ShoppingCartDomain.cs

public class ShoppingCartDomain
{
    private readonly ShoppingCart _shoppingCart;

    public ShoppingCartDomain(ShoppingCart shoppingCart)
    {
        _shoppingCart = shoppingCart;
    }

    public void AddItem(Guid productId, int quantity, decimal price)
    {
        var existingItem = _shoppingCart.Items.SingleOrDefault(x => x.ProductId == productId);
        if (existingItem != null)
        {
            existingItem.Quantity += quantity;
        }
        else
        {
            _shoppingCart.Items.Add(new ShoppingCartItem(_shoppingCart.ShoppingCartId, productId, quantity, price));
        }
    }

    public void RemoveItem(Guid productId)
    {
        var product = _shoppingCart.Items.SingleOrDefault(x => x.ProductId == productId);
        if (product != null)
        {
            _shoppingCart.Items.Remove(product);
        }
    }
}