public class Customer : 
Entity,  // knowing  這屬於知道
IAggregateRoot  // knowing 繼承聚合根屬於知道
{
    public Guid Id { get; private set; } // knowing

    public string Email { get; private set; } // knowing

    public string Name { get; private set; } // knowing

    private readonly List<Order> _orders; // knowing

    private Customer()
    {
        this._orders = new List<Order>();
    }

    // doing something itself 這屬於做某些事情
    public Customer(string email, string name, ICustomerUniquenessChecker customerUniquenessChecker)
    {
        this.Email = email;
        this.Name = name;

        // doing - initiate and coordinate actions with other objects
        var isUnique = customerUniquenessChecker.IsUnique(this); 
        if (!isUnique)
        {
            throw new BusinessRuleValidationException("Customer with this email already exists.");
        }

        this.AddDomainEvent(new CustomerRegisteredEvent(this));
    }
    
    // doing something itself
    public void AddOrder(Order order)
    {
        // doing - initiate and coordinate actions with other objects
        if (this._orders.Count(x => x.IsOrderedToday()) >= 2) 
        {
            throw new BusinessRuleValidationException("You cannot order more than 2 orders on the same day");
        }

        this._orders.Add(order);

        this.AddDomainEvent(new OrderAddedEvent(order));
    }

    // doing something itself
    public void ChangeOrder(
        Guid orderId, 
        List<OrderProduct> products,
        List<ConversionRate> conversionRates)
    {
        var order = this._orders.Single(x => x.Id == orderId);
        
        // doing - initiate and coordinate actions with other objects
        order.Change(products, conversionRates); 

        this.AddDomainEvent(new OrderChangedEvent(order));
    }

    // doing something itself
    public void RemoveOrder(Guid orderId)
    {
        var order = this._orders.Single(x => x.Id == orderId);
        
        // doing - initiate and coordinate actions with other objects
        order.Remove(); 

        this.AddDomainEvent(new OrderRemovedEvent(order));
    }
    
    // doing something itself
    public GetOrdersTotal(Guid orderId) 
    {
        return this._orders.Sum(x => x.Value);
    }
}