IEqualityComparer for SequenceEqual

Question

In C#, is there an IEqualityComparer<IEnumerable> that uses the SequenceEqual method to determine equality?

Not exactly what you ask, but if you restrict yourself to `T[]` (not general `IEnumerable`), you can use `StructuralComparisons.StructuralEqualityComparer.Equals(arr1, arr2)` and `StructuralComparisons.StructuralEqualityComparer.GetHashCode(arr)`. Not the best solution from the BCL, and not so type-safe (you can wrap it in your own class deriving from `EqualityComparer` to make sure it is use on `T[]` only), but it exists. — Jeppe Stig Nielsen, Jun 01 '19 at 18:24

score 26 · Accepted Answer · edited Sep 21 '17 at 20:14

26

There is no such comparer in .NET Framework, but you can create one:

public class IEnumerableComparer<T> : IEqualityComparer<IEnumerable<T>>
{
    public bool Equals(IEnumerable<T> x, IEnumerable<T> y)
    {
        return Object.ReferenceEquals(x, y) || (x != null && y != null && x.SequenceEqual(y));
    }

    public int GetHashCode(IEnumerable<T> obj)
    {
        // Will not throw an OverflowException
        unchecked
        {
            return obj.Where(e => e != null).Select(e => e.GetHashCode()).Aggregate(17, (a, b) => 23 * a + b);
        }
    }
}

In the above code, I iterate over all items of the collection in the GetHashCode. I don't know if it's the wisest solution but this what is done in the internal HashSetEqualityComparer.

edited Sep 21 '17 at 20:14

Ben Gripka

16,012
6
45
41

answered Feb 03 '13 at 18:27

Cédric Bignon

12,892
3
39
51

FYI the current hashing in `HashSetEqualityComparer` is not what you have here. It's not what you want anyway. It doesn't compare a *sequence* but a *set*- its hash purposely collides between sequences that have the same items in different orders. Your code here is fine. – jnm2 Jan 06 '17 at 18:08
Good implementation, can the unchecked be better off only enclosing the actual calculation which can overflow? – too Apr 05 '19 at 10:16
Note that depending on what you plan to do with the thing, it may be reasonable to simply implement the hash code as `int GetHashCode(IEnumnerable object) => 0;` – dlf Dec 13 '19 at 13:38

score 1 · Answer 2 · edited Jun 16 '21 at 07:40

1

Created a NuGet package based on Cédric Bignon's answer:

Assembly package: https://www.nuget.org/packages/OBeautifulCode.Collection/

Code-file-only package: https://www.nuget.org/packages/OBeautifulCode.Collection.Recipes.EnumerableEqualityComparer/

var myEqualityComparer = new EnumerableEqualityComparer<string>();

edited Jun 16 '21 at 07:40

Pang

9,564
146
81
122

answered Jun 01 '19 at 18:02

Suraj

35,905
47
139
250

IEqualityComparer for SequenceEqual

2 Answers2

Linked