override GetHashCode

  • Thread starter Thread starter Andrew Robinson
  • Start date Start date
A

Andrew Robinson

I have a class that has three properties: two of type int and one of type
string.

Is this the best method when overriding the GetHashCode() ? I am guessing
not... any thing better?

public override int GetHashCode() {
string hash = craneCounterweightID.ToString() + ":" +
trailerID.ToString() + ":" + craneConfigurationTypeCode;
return hash.GetHashCode();
}



Thanks,
 
Andrew Robinson said:
I have a class that has three properties: two of type int and one of type
string.

Is this the best method when overriding the GetHashCode() ? I am guessing
not... any thing better?

public override int GetHashCode() {
string hash = craneCounterweightID.ToString() + ":" +
trailerID.ToString() + ":" + craneConfigurationTypeCode;
return hash.GetHashCode();
}
Click to expand...

No, the above is relatively slow and doesn't help a lot. Some people
use XOR (eg a ^ b ^ c) but I prefer the kind of method shown in Josh
Bloch's "Effective Java":

public override int GetHashCode()
{
int hash = 23;
hash = hash*37 + craneCounterweightID;
hash = hash*37 + trailerID;
hash = hash*37 + craneConfigurationTypeCode.GetHashCode();
return hash;
}

The 23 and 37 are arbitrary numbers which are co-prime.

The benefit of the above over the XOR method is that if you have a type
which has two values which are frequently the same, XORing those values
will always give the same result (0) whereas the above will
differentiate between them unless you're very unlucky.
 
I have a class that has three properties: two of type int and one of type
string.

Is this the best method when overriding the GetHashCode() ? I am guessing
not... any thing better?

public override int GetHashCode() {
string hash = craneCounterweightID.ToString() + ":" +
trailerID.ToString() + ":" + craneConfigurationTypeCode;
return hash.GetHashCode();

}

Thanks,
Click to expand...

Hi,

XOR'ing the constituent fields provides a fairly good and fast
implementation.

public override int GetHashCode()
{
return
craneCounterweightID.GetHashCode() ^
trailerID.GetHashCode() ^
craneConfigurationTypeCode.GetHashCode();
}

One problem with XOR is that it is commutative. That means if the
values for craneCounterweightID and trailerID are swapped then the
returned code will remain the same. Addition and multiplication are
commutative as well so using either of those operators by themselves
will cause the same behavior not to mention that they probably
wouldn't produce a good distribution of values anyway. Basically, you
need to remove the "commutativeness" from the formula and still
maintain a good distribution. Jon posted this example some time ago.

public override int GetHashCode()
{
int result = 17;
result = result*37 + craneCounterweightID.GetHashCode();
result = result*37 + trailerID.GetHashCode();
result = result*37 + craneConfigurationTypeCode.GetHashCode();
return result;
}

Brian
 
Hi Andrew,

I agree to what Jon has suggested.

In addition, while we override the GetHashCode method in a derived class,
we'd better also override the Equals method to guarantee that the two
objects considered equal have the same hash code; otherwise, Hashtable
might not work correctly.

Hope this helps.


Sincerely,
Linda Liu
Microsoft Online Community Support

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Hi Andrew,

How about the problem now?

If you have any question, please feel free to let me know.

Thank you for using our MSDN Managed Newsgroup Support Service!


Sincerely,
Linda Liu
Microsoft Online Community Support
 
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