private void btnAdd_Click(object sender, EventArgs e)
{
ProductViewModel productViewModell = (ProductViewModel)cbProducts.SelectedItem;
carItem = productViewModell.ConvertToCarItemPurchase();
carItem.Quantity = int.Parse(txtQuantity.Text);
shoppingcar.AddPurchase(carItem, int.Parse(productViewModell.Stock));
RefreshData();
}