RenderViewToString.cs

    public static class ControllerExtensions
    {
        public static string RenderRazorViewToString(this Controller c, string viewName, object model)
        {
            c.ViewData.Model = model;
            using (var sw = new StringWriter())
            {
                var viewResult = ViewEngines.Engines.FindPartialView(c.ControllerContext, viewName);
                var viewContext = new ViewContext(c.ControllerContext, viewResult.View, c.ViewData, c.TempData, sw);

                viewResult.View.Render(viewContext, sw);
                viewResult.ViewEngine.ReleaseView(c.ControllerContext, viewResult.View);

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