struct FullItem
    {
        public String name;
        public int code;
        public int quantity;
        public double price;
        public double weight;
    }

. . .

private ArrayList Purchase = new ArrayList();

. . .

FullItem FullPurchase;
FullPurchase.code = purchase.Code;
FullPurchase.name = purchase.Name;
FullPurchase.price = purchase.Price;

if (purchase.ByWeight == true)
{
      FullPurchase.weight = 100 / 1000;  // add wieght input
      FullPurchase.price = FullPurchase.weight * FullPurchase.price;
}
            
      FullPurchase.quantity = 1; // add something for getting Q numbers

      Purchase.Add(FullPurchase);  //----this line has the error-------

. . .

Error	1	Use of unassigned local variable 'FullPurchase'

FullItem FullPurchase = new FullItem();

FullPurchase.code = purchase.Code;