public void addItem(Product product, int quantity)
{
if (product == null)
{
throw new ArgumentNullException("Product is Null");
}
if (quantity > 0)
{
if (ShoppingCartItems.ContainsKey(product))
{
ShoppingCartItems[product] += quantity;
}
else
{
ShoppingCartItems.Add(product, quantity);
}
}
}