RenderViewToString.cs

public static string RenderViewToString(ControllerContext context, string viewName, object model)
		{
			if (string.IsNullOrEmpty(viewName))
				viewName = context.RouteData.GetRequiredString("action");

			ViewDataDictionary viewData = new ViewDataDictionary(model);

			using (StringWriter sw = new StringWriter())
			{
				ViewEngineResult viewResult = ViewEngines.Engines.FindPartialView(context, viewName);
				ViewContext viewContext = new ViewContext(context, viewResult.View, viewData, new TempDataDictionary(), sw);
				viewResult.View.Render(viewContext, sw);

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