How to NOT expose a read only collection in C#

These days I had the opportunity to make a review over an ecommerce application. They tried to use CQRS and they almost succeeded. 
I notified a problem on their queries classes that can become a big problem in time, if you want to sell this solution as a platform. Also, from some perspective, these problems also violate the CQRS principle.
Let’s see some code now:

public class Order
{
    Collection<OrderItem> _items;   

    public IEnumerable<OrderItem> Items
    {
        get
        {
            return _items;
        }
    }
    ...
}

What do you see here strange?  We want to expose the OrderItem collection as a read only collection. To do this, we convert it to IEnumerable.
Hmmm… sounds good? Nope. Big mistake! Nobody stop us to convert the Items back to an ICollection and Add/Remove items from it.

Order order = new Order();
…
var orderItems = (Collection<OrderItem>)oder.Items;

What should we do? .NET framework has specials collections that can be used in these situations. I this case we should use IReadOnlyCollection<T>. This class will let the user to access items but he will not be able to modify the list (add or remove items from the collection).
To obtain the list as a read only collection we will need to use the AsReadOnly() method:

public class Order
{
    Collection<OrderItem> _items;   

    public IReadOnlyCollection<OrderItem> Items
    {
        get
        {
            return _items. AsReadOnly();
        }
    }
   
    ...
}

Under the hood, this method will create a new collection that point to our original items.
When expose items as “read-only”, you should always double check. There are many methods that permit us to cast items.