Return ReadOnlyCollection from IList<>

Question

OK, so List<> contains the AsReadOnly() which gives you the ReadOnlyCollection. What I need is to have a field of IList type, and a property which would return a ReadOnlyCollection for this list.

Example class:

class Test
{
   private IList<Abc> list;

   public AddToList(Abc someItem) { /* adds inside the list */... }

   public ReadOnlyCollection<Abc> List { get { return ??? } } // <- no "set" here!
}

The scenario is following: I need to have some custom logic inside my class when the item is added into the list, and I want to restrict adding to this list by calling AddToList(someitem), while not allowing the usage of list.Add(someItem). The problem is that I use NHibernate which requires the IList interface, so I cannot cast / call the AsReadOnly() on the IList (it does not contain this method).

What way would you recommend to solve this situation? I simply need a way for NHibernate to set the needed collection in some way, but I also need to restrict users.

Answer 1

You can just emulate AsReadOnly():

public ReadOnlyCollection<Abc> List
{
    get { return new ReadOnlyCollection<Abc>(list); }
}

UPDATE:
This doesn't create a copy of list. ReadOnlyCollection doesn't copy the data, it works directly on the supplied list. See documentation:

A collection that is read-only is simply a collection with a wrapper that prevents modifying the collection; therefore, if changes are made to the underlying collection, the read-only collection reflects those changes.
This constructor is an O(1) operation.