/**
  * AddItem() - Adds an item to the shopping cart
  */
 public void AddItem(Produkt product)
 {
     // if list is empty simply add new object
     if (warenkorb.Items.Count == 0)
     {
         // Create a new item to add to the cart
         WarenkorbItemViewModel newItem = new WarenkorbItemViewModel(product);
         newItem.Menge = 1;
         warenkorb.Items.Add(newItem);
     }
     else
     {
         // if item with this productId already exists, just increase quantity
         foreach (WarenkorbItemViewModel item in warenkorb.Items)
         {
             if (item.Produkt.Produkt_ID == product.Produkt_ID)
             {
                 item.Menge++;
                 return;
             }
         }
         WarenkorbItemViewModel newItem = new WarenkorbItemViewModel(product);
         newItem.Menge = 1;
         warenkorb.Items.Add(newItem);
     }
 }
        //Removes an item from the shopping cart
        public void RemoveItem(Produkt product)
        {
            WarenkorbItemViewModel removedItem = new WarenkorbItemViewModel(product);

            warenkorb.Items.Remove(removedItem);
        }