JSON.NET 空值处理, 数字转字符,时间格式化

        public static string ToJsonString(this Object obj)
        {
            IsoDateTimeConverter idtc = new IsoDateTimeConverter();
            idtc.DateTimeFormat = "yyyy-MM-dd HH:mm:ss";
            JsonSerializerSettings jsSettings = new JsonSerializerSettings();
            jsSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;
            jsSettings.DateFormatHandling = Newtonsoft.Json.DateFormatHandling.MicrosoftDateFormat;

            //jsSettings.NullValueHandling = NullValueHandling.Include;//包含空值
            //jsSettings.DateTimeZoneHandling = DateTimeZoneHandling.Local;//本地时间
            jsSettings.Converters.Add(idtc); //// 时间处理添加到设置中
           // jsSettings.DateFormatString = "yyyy-MM-dd HH:mm:ss";
            return JsonConvert.SerializeObject(obj, jsSettings);
        }

DateFormatString 这个老报错，

错误描述：找不到方法: 'Void Newtonsoft.Json.JsonSerializerSettings.set_DateFormatString(System.String)'.