public PurchaseItem(ISellableProduct product, int quantity)
{
Guard.Against.Null(product, nameof(product));
Guard.Against.NegativeOrZero(quantity, nameof(quantity));
Product = product;
Quantity = quantity;
}
/// <summary>
/// Adds a product.
/// </summary>
/// <param name="product"></param>
public void AddProduct(ISellableProduct product)
{
Products.Add(product);
}