XML сериализация и ошибка "it does not have a parameterless constructor" - C#

 [Serializable]
    public class Obj
    {
        
        public Dictionary<ushort,Beton> beton;
        public Dictionary<ushort, Kirpich> kirpich;
        public Dictionary<ushort, Lestnica> lestnica;
        public Dictionary<ushort, Gold> gold;
        public Dictionary<ushort, Perekladina> perekladina;
        public Dictionary<ushort, Type> idType = new Dictionary<ushort, Type>();
        public Dictionary<ushort,Respown> respown = new Dictionary<ushort,Respown>();
        public Dictionary<ushort, Hunters> hunters = new Dictionary<ushort, Hunters>();
        public Obj(Dictionary<ushort, Beton> beton, Dictionary<ushort, Kirpich> kirpich, Dictionary<ushort, Lestnica> lestnica, Dictionary<ushort, Gold> gold, Dictionary<ushort, Perekladina> perekladina, Dictionary<ushort, Respown> respown, Dictionary<ushort,Hunters> hunters)
        {
            this.beton = beton;
            this.kirpich = kirpich;
            this.gold = gold;
            this.lestnica = lestnica;
            this.perekladina = perekladina;
            this.respown = respown;
        }
    }

FileStream strwr = new FileStream(@"data.dat", FileMode.Create);
            XmlSerializer xml = new XmlSerializer(typeof(Obj));
            xml.Serialize(strwr, obj);