Hashing an array in c#

Question

Short question

How to implement GetHashCode for an Array.

Details

I have an object that overrides Equals, checking that:

this.array[n] == otherObject.array[n]

for all n in array.

Naturally I should implement the complementary GetHashCode. I was wondering if there is .NET way to do this, or if I should implement my own, something like

hash = hash ^ array[n]

Clarification

My object contains an array, and I'm interested on GetHashCode for the elements of the array. My code for array equivalence is for example only - like my question says but maybe I wasn't clear, I'm interested in GetHashCode (not Equals). I say I naturally should implement the complementary GetHashCode because it is a requirement of .NET to implement this once Equals is overridden (for Dictionary etc. to function correctly). Thanks.

Answer 1

To compute a hash code using the elements of an array, you can cast the array to IStructuralEquatable and then call the GetHashCode(IEqualityComparer) method, passing a comparer for the type of elements in the array.

(The cast is necessary because the Array class implements the method explicitly.)

For example, if your object has an int array, then you can implement GetHashCode like this:

public override int GetHashCode()
{
    return ((IStructuralEquatable)this.array).GetHashCode(EqualityComparer<int>.Default);
}

In case you're curious, here's how the Array class implements the GetHashCode method (from the Reference Source):

internal static int CombineHashCodes(int h1, int h2) {
    return (((h1 << 5) + h1) ^ h2);
}

int IStructuralEquatable.GetHashCode(IEqualityComparer comparer) {
    if (comparer == null)
        throw new ArgumentNullException("comparer");
    Contract.EndContractBlock();

    int ret = 0;

    for (int i = (this.Length >= 8 ? this.Length - 8 : 0); i < this.Length; i++) {
        ret = CombineHashCodes(ret, comparer.GetHashCode(GetValue(i)));
    }

    return ret;
}

As you can see, the current implementation only uses the last eight elements of the array.

Answer 2

It depends on what you want ...

One option as Michael answered above is to have a hashcode based on the array elements. This will be in line with your Equals value semantics. However because "As a guideline, the hash of an object must be the same over the object's entire lifetime" you will have to ensure your array does not change after getting its hashcode. To have a non immutable container with a demand that it never changes sounds error prone to me.

Your other (IMO better option) is to switch to an immutable container (ie ImmutableArray) then a value-based hashcode makes sense. You can either use IStructuralEquatable as above or more generally:

    public override GetHashCode() =>
        Value.Aggregate(0, (total, next) => HashCode.Combine(total, next));

which will work for other immutable collections as well.

Answer 3

I don't agree you should naturally implement GetHashCode on an array
You would have to update it with every change
Or calculate it on the fly
I would compare directly on the fly
SequenceEquals will use the default equality comparer so you should also implement

public bool Equals

0n the objects in the array

Enumerable.SequenceEqual
Has an example

public static void SequenceEqualEx1()
{
    Pet pet1 = new Pet { Name = "Turbo", Age = 2 };
    Pet pet2 = new Pet { Name = "Peanut", Age = 8 };

    // Create two lists of pets.
    List<Pet> pets1 = new List<Pet> { pet1, pet2 };
    List<Pet> pets2 = new List<Pet> { pet1, pet2 };

    bool equal = pets1.SequenceEqual(pets2);

    Console.WriteLine(
        "The lists {0} equal.",
        equal ? "are" : "are not");
}

Answer 4

Using the current framework one could consider using

int value=0;
for (var i = 0;i< this.array.Length; i++)
{
    value=HashCode.Combine(this.array[i],value);
}