// </summary>
+        public IList<TypeInfo> Controllers { get; } = new List<TypeInfo>();
+    }
+}

src/Microsoft.AspNetCore.Mvc.Core/Controllers/ControllerFeatureProvider.cs

@@ -0,0 +1,79 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Reflection;
+using Microsoft.AspNetCore.Mvc.ApplicationParts;
+
+namespace Microsoft.AspNetCore.Mvc.Controllers
+{
+    /// <summary>
+    /// Discovers controllers from a list of <see cref="ApplicationPart"/> instances.
+    /// </summary>
+    public class ControllerFeatureProvider : IApplicationFeatureProvider<ControllerFeature>
+    {
+        private const string ControllerTypeNameSuffix = "Controller";
+
+        /// <inheritdoc />
+        public void PopulateFeature(
+            IEnumerable<ApplicationPart> parts,
+            ControllerFeature feature)
+        {
+            foreach (var part in parts.OfType<IApplicationPartTypeProvider>())
+            {
+                foreach (var type in part.Types)
+                {
+                    if (IsController(type) && !feature.Controllers.Contains(type))
+                    {
+                        feature.Controllers.Add(type);
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Determines if a given <paramref name="typeInfo"/> is a controller.
+        /// </summary>
+        /// <param name="typeInfo">The <see cref="TypeInfo"/> candidate.</param>
+        /// <returns><code>true</code> if the type is a controller; otherwise <code>false</code>.</returns>
+        protected virtual bool IsController(TypeInfo typeInfo)
+        {
+            if (!typeInfo.IsClass)
+            {
+                return false;
+            }
+
+            if (typeInfo.IsAbstract)
+            {
+                return false;
+            }
+
+            // We only consider public top-level classes as controllers. IsPublic returns false for nested
+            // classes, regardless of visibility modifiers
+            if (!typeInfo.IsPublic)
+            {
+                return false;
+            }
+
+            if (typeInfo.ContainsGenericParameters)
+            {
+                return false;
+            }
+
+            if (typeInfo.IsDefined(typeof(NonControllerAttribute)))
+            {
+                return false;
+            }
+
+            if (!typeInfo.Name.EndsWith(ControllerTypeNameSuffix, StringComparison.OrdinalIgnoreCase) &&
+                !typeInfo.IsDefined(typeof(ControllerAttribute)))
+            {
+                return false;
+            }
+
+            return true;
+        }
+    }
+}

src/Microsoft.AspNetCore.Mvc.Core/Controllers/DefaultControllerTypeProvider.cs

@@ -2,13 +2,13 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.ApplicationParts;
+using Microsoft.AspNetCore.Mvc.Controllers;
 using Microsoft.AspNetCore.Mvc.Formatters;
-using Microsoft.AspNetCore.Mvc.Internal;
+using Microsoft.Extensions.DependencyInjection.Extensions;
 
 namespace Microsoft.Extensions.DependencyInjection
 {
@@ -110,70 +110,22 @@ public static IMvcBuilder ConfigureApplicationPartManager(
         }
 
         /// <summary>
-        /// Register the specified <paramref name="controllerTypes"/> as services and as a source for controller
-        /// discovery.
+        /// Registers discovered controllers as services in the <see cref="IServiceCollection"/>.
         /// </summary>
         /// <param name="builder">The <see cref="IMvcBuilder"/>.</param>
-        /// <param name="controllerTypes">A sequence of controller <see cref="Type"/>s to register.</param>
         /// <returns>The <see cref="IMvcBuilder"/>.</returns>
-        public static IMvcBuilder AddControllersAsServices(
-           this IMvcBuilder builder,
-           params Type[] controllerTypes)
+        public static IMvcBuilder AddControllersAsServices(this IMvcBuilder builder)
         {
-            return builder.AddControllersAsServices(controllerTypes.AsEnumerable());
-        }
+            var feature = new ControllerFeature();
+            builder.PartManager.PopulateFeature(feature);
 
-        /// <summary>
-        /// Register the specified <paramref name="controllerTypes"/> as services and as a source for controller
-        /// discovery.
-        /// </summary>
-        /// <param name="builder">The <see cref="IMvcBuilder"/>.</param>
-        /// <param name="controllerTypes">A sequence of controller <see cref="Type"/>s to register.</param>
-        /// <returns>The <see cref="IMvcBuilder"/>.</returns>
-        public static IMvcBuilder AddControllersAsServices(
-           this IMvcBuilder builder,
-           IEnumerable<Type> controllerTypes)
+            foreach (var controller in feature.Controllers.Select(c => c.AsType()))
         {
-            if (builder == null)
-            {
-                throw new ArgumentNullException(nameof(builder));
+                builder.Services.TryAddTransient(controller, controller);
             }
 
-            ControllersAsServices.AddControllersAsServices(builder.Services, controllerTypes);
-            return builder;
-        }
-
-        /// <summary>
-        /// Registers controller types from the specified <paramref name="controllerAssemblies"/> as services and as a source
-        /// for controller discovery.
-        /// </summary>
-        /// <param name="builder">The <see cref="IMvcBuilder"/>.</param>
-        /// <param name="controllerAssemblies">Assemblies to scan.</param>
-        /// <returns>The <see cref="IMvcBuilder"/>.</returns>
-        public static IMvcBuilder AddControllersAsServices(
-            this IMvcBuilder builder,
-            params Assembly[] controllerAssemblies)
-        {
-            return builder.AddControllersAsServices(controllerAssemblies.AsEnumerable());
-        }
-
-        /// <summary>
-        /// Registers controller types from the specified <paramref name="controllerAssemblies"/> as services and as a source
-        /// for controller discovery.
-        /// </summary>
-        /// <param name="builder">The <see cref="IMvcBuilder"/>.</param>
-        /// <param name="controllerAssemblies">Assemblies to scan.</param>
-        /// <returns>The <see cref="IMvcBuilder"/>.</returns>
-        public static IMvcBuilder AddControllersAsServices(
-            this IMvcBuilder builder,
-            IEnumerable<Assembly> controllerAssemblies)
-        {
-            if (builder == null)
-            {
-                throw new ArgumentNullException(nameof(builder));
-            }
+            builder.Services.Replace(ServiceDescriptor.Transient<IControllerActivator, ServiceBasedControllerActivator>());
 
-            ControllersAsServices.AddControllersAsServices(builder.Services, controllerAssemblies);
             return builder;
         }
     }