Producing the same hashcode if 2 array objects contain the same values

Question

I am trying to calculate a hash code for one of my classes, the problem is: this class has a byte[] which returns a different hash for every object. I have made 2 objects of the class and both are instantiated with a new byte[0].

When overriding the Equals operator I've used Enumberable.SquenceEqual() to make sure the content of both array's are the same. But how would I make sure the arrays return the same hashcode if their contents are the same?

my code:

    public override bool Equals(object obj)
    {
        return Equals(obj as MessageType);
    }

    {
        if (messageType != null)
        {
            return (this.Identification == messageType.Identification) &&
                (this.ActivateWindow == messageType.ActivateWindow) &&
                (this.Logging == messageType.Logging) &&
                (Enumerable.SequenceEqual(this.Pictogram, messageType.Pictogram) == true) &&
                (this.Priority == messageType.Priority) &&
                (this.Procedure == messageType.Procedure);
        }

        return false;
    }

    public override int GetHashCode()
    {
        var result = this.Identification != null ? this.Identification.GetHashCode() : 0;
        result = (result * 397) ^ this.ActivateWindow.GetHashCode();
        result = (result * 397) ^ this.Logging.GetHashCode();
        result = (result * 397) ^ ((this.Pictogram != null) ? this.Pictogram.GetHashCode() : 0);
        result = (result * 397) ^ this.Priority.GetHashCode();
        result = (result * 397) ^ ((this.Procedure != null) ? this.Procedure.GetHashCode() : 0);
        return result;
    }

Dmitrii Bychenko · Accepted Answer · 2016-11-22 10:25:40Z

1

As far as I understand the question you should use Enumerable.SequenceEqual in Equals and some kind of aggregation within GetHashCode():

   byte[] extraData = ...

   ...

   public bool Equals(MessageType other) {
     ...

     if (!Enumerable.SequenceEqual(extraData, other.extraData))
       return false;

     ...  
   } 

   public override int GetHashCode() {
     unchecked { // we don't want IntegerOverflow exceptions to be thrown
       int result = ...

       ...  
       // let's combine hashes with xor
       result ^= extraData == null 
        ? 0
        : extraData.Aggerate(0, (s, a) => s ^ a); // ...and aggerate with xor as well

       ...

       return result;
     }
   }

In case extraData can be quite long (an appalling 1 GB for instance) you may want to restrict the computation to, say, 10 first items:

   result ^= extraData == null 
     ? 0
     : extraData.Take(10).Aggerate(0, (s, a) => s ^ a);

edited Nov 22, 2016 at 10:25

answered Nov 22, 2016 at 9:55

Dmitrii Bychenko

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2 Comments

Vincent Over a year ago

what do you mean with? "// ...and aggerate with xor as well" would this simply do the trick: result = (result * 397) ^ ((this.Pictogram != null) ? this.Pictogram.Aggregate(0, (s, a) => s ^ a) : 0); ?

Dmitrii Bychenko Over a year ago

@Vincent Advocaat: there're many formulae you can use to combine hashes; result = (result * 397) ^ ... is OK as well and the trick does work; just not forget to wrap the entire GetHashCode into unchecked {...} (you don't want integer overflow exception to be thrown)

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