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.

Answer 2

try

return new ReadOnlyCollection<Abc> (list);

Answer 3

Use following way,

public class Test
    {

        private IList<SubTest>      _subTests;


        public virtual void AddSubTest(SubTest SubTest)
        {
            if (_subTests == null)
                _subTests = new List<SubTest>();

            _subTests.Add(SubTest);

            return this;
        }


        public virtual IReadOnlyList<SubTest> SubTests
        {
            get
            {
                if (_subTests == null)
                    _subTests = new List<SubTest>();

                return _subTests.AsReadOnly();
            }
        }




        public void RemoveSubTest( SubTest SubTest )
        {
            if( _subTests == null ) 
                return;

            _subTests.Remove( SubTest );            
        }

    }

use following mapping, and it would sets the value to the field not to the property which is read only list

<bag 
      name="SubTests" table="SubTest" lazy="true" access="field.camelcase-underscore"
      optimistic-lock="false"
      >
      <key column ="TestID" />
      <many-to-many class="SubTest, Domain" column="SubTestID" />
    </bag>

Answer 4

You can also return IEnumerable<Abc> which is read only iterator and provide methods for adding and removing elements.

Answer 5

In my opinion the best way to do this is to return IEnumerable<Abc> using the method suggested by Jon Skeet:

public IEnumerable<Abc> Abcs {
    get { return list.Skip(0); }
}

If you're not worried about consumers casting the IEnumerable<T> back to IList<T> you can just return IEnumerable<T>.

ReadOnlyCollection<T> has some serious flaws, the primary one being that it implements IList<T> so it does have Add and Remove methods. This can lead to runtime errors if consumers neglect to check its ReadOnly property. I strongly recommend against using it.