c# - GetHashCode Extension Method

Question

After reading all the questions and answers on StackOverflow concerning overriding GetHashCode() I wrote the following extension method for easy and convenient overriding of GetHashCode():

public static class ObjectExtensions
{
    private const int _seedPrimeNumber = 691;
    private const int _fieldPrimeNumber = 397;
    public static int GetHashCodeFromFields(this object obj, params object[] fields) {
        unchecked { //unchecked to prevent throwing overflow exception
            int hashCode = _seedPrimeNumber;
            for (int i = 0; i < fields.Length; i++)
                if (fields[i] != null)
                    hashCode *= _fieldPrimeNumber + fields[i].GetHashCode();
            return hashCode;
        }
    }
}

(I basically only refactored the code that someone posted there, because I really like that it can be used generally)

which I use like this:

    public override int GetHashCode() {
        return this.GetHashCodeFromFields(field1, field2, field3);
    }

Do you see any problems with this code?

Answer 1

That looks like a solid way to do it.

My only suggestion is that if you're really concerned about performance with it, you may want to add generic versions for several common cases (ie. probably 1-4 args). That way, for those objects (which are most likely to be small, key-style composite objects), you won't have the overhead of building the array to pass to the method, the loop, any boxing of generic values, etc. The call syntax will be exactly the same, but you'll run slightly more optimized code for that case. Of course, I'd run some perf tests over this before you decide whether it's worth the maintenance trade-off.

Something like this:

public static int GetHashCodeFromFields<T1,T2,T3,T4>(this object obj, T1 obj1, T2 obj2, T3 obj3, T4 obj4) {
    int hashCode = _seedPrimeNumber;
    if(obj1 != null)
        hashCode *= _fieldPrimeNumber + obj1.GetHashCode();
    if(obj2 != null)
        hashCode *= _fieldPrimeNumber + obj2.GetHashCode();
    if(obj3 != null)
        hashCode *= _fieldPrimeNumber + obj3.GetHashCode();
    if(obj4 != null)
        hashCode *= _fieldPrimeNumber + obj4.GetHashCode();
    return hashCode;
}