public GSM(string model, string manufacturer, decimal? price, string owner, Battery battery, Display display)
 {
     this.Model = model;
     this.Manufacturer = manufacturer;
     this.Price = price;
     this.Owner = owner;
     this.GsmBattery = battery;
     this.GsmDisplay = display;
 }
 public GSM(string model, string manufacturer, decimal? price, string owner)
 {
     this.Model = model;
     this.Manufacturer = manufacturer;
     this.Price = price;
     this.Owner = owner;
     this.GSMBattery = new Battery();
     this.GSMDisplay = new Display();
 }