ASP.NET MVC partial view to string

protected string RenderPartialViewToString(string viewName, object model)
{
   if (string.IsNullOrEmpty(viewName))
                viewName = ControllerContext.RouteData.GetRequiredString("action");

    ViewData.Model = model;

   using (StringWriter sw = new StringWriter())
  {
     ViewEngineResult viewResult = 

           ViewEngines.Engines.FindPartialView(ControllerContext, viewName);
     ViewContext viewContext = 

          new ViewContext(ControllerContext, viewResult.View, ViewData, TempData, sw);
      viewResult.View.Render(viewContext, sw);

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