Empty ILookup

Question

I have a method that returns an ILookup. In some cases I want to return an empty ILookup as an early exit. What is the best way of constructing an empty ILookup?

Answer 1

Further to the answers from mquander and Vasile Bujac, you could create a nice, straightforward singleton-esque EmptyLookup<K,E> class as follows. (In my opinion, there doesn't seem much benefit to creating a full ILookup<K,E> implementation as per Vasile's answer.)

var empty = EmptyLookup<int, string>.Instance;

// ...

public static class EmptyLookup<TKey, TElement>
{
    private static readonly ILookup<TKey, TElement> _instance
        = Enumerable.Empty<TElement>().ToLookup(x => default(TKey));

    public static ILookup<TKey, TElement> Instance
    {
        get { return _instance; }
    }
}

Answer 2

There's no built-in, so I'd just write an extension method that runs something along the lines of new T[0].ToLookup<K, T>(x => default(K));

I strongly doubt returning null would be more correct here. It's almost never the case that you want to return null from a method which returns a collection (as opposed to an empty collection.) I could not possibly disagree more with people who are suggesting that.

Answer 3

You can create a singleton class for empty lookups.

using System.Linq;

public sealed class EmptyLookup<T, K> : ILookup<T, K> 
{
        public static readonly EmptyLookup<T, K> Instance { get; }
            = new EmptyLookup<T, K>();

        private EmptyLookup() { }

        public bool Contains(T key) => false;

        public int Count => 0;

        public IEnumerable<K> this[T key] => Enumerable.Empty<K>();

        public IEnumerator<IGrouping<T, K>> GetEnumerator()
          => Enumerable.Empty<IGrouping<K, V>>().GetEnumerator();

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() => GetEnumerator();
 }

then you can write code like this:

var x = EmptyLookup<int, int>.Instance;

The benefit of creating a new class is that you can use the "is" operator and check for type equality:

if (x is EmptyLookup<,>) {
 // ....
}

Answer 4

Based on LukeH answer, I'd create a static class Lookup with an Empty<TKey, TElement> method. In this way you could use is just the same way as Enumerable.Empty<T>.

public static class Lookup
{
    public static ILookup<TKey, TElement> Empty<TKey, TElement>()
        => Enumerable.Empty<TElement>().ToLookup(x => default(TKey));
}

Example Usage: Lookup.Empty<string, string>()

Answer 5

Create an empty list, then execute ToLookup() on it, like this:

List<Point> items = new List<Point>();
ILookup<int, int> lookup = items.ToLookup(p => p.X, p => p.Y);

Good luck!

Answer 6

Thanks @mqp for the good idea. I can propose several extension methods based on that approach:

public static class IEnumerableExtensions
{
    public static ILookup<TKey, TElement> ToEmptyLookup<TKey, TElement>(this IEnumerable<TElement> elements) => new TElement[0].ToLookup(k => default(TKey));
    public static ILookup<TKey, TElement> ToEmptyLookup<TKey, TElement>(this IDictionary<TKey, TElement> elements) => new TElement[0].ToLookup(k => default(TKey));
    public static ILookup<TKey, TElement> ToEmptyLookup<TKey, TElement>(this IGrouping<TKey, TElement> elements) => new TElement[0].ToLookup(k => default(TKey));
    public static ILookup<TKey, TElement> ToEmptyLookup<TKey, TElement>(this IEnumerable<ILookup<TKey, TElement>> elements) => new TElement[0].ToLookup(k => default(TKey));
}

Answer 7

Or something more in the spirit of LINQ:

    public static class Utility
{

    public static ILookup<TKey, TElement> EmptyLookup<TKey, TElement>(Func<TKey, TKey> keySelector,
                                                                      Func<TKey, TElement> elementSelector)
    {
        return Enumerable.Empty<TKey>().ToLookup(keySelector, elementSelector);
    }

}

Answer 8

You can return null

or Exception

But you should note it in class comment

Added:+ This is more obvious than some extension method