.net - 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.