Content Delivery API: render any block inside XhtmlString property

Hi everyone,

Our team is building a single-page application retrieving content from EPiServer Content Delivery API. A key requirement from the editors is that they should be able to use blocks inside XhtmlString properties (TinyMCE text editor). We're having trouble getting content from blocks included in XhtmlString properties in our API's reponses.

We have tried combining the steps in the two following articles in order to override the default serialization of XhtmlString properties and render the blocks inside them by using EPiServer's "RenderXhtmlString" extension method for HtmlHelper instances.

1. https://talk.alfnilsson.se/2018/04/24/tweaking-and-extending-serialization-from-episerver-content-delivery-api/
2. https://antecknat.se/blog/2016/03/07/xhtmlstring-render-blocks-and-convert-link-to-external-without-htmlhelper/

Here are snippets of our code:

Custom property model for XhtmlString

    public class CustomXhtmlStringPropertyModel : PropertyModel<XhtmlString, PropertyXhtmlString>
    {
        public CustomXhtmlStringPropertyModel(PropertyXhtmlString propertyXhtmlString) : base(propertyXhtmlString)
        {
            ExternalValue = propertyXhtmlString.XhtmlString.ConvertToExternal();
        }
        public string ExternalValue { get; set; }
    }

Extension method "ConvertToExternal"

        public static string ConvertToExternal(this XhtmlString xhtmlstring)
        {
            if (string.IsNullOrWhiteSpace(xhtmlstring?.ToHtmlString()) || xhtmlstring.IsEmpty)
            {
                return string.Empty;
            }
            var routeData = new RouteData();
            routeData.Values.Add("controller", "a");
            routeData.Values.Add("subject", "a");
            var hh = new HtmlHelper(new ViewContext()
            {
                HttpContext = new HttpContextWrapper(HttpContext.Current),
                ViewData = new ViewDataDictionary(),
                TempData = new TempDataDictionary(),
                Controller = new DummyController(),
                RouteData = routeData
            }, new ViewPage());
            string result;
            using (var writer = new StringWriter())
            {
                hh.ViewContext.Writer = writer;
                hh.RenderXhtmlString(xhtmlstring);
                writer.Flush();
                result = writer.ToString();
            }
            return result;
        }

The extension method "ConvertToExternal" works as expected (by including content for blocks inside the XhtmlString) when using it in a traditional ASP.NET MVC EPiServer solution. However, it does not include any content from blocks when being used through EPiServer's Content Delivery API.

Any suggestion as to what we are doing wrong, or other solutions to our problem, is much appreciated.

- Thomas