在给定对象y的情况下，我可以在散列集中检索存储的值x，其中x.Equals（y）

Question

[TestFixture]
class HashSetExample
{
    [Test]
    public void eg()
    {
        var comparer = new OddEvenBag();
        var hs = new HashSet<int>(comparer);

        hs.Add(1);

        Assert.IsTrue(hs.Contains(3));
        Assert.IsFalse(hs.Contains(0));

        // THIS LINE HERE
        var containedValue = hs.First(x => comparer.Equals(x, 3)); // i want something faster than this
        Assert.AreEqual(1, containedValue);
    }

    public class OddEvenBag : IEqualityComparer<int>
    {
        public bool Equals(int x, int y)
        {
            return x % 2 == y % 2;
        }

        public int GetHashCode(int obj)
        {
            return obj % 2;
        }
    }
}

As well as checking if hs contains an odd number, I want to know what odd number if contains. Obviously I want a method that scales reasonably and does not simply iterate-and-search over the entire collection.

Another way to rephrase the question is, I want to replace the line below THIS LINE HERE with something efficient (say O(1), instead of O(n)).

Towards what end? I'm trying to intern a laaaaaaaarge number of immutable reference objects similar in size to a Point3D. Seems like using a HashSet<Foo> instead of a Dictionary<Foo,Foo> saves about 10% in memory. No, obviously this isn't a game changer but I figured it would not hurt to try it for a quick win. Apologies if this has offended anybody.

Edit: Link to similar/identical post provided by Balazs Tihanyi in comments, put here for emphasis.

Answer 1

The simple answer is no, you can't.

If you want to retrieve the object you will need to use a HashSet . There just isn't any suitable method in the API to do what you are asking for otherwise.

One optimization you could make though if you must use a Set for this is to first do a contains check and then only iterate over the Set if the contains returns true. Still you would almost certainly find that the extra overhead for a HashMap is tiny (since essentially it's just another object reference).