RenderPartialViewToString.cs

        public static String RenderRazorViewToString(ControllerContext controllerContext, String viewName, Object model)
        {
            controllerContext.Controller.ViewData.Model = model;

            using (var sw = new StringWriter())
            {
                var viewResult = ViewEngines.Engines.FindPartialView(controllerContext, viewName);
                var viewContext = new ViewContext(controllerContext, viewResult.View, controllerContext.Controller.ViewData, controllerContext.Controller.TempData, sw);
                viewResult.View.Render(viewContext, sw);
                viewResult.ViewEngine.ReleaseView(controllerContext, viewResult.View);
                return sw.GetStringBuilder().ToString();
            }
        }