Render Episerver Blocks and ContentArea for use in web api https://devblog.gosso.se/?p=1463

ContentRendererService.cs

using EPiServer;
using EPiServer.Core;
using EPiServer.DataAbstraction;
using EPiServer.Framework.Web;
using EPiServer.Globalization;
using EPiServer.Logging;
using EPiServer.ServiceLocation;
using EPiServer.Web;
using EPiServer.Web.Mvc;
using System.Collections.Generic;
using System.IO;
using System.Web;
using System.Web.Mvc;
using System.Web.Routing;

namespace Gosso.Business.Services
{
    public class ContentRendererService
    {
        private readonly ITemplateResolver _templateResolver;
        private readonly IContentLoader _contentLoader;
        private readonly IContentRenderer _contentRenderer;

        public ContentRendererService(ITemplateResolver templateResolver, IContentLoader contentLoader, IContentRenderer contentRenderer)
        {
            _templateResolver = templateResolver;
            _contentLoader = contentLoader;
            _contentRenderer = contentRenderer;
        }

        public string Render(ContentReference contentReference, IEnumerable<string> tags = null)
        {
            //Find the content in question
            IContent matchedContent = _contentLoader.Get<IContent>(contentReference);

            if (matchedContent == null)
                throw new ContentNotFoundException("Content was not found");

            //Resolve the right Template based on Episervers own templating engine
            TemplateModel model = _templateResolver.Resolve(HttpContext.Current, matchedContent.GetOriginalType(), TemplateTypeCategories.MvcPartial | TemplateTypeCategories.MvcPartialController | TemplateTypeCategories.MvcController, tags ?? new string[0]);

            //Resolve the controller
            ControllerBase contentController = null;
            if (model.TemplateType != null)
                contentController = ServiceLocator.Current.GetInstance(model.TemplateType) as ControllerBase;

            if (contentController == null) // if view without controller
                contentController = new CustomController(model.Name);

            //Mimic the routing of our rendition 
            RouteData routeData = new RouteData();
            routeData.Values.Add("currentContent", matchedContent);
            routeData.Values.Add("controllerType", model.TemplateType);
            routeData.Values.Add("language", ContentLanguage.PreferredCulture.Name);
            routeData.Values.Add("controller", model.Name.Replace("Controller", ""));
            routeData.Values.Add("action", "Index");
            routeData.Values.Add("node", matchedContent.ContentLink.ID);
            routeData.DataTokens["contextmode"] = ContextMode.Default;//important to declare that the rendering should not be "edit"-mode, we dont want that, and it will crash
            //Create a fake context, that can be executed based on the route
            ViewContext viewContext = new ViewContext(new ControllerContext(new HttpContextWrapper(HttpContext.Current), routeData, contentController), new FakeView(), new ViewDataDictionary(), new TempDataDictionary(), new StringWriter());

            HtmlHelper helper = new HtmlHelper(viewContext, new ViewPage());

            //Render in our fake context
            _contentRenderer.Render(helper, new PartialRequest(), matchedContent, model);
            //helper.RenderContentData(matchedContent, false);//another way to execute it

            //Derive the output based on our template and view engine
            return viewContext.Writer.ToString();
        }

        public IEnumerable<string> RenderContentToHtml(IEnumerable<ContentAreaItem> filteredItems)
        {
            foreach (var item in filteredItems)
            {
                var tag = item.LoadDisplayOption()?.Tag;
                if (tag != null)
                {
                    yield return Render(item.ContentLink, new string[1] { tag });
                }
                else
                {
                    yield return Render(item.ContentLink);
                }
            }
        }
    }
    public class FakeView : IView
    {
        public void Render(ViewContext viewContext, TextWriter writer)
        {
        }
    }

    public class CustomController : Controller
    {
        private readonly string name;

        public CustomController(string name)
        {
            this.name = name;
        }
        public ActionResult Index()
        {
            return PartialView(name);
        }
    }
}