How to use @Html.Action instead of ViewComponent in ASP.NET Core

Question

Is there any way to implement @Html.Action method in ASP.NET Core (like it was in ASP.NET MVC)? I know about the ViewComponent feature of ASP.NET Core. But there is a scenario when we have to use Action method.

Answer 1

This is the way to implement this as a HtmlHelper extension. You can use it as follows:

The last parameter is an anonymous type.

@Html.Action("Action");

@Html.Action("Action", new { string a = "a", int i = 5 }

@Html.Action("Action", "Controller");

@Html.Action("Action", "Controller", new (string a = "a", int i = 5 }

using System;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Html;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Infrastructure;
using Microsoft.AspNetCore.Routing;

namespace Microsoft.AspNetCore.Mvc.Rendering
{
    public static class HtmlHelperViewExtensions
    {
        public static IHtmlContent Action(this IHtmlHelper helper, string action, object parameters = null)
        {
            var controller = (string)helper.ViewContext.RouteData.Values["controller"];
            return Action(helper, action, controller, parameters);
        }

        public static IHtmlContent Action(this IHtmlHelper helper, string action, string controller, object parameters = null)
        {
            var area = (string)helper.ViewContext.RouteData.Values["area"];
            return Action(helper, action, controller, area, parameters);
        }

        public static IHtmlContent Action(this IHtmlHelper helper, string action, string controller, string area, object parameters = null)
        {
            if (action == null)
                throw new ArgumentNullException(nameof(controller));
            if (controller == null)
                throw new ArgumentNullException(nameof(action));

            var task = ActionAsync(helper, action, controller, area, parameters);
            return task.Result;
        }

        private static async Task<IHtmlContent> ActionAsync(this IHtmlHelper helper, string action, string controller, string area, object parameters = null)
        {
            // fetching required services for invocation
            var currentHttpContext = helper.ViewContext.HttpContext;
            var httpContextFactory = GetServiceOrFail<IHttpContextFactory>(currentHttpContext);
            var actionInvokerFactory = GetServiceOrFail<IActionInvokerFactory>(currentHttpContext);
            var actionSelector = GetServiceOrFail<IActionDescriptorCollectionProvider>(currentHttpContext);

            // creating new action invocation context
            var routeData = new RouteData();
            var routeParams = new RouteValueDictionary(parameters ?? new { });
            var routeValues = new RouteValueDictionary(new { area, controller, action });
            var newHttpContext = httpContextFactory.Create(currentHttpContext.Features);

            newHttpContext.Response.Body = new MemoryStream();

            foreach (var router in helper.ViewContext.RouteData.Routers)
                routeData.PushState(router, null, null);

            routeData.PushState(null, routeValues, null);
            routeData.PushState(null, routeParams, null);

            var actionDescriptor = actionSelector.ActionDescriptors.Items.First(i => i.RouteValues["Controller"] == controller && i.RouteValues["Action"] == action);
            var actionContext = new ActionContext(newHttpContext, routeData, actionDescriptor);

            // invoke action and retreive the response body
            var invoker = actionInvokerFactory.CreateInvoker(actionContext);
            string content = null;

            await invoker.InvokeAsync().ContinueWith(task =>
            {
                if (task.IsFaulted)
                {
                    content = task.Exception.Message;
                }
                else if (task.IsCompleted)
                {
                    newHttpContext.Response.Body.Position = 0;
                    using (var reader = new StreamReader(newHttpContext.Response.Body))
                        content = reader.ReadToEnd();
                }
            });

            return new HtmlString(content);
        }

        private static TService GetServiceOrFail<TService>(HttpContext httpContext)
        {
            if (httpContext == null)
                throw new ArgumentNullException(nameof(httpContext));

            var service = httpContext.RequestServices.GetService(typeof(TService));

            if (service == null)
                throw new InvalidOperationException($"Could not locate service: {nameof(TService)}");

            return (TService)service;
        }
    }
}

Answer 2

While it does work and renders the content correctly, when you try to access the IHttpContextAccessor.HttpContext property afterwards it is empty for some reason. I.e. you are opening another controller method which has a reference to IHttpContextAccessor via dependency injection and try to access the HttpContext property after you have renderend an Html.Action-element inside a partial view for example. If I remove the Html.Action-Element, HttpContext is filled correctly. I assume it somehow destroys the context.