ViewRenderingService.cs

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

namespace PS.Services
{
   public interface IViewRenderingService
   {
      Task<string> RenderViewToStringAsync(Controller controller, string viewName, object model);
   }
    public class ViewRenderingService : IViewRenderingService
    {
      private readonly ICompositeViewEngine viewEngine;
      private readonly ITempDataProvider tempDataProvider;
      public ViewRenderingService(ICompositeViewEngine viewEngineInject, ITempDataProvider tempDataProviderInject)
      {
         viewEngine = viewEngineInject;
         tempDataProvider = tempDataProviderInject;
      }

      public async Task<string> RenderViewToStringAsync(Controller controller, string viewName, object model)
      {
         controller.ViewData.Model = model;
         using (StringWriter sw = new StringWriter())
         {
            ViewEngineResult viewResult = viewEngine.FindView(controller.ControllerContext, viewName, false);

            ViewContext viewContext = new ViewContext(
                controller.ControllerContext,
                viewResult.View,
                controller.ViewData,
                new TempDataDictionary(
                    controller.ControllerContext.HttpContext,
                    tempDataProvider),
                sw,
                new HtmlHelperOptions()
            );
            await viewResult.View.RenderAsync(viewContext);
            return sw.GetStringBuilder().ToString();
         }
      }
   }
}