private void btnAdd_Click(object sender, EventArgs e)
{
ProductViewModel productViewModel = cbproducts.SelectedItem as ProductViewModel;
carItemSale = productViewModel.ConvertToCarItem();
carItemSale.Quantity = int.Parse(txtQuantity.Text);
carItemSale.Fkuser = SessionUser.Instance.Currentuser.Id;
shoppingSale.AddSale(carItemSale, int.Parse(productViewModel.Stock));
RefreshData();
}