How to create an empty IReadOnlyCollection

I don't think there's anything like Enumerable.Empty for read-only collections, but:

• List<T> already implements IReadOnlyCollection<T> so you can avoid one object allocation by not calling AsReadOnly() and simply casting the list instead. This is less "safe" in theory but hardly matters in practice.

• Alternatively, you could cache the returned ReadOnlyCollection to avoid any object allocation whatsoever (except for the cached object).

As far as I know there is no built in way(Interested to know if one). That said, you can use the following:

IReadOnlyCollection<TValue> readonlyCollection = new ReadOnlyCollection<TValue>(new TValue[] { });

Optionally you can cache the results as it is a ReadOnlyCollection over empty array, It will always be the same no matter how many instances you have.

EDIT: The new .Net 4.6 adds an API to get an empty array: Array.Empty<T> and arrays implement IReadOnlyCollection<T>. This also reduces allocations as it only creates an instance once:

IReadOnlyCollection<int> emptyReadOnlyCollection = Array.Empty<int>();

---

What I ended up doing is mimicking the implementation of Enumerable.Empty using new TElement[0]:

public static class ReadOnlyCollection
{
    public static IReadOnlyCollection<TResult> Empty<TResult>()
    {
        return EmptyReadOnlyCollection<TResult>.Instance;
    }

    private static class EmptyReadOnlyCollection<TElement>
    {
        static volatile TElement[] _instance;

        public static IReadOnlyCollection<TElement> Instance
        {
            get { return _instance ?? (_instance = new TElement[0]); }
        }
    }
}

Usage:

IReadOnlyCollection<int> emptyReadOnlyCollection = ReadOnlyCollection.Empty<int>();