//Constructor
public MobilePhoneDevice(string manufacturer, string model, decimal? price = null,
string owner = null, Battery battery = null, Display display = null)
{
this.Manufacturer = manufacturer;
this.Model = model;
this.Price = price;
this.Owner = owner;
this.Battery = battery;
this.Display = display;
}
public GSM(string model, string manufacturer, int price = 0,
string owner = null, Battery battery = null, Display display = null)
{
this._model = model;
this._manufacturer = manufacturer;
this._price = price;
this._owner = owner;
this._battery = battery;
this._display = display;
this.CallHistory = new List<Call>();
}