IEnumerable vs IReadonlyCollection vs ReadonlyCollection for exposing a list member

One important aspect seems to be missing from the answers so far:

When an IEnumerable<T> is returned to the caller, they must consider the possibility that the returned object is a "lazy stream", e.g. a collection built with "yield return". That is, the performance penalty for producing the elements of the IEnumerable<T> may have to be paid by the caller, for each use of the IEnumerable. (The productivity tool "Resharper" actually points this out as a code smell.)

By contrast, an IReadOnlyCollection<T> signals to the caller that there will be no lazy evaluation. (The Count property, as opposed to the Count extension method of IEnumerable<T> (which is inherited by IReadOnlyCollection<T> so it has the method as well), signals non-lazyness. And so does the fact that there seem to be no lazy implementations of IReadOnlyCollection.)

This is also valid for input parameters, as requesting an IReadOnlyCollection<T> instead of IEnumerable<T> signals that the method needs to iterate several times over the collection. Sure the method could create its own list from the IEnumerable<T> and iterate over that, but as the caller may already have a loaded collection at hand it would make sense to take advantage of it whenever possible. If the caller only has an IEnumerable<T> at hand, he only needs to add .ToArray() or .ToList() to the parameter.

What IReadOnlyCollection does not do is prevent the caller to cast to some other collection type. For such protection, one would have to use the class ReadOnlyCollection<T>.

In summary, the only thing IReadOnlyCollection<T> does relative to IEnumerable<T> is add a Count property and thus signal that no lazyness is involved.

Talking about class libraries, I think IReadOnly* is really useful, and I think you're doing it right :)

It's all about immutable collection... Before there were just immutables and to enlarge arrays was a huge task, so .net decided to include in the framework something different, mutable collection, that implement the ugly stuff for you, but IMHO they didn't give you a proper direction for immutable that are extremely useful, especially in a high concurrency scenario where sharing mutable stuff is always a PITA.

If you check other today languages, such as objective-c, you will see that in fact the rules are completely inverted! They quite always exchange immutable collection between different classes, in other words the interface expose just immutable, and internally they use mutable collection (yes, they have it of course), instead they expose proper methods if they want let the outsiders change the collection (if the class is a stateful class).

So this little experience that I've got with other languages pushes me to think that .net list are so powerful, but the immutable collection were there for some reason :)

In this case is not a matter of helping the caller of an interface, to avoid him to change all the code if you're changing internal implementation, like it is with IList vs List, but with IReadOnly* you're protecting yourself, your class, to being used in not a proper way, to avoid useless protection code, code that sometimes you couldn't also write (in the past in some piece of code I had to return a clone of the complete list to avoid this problem).

It seems that you can just return an appropriate interface:

...
    private readonly List<WorkItem> workItems = new List<WorkItem>();

    // Usually, there's no need the property to be virtual 
    public virtual IReadOnlyList<WorkItem> WorkItems {
      get {
        return workItems;
      }
    }
...

Since workItems field is in fact List<T> so the natural idea IMHO is to expose the most wide interface which is IReadOnlyList<T> in the case