Returning a ReadOnlyCollection from a method with an IList return type

Here I have the following bit of code:

    private IList<IState> _states = new List<IState>();
    private ReadOnlyCollection<IState> _statesViewer;

    public IList<IState> States { get { return _statesViewer; } }

I believe that generally it is preferable to return interfaces rather than the concret开发者_JS百科e classes themselves, but in this case, shouldn't I set as the return type of the States property a ReadOnlyCollection?

Any user of my library will think it is possible to anything you can do with an IList if I set it as so, and that means adding elements. That is not true and I'm definitely breaking the contract exposing it as an IList.

Am I right with this view or there is something else I am missing here?

Do whatever makes the API clearest. If the caller only needs to enumerate it you could expose as IEnumerable<T>, but I wouldn't be concerned about exposing it as ReadOnlyCollection. You could declare a custom type (interface or class) with just an indexer and enumerator, of course

If it was me, I would simply expose it as

IEnumerable<IState>

IEnumerable<T> is a good choice for any public property that represents a sequence.

It's the smallest contract possible that is still a sequence, which helps you stay decoupled.

It enables a rich collection of operations in Linq for objects, so you're offering your consumer a lot of power.

For some cases i take the IEnumerable<IState> if people are only allowed to run over the list. But if i need some more built-in functionality for the outer world like index operator, Count, Contains, IndexOf, etc. i'll give them an ReadOnlyCollection as IList and write within the documentation that it is read only.