如何创建一个空的 IReadOnlyCollection

Question

I'm creating an extension method for MultiValueDictionary to encapsulate frequent ContainsKey checks and I was wondering what was the best way to create an empty IReadOnlyCollection ?.

What I've used so far is new List<TValue>(0).AsReadOnly() but there must be a better way, an equivilant to IEnumerable 's Enumerable.Empty

public static IReadOnlyCollection<TValue> GetValuesOrEmpty<TKey, TValue>(this MultiValueDictionary<TKey, TValue> multiValueDictionary, TKey key)
{            
    IReadOnlyCollection<TValue> values;
    return !multiValueDictionary.TryGetValue(key, out values) ? new List<TValue>(0).AsReadOnly() : values;
}

Answer 1

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>();

Answer 2

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).

Answer 3

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.

Answer 4

return new List<XElement>().AsReadOnly();

Answer 5

How's about this which has a similar syntax to Enumerable.Empty:

/// <summary>
/// Contains a method used to provide an empty, read-only collection.
/// </summary>
public static class ReadOnlyCollection
{
    /// <summary>
    /// Returns an empty, read-only collection that has the specified type argument.
    /// </summary>
    /// <typeparam name="T">
    /// The type to assign to the type parameter of the returned generic read-only collection.
    /// </typeparam>
    /// <returns>
    /// An empty, read-only collection whose type argument is T.
    /// </returns>
    public static IReadOnlyCollection<T> Empty<T>()
    {
        return CachedValueProvider<T>.Value;
    }

    /// <summary/>
    static class CachedValueProvider<T>
    {
        /// <summary/>
        public static readonly IReadOnlyCollection<T> Value = new T[0];
    }
}

Used like this:

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