Good GetHashCode() override for List of Foo objects respecting the order

c# algorithm hashcode gethashcode
Ben B. picture Ben B. · Nov 11, 2011 · Viewed 26k times · Source

EnumerableObject : IEnumerable<Foo>

wraps a List<Foo>

If EnumerableObject a.SequenceEquals( EnumerableObject b), then they are equal.

Therefore, a GetHashCode must be implemented. The problem is XORing each element in the list will return the same hash code for any list with all and only the same elements, regardless of order. This is Okay in terms of it working, but will result in many collisions, which will slow down retrieval, etc.

What is a good, fast GetHashCode method for lists of objects that is order dependent?

Answer

Jon Skeet picture Jon Skeet · Nov 11, 2011

I'd do it the same way I normally combine hash codes - with an addition and a multiplication:

public override int GetHashCode()
{
    unchecked
    {
        int hash = 19;
        foreach (var foo in foos)
        {
            hash = hash * 31 + foo.GetHashCode();
        }
        return hash;
    }
}

(Note that you shouldn't add anything to the list after this has been used for the key in a hash table of any description, as the hash will change. This also assumes that there are no null entries - if there could be, you need to take account of that.)

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