ControllerlessMvcRouteHandler.cs

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Infrastructure;
using Microsoft.AspNetCore.Mvc.Internal;
using Microsoft.AspNetCore.Routing;
using Microsoft.Extensions.Logging;
using System;
using System.Diagnostics;
using System.Threading.Tasks;

namespace ControllerlessDemo
{
    // "controllerless" router - if no controller/action found, route to a default controller that simply renders the view, 
    // otherwise invoke controller/action as normal - replaces MvcRouteHandler
    public class ControllerlessMvcRouteHandler : IRouter
    {
        private IActionContextAccessor _actionContextAccessor;
        private IActionInvokerFactory _actionInvokerFactory;
        private IActionSelector _actionSelector;
        private ILogger _logger;
        private DiagnosticSource _diagnosticSource;

        public ControllerlessMvcRouteHandler(
            IActionInvokerFactory actionInvokerFactory,
            IActionSelector actionSelector,
            DiagnosticSource diagnosticSource,
            ILoggerFactory loggerFactory)
            : this(actionInvokerFactory, actionSelector, diagnosticSource, loggerFactory, actionContextAccessor: null)
        {
        }

        public ControllerlessMvcRouteHandler(
            IActionInvokerFactory actionInvokerFactory,
            IActionSelector actionSelector,
            DiagnosticSource diagnosticSource,
            ILoggerFactory loggerFactory,
            IActionContextAccessor actionContextAccessor)
        {
            // The IActionContextAccessor is optional. We want to avoid the overhead of using CallContext
            // if possible.
            _actionContextAccessor = actionContextAccessor;

            _actionInvokerFactory = actionInvokerFactory;
            _actionSelector = actionSelector;
            _diagnosticSource = diagnosticSource;
            _logger = loggerFactory.CreateLogger<ControllerlessMvcRouteHandler>();
        }

        public VirtualPathData GetVirtualPath(VirtualPathContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException(nameof(context));
            }

            // We return null here because we're not responsible for generating the url, the route is.
            return null;
        }

        public Task RouteAsync(RouteContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException(nameof(context));
            }

            var candidates = _actionSelector.SelectCandidates(context);

            // no controller/action available to execute route - run it controllerless
            if (candidates == null || candidates.Count == 0)
            {
                MakeRouteControllerless(context);
                candidates = _actionSelector.SelectCandidates(context); 
            }

            var actionDescriptor = _actionSelector.SelectBestCandidate(context, candidates);
            if (actionDescriptor == null)
            {
                _logger.LogDebug(3, "No actions matched the current request");
                return TaskCache.CompletedTask;
            }

            context.Handler = (c) =>
            {
                var routeData = c.GetRouteData();

                var actionContext = new ActionContext(context.HttpContext, routeData, actionDescriptor);
                if (_actionContextAccessor != null)
                {
                    _actionContextAccessor.ActionContext = actionContext;
                }

                var invoker = _actionInvokerFactory.CreateInvoker(actionContext);
                if (invoker == null)
                {
                    throw new InvalidOperationException($"An action invoker could not be created for action '{actionDescriptor.DisplayName}'.");
                }

                return invoker.InvokeAsync();
            };

            return TaskCache.CompletedTask;
        }

        private void MakeRouteControllerless(RouteContext context)
        {
            var controller = context.RouteData.Values["controller"].ToString();
            var action = context.RouteData.Values["action"].ToString();

            context.RouteData.Values["x-old-controller"] = controller;
            context.RouteData.Values["x-old-action"] = action;
            context.RouteData.Values["controller"] = "default";
            context.RouteData.Values["action"] = "default";
            context.RouteData.Values["viewName"] = $"~/views/{controller}/{action}.cshtml";
            
        }
    }
}