Creates an IAsyncRenderMvcController which can be used instead of IRenderMvcController if developers want access to an async Index action.

src/Umbraco.Web/Mvc/DefaultRenderMvcControllerResolver.cs

@@ -36,6 +36,7 @@ namespace Umbraco.Web.Mvc
         /// <summary>
         /// Returns an instance of the default controller instance.
         /// </summary>
+        [Obsolete("This method will be removed in future versions and should not be used to resolve a controller instance, the IControllerFactory is used for that purpose")]
         public IRenderMvcController GetControllerInstance()
         {
             //try the dependency resolver, then the activator
@@ -64,9 +65,10 @@ namespace Umbraco.Web.Mvc
         /// <param name="type"></param>
         private void ValidateType(Type type)
         {
-            if (TypeHelper.IsTypeAssignableFrom<IRenderMvcController>(type) == false)
+            if (TypeHelper.IsTypeAssignableFrom<IRenderMvcController>(type) == false
+                && TypeHelper.IsTypeAssignableFrom<IAsyncRenderMvcController>(type) == false)
             {
-                throw new InvalidOperationException("The Type specified (" + type + ") is not of type " + typeof(IRenderMvcController));
+                throw new InvalidOperationException("The Type specified (" + type + ") is not of type " + typeof(IRenderMvcController) + " or of type " + typeof(IAsyncRenderMvcController));
             }
         }
 

src/Umbraco.Web/Mvc/IAsyncRenderMvcController.cs

@@ -0,0 +1,24 @@
+using System.Threading.Tasks;
+using System.Web.Mvc;
+using Umbraco.Web.Models;
+
+namespace Umbraco.Web.Mvc
+{
+    /// <summary>
+    /// The interface that can be implemented for an async controller to be designated to execute for route hijacking
+    /// </summary>
+    public interface IAsyncRenderMvcController : IController
+    {
+        /// <summary>
+        /// The default action to render the front-end view
+        /// </summary>
+        /// <param name="model"></param>
+        /// <returns></returns>
+        /// <remarks>
+        /// Ideally we could have made a marker interface as a base interface for both the IRenderMvcController and IAsyncRenderMvcController
+        /// however that would require breaking changes in other classes like DefaultRenderMvcControllerResolver since the result would have 
+        /// to be changed. Instead we are hiding the underlying interface method.
+        /// </remarks>
+        Task<ActionResult> Index(RenderModel model);
+    }
+}

src/Umbraco.Web/Mvc/IRenderMvcController.cs

@@ -1,5 +1,6 @@
 using System.Web.Http.Filters;
 using System.Web.Mvc;
+using System.Web.Routing;
 using System.Windows.Forms;
 using Umbraco.Web.Models;
 

src/Umbraco.Web/Mvc/RenderActionInvoker.cs

@@ -26,8 +26,9 @@ namespace Umbraco.Web.Mvc
 			//now we need to check if it exists, if not we need to return the Index by default
 			if (ad == null)
 			{
-                //check if the controller is an instance of IRenderMvcController
-				if (controllerContext.Controller is IRenderMvcController)
+                //check if the controller is an instance of IRenderMvcController or IAsyncRenderMvcController and find the index
+                if (controllerContext.Controller is IRenderMvcController 
+                    || controllerContext.Controller is IAsyncRenderMvcController)
 				{
 					return controllerDescriptor.FindAction(controllerContext, "Index");
                 }

src/Umbraco.Web/Mvc/RenderRouteHandler.cs

@@ -310,8 +310,8 @@ namespace Umbraco.Web.Mvc
             //check if that controller exists
             if (controllerType != null)
             {
-                //ensure the controller is of type 'IRenderMvcController' and ControllerBase
-                if (TypeHelper.IsTypeAssignableFrom<IRenderMvcController>(controllerType)
+                //ensure the controller is of type IRenderMvcController/IAsyncRenderMvcController and ControllerBase
+                if ((TypeHelper.IsTypeAssignableFrom<IRenderMvcController>(controllerType) || TypeHelper.IsTypeAssignableFrom<IAsyncRenderMvcController>(controllerType))
                     && TypeHelper.IsTypeAssignableFrom<ControllerBase>(controllerType))
                 {
                     //set the controller and name to the custom one
@@ -325,10 +325,11 @@ namespace Umbraco.Web.Mvc
                 else
                 {
                     LogHelper.Warn<RenderRouteHandler>(
-                        "The current Document Type {0} matches a locally declared controller of type {1}. Custom Controllers for Umbraco routing must implement '{2}' and inherit from '{3}'.",
+                        "The current Document Type {0} matches a locally declared controller of type {1}. Custom Controllers for Umbraco routing must implement either '{2}' or '{3}' and inherit from '{4}'.",
                         () => publishedContentRequest.PublishedContent.DocumentTypeAlias,
                         () => controllerType.FullName,
                         () => typeof(IRenderMvcController).FullName,
+                        () => typeof(IAsyncRenderMvcController).FullName,
                         () => typeof(ControllerBase).FullName);
 
                     //we cannot route to this custom controller since it is not of the correct type so we'll continue with the defaults

src/Umbraco.Web/Umbraco.Web.csproj

@@ -304,6 +304,7 @@
     <Compile Include="Media\EmbedProviders\Flickr.cs" />
     <Compile Include="Models\ContentEditing\SimpleNotificationModel.cs" />
     <Compile Include="Models\PublishedContentWithKeyBase.cs" />
+    <Compile Include="Mvc\IAsyncRenderMvcController.cs" />
     <Compile Include="PropertyEditors\DatePreValueEditor.cs" />
     <Compile Include="RequestLifespanMessagesFactory.cs" />
     <Compile Include="Scheduling\LatchedBackgroundTaskBase.cs" />