private void BtnSave_Click(object sender, RoutedEventArgs e)
        {
            if (IsValid() == false)
            {
                return;
            }
            switch (operation)
            {
            case Operation.ADD:

                FurnitureDAO.Create(furniture);
                break;

            case Operation.EDIT:

                foreach (var f in Project.Instance.FurnitureList)
                {
                    if (f.ID == furniture.ID)
                    {
                        var pricePrim = furniture.Price;
                        f.Name        = furniture.Name;
                        f.ProductCode = furniture.ProductCode;
                        f.Price       = furniture.Price;
                        if (f.Sales != null)
                        {
                            foreach (var s in f.Sales)
                            {
                                f.PriceOnSale = pricePrim - (pricePrim / 100 * s.Discount);
                                pricePrim     = f.PriceOnSale;
                            }
                        }
                        f.Quantity      = furniture.Quantity;
                        f.FurnitureType = furniture.FurnitureType;
                        f.Deleted       = furniture.Deleted;
                        break;
                    }
                    FurnitureDAO.Update(furniture);
                }
                break;
            }
            this.Close();
        }