Accessing subclass properties

Question

 

I have a base class called StockItem and one called EcoStockItem and a Plate subclass. EcoStockItem has a Juice object that is a subclass to EcoStockItem. In another class, I have the method AddItem (StockItem p). From the Form1 class should I send the StockItem item as a parameter to the method AddItem? I could not access the other property from subclasses. How should I do it? Create an object of StockItem to inherit all properties from subclasses and add it to a StockItem array?                  newStockItem = new StockItem();             newStockItem.Id = id; newStockItem.Name = name; newStockItem.StockCount = stockcount; newEcoStockItem =new EcoStockItem ();       newEcoStockItem.Markning = markning; newPlate = new Plate();           newPlate.Typ = typ; newJuice = new Juice();           newJuice.Typ = jtyp; // StockItem p = ????     Stock stock = new Stock();     stock.AddItem(p);     and method:        public StockItem[] StockItems = new StockItem[10];     public void AddItem(StockItem item)     { int index = GetCount();     StockItems[index] = item; Form1.myForm.textBoxTest.Text = StockItems[index].ToString();     }