Order.cs

public class Order : BaseEntity, IAggregateRoot
{
    private Order()
    {
        // required by EF
    }

    public Order(string buyerId, Address shipToAddress, List<OrderItem> items)
    {
        Guard.Against.NullOrEmpty(buyerId, nameof(buyerId));
        Guard.Against.Null(shipToAddress, nameof(shipToAddress));
        Guard.Against.Null(items, nameof(items));

        BuyerId = buyerId;
        ShipToAddress = shipToAddress;
        _orderItems = items;
    }

    public string BuyerId { get; private set; }
    public DateTimeOffset OrderDate { get; private set; } = DateTimeOffset.Now;
    public Address ShipToAddress { get; private set; }

    // DDD Patterns comment
    // Using a private collection field, better for DDD Aggregate's encapsulation
    // so OrderItems cannot be added from "outside the AggregateRoot" directly to the collection,
    // but only through the method Order.AddOrderItem() which includes behavior.
    private readonly List<OrderItem> _orderItems = new List<OrderItem>();

    // Using List<>.AsReadOnly() 
    // This will create a read only wrapper around the private list so is protected against "external updates".
    // It's much cheaper than .ToList() because it will not have to copy all items in a new collection. (Just one heap alloc for the wrapper instance)
    //https://msdn.microsoft.com/en-us/library/e78dcd75(v=vs.110).aspx 
    public IReadOnlyCollection<OrderItem> OrderItems => _orderItems.AsReadOnly();

    public decimal Total()
    {
        var total = 0m;
        foreach (var item in _orderItems)
        {
            total += item.UnitPrice * item.Units;
        }
        return total;
    }
}

public class OrderItem : BaseEntity
{
    public CatalogItemOrdered ItemOrdered { get; private set; }
    public decimal UnitPrice { get; private set; }
    public int Units { get; private set; }

    private OrderItem()
    {
        // required by EF
    }

    public OrderItem(CatalogItemOrdered itemOrdered, decimal unitPrice, int units)
    {
        ItemOrdered = itemOrdered;
        UnitPrice = unitPrice;
        Units = units;
    }
}