private void Button_Click(object sender, RoutedEventArgs e) { api AddShopListItemApi = new api(); Shoppinglist shoppinglist = (Shoppinglist)shoppinglists.SelectedItem; Product product = finalProductList[productlist.SelectedIndex]; MessageBox.Show(product.naam); MessageBox.Show(shoppinglist.Name); if (AddShopListItemApi.AddShoppinglistItem(shoppinglist.Id, product.Id)) { MessageBox .Show("gelukt"); } else { MessageBox.Show("nee"); } }