ASP Core 2 Render a View to a String

CustomController.cs

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewEngines;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using System.IO;
using System.Threading.Tasks;

namespace Website.Infrastructure {
    public class CustomController : Controller {
        public async Task<string> RenderViewAsync<TModel>(string viewName, TModel model, bool partial = false) {
            if (string.IsNullOrEmpty(viewName)) {
                viewName = ControllerContext.ActionDescriptor.ActionName;
            }

            ViewData.Model = model;

            using (var writer = new StringWriter()) {
                IViewEngine viewEngine = HttpContext.RequestServices.GetService(typeof(ICompositeViewEngine)) as ICompositeViewEngine;
                ViewEngineResult viewResult = viewEngine.FindView(ControllerContext, viewName, !partial);

                if (viewResult.Success == false) {
                    return $"A view with the name {viewName} could not be found";
                }

                ViewContext viewContext = new ViewContext(
                    ControllerContext,
                    viewResult.View,
                    ViewData,
                    TempData,
                    writer,
                    new HtmlHelperOptions()
                );

                await viewResult.View.RenderAsync(viewContext);

                return writer.GetStringBuilder().ToString();
            }
        }
    }
}