Question

I am stumped by a seemingly simple problem. I have two objects that I am comparing with a !=.

When I run the application, a != b is true.
When I put a breakpoint and do a Watch, a.GetHashCode() == b.GetHashCode() is true.

These two (reference type) objects are defined in a different assembly, but I cannot find an override to the != method (although GetHashCode is overridden). Is there another explanation for this? Could it be possible that a GetHashCode for two objects could be the same, but a not-overriden != would return true?

Thanks.

Answer 1

When two objects that are different return the same code it is called a “collision”. With only ~4 billion possible integer values, and more than 4 billion possible values of [your class name here] some collisions are inevitable. This is why a hash based structure (i.e. Dictionary) can’t rely entirely on GetHashCode, it also needs a sensible Equals implementation to be effective. The Equals method is what is used to resolve these collisions.

Of course it’s also possible that the creator of the class overwrite either GetHashCode or Equals and in some way made a mistake that in some way violated the “contract” for generating hash codes. Here is one list of guidelines to keep in mind when creating your GetHashCode methods. Remember that there is a fairly small set of things that you have to do, and another set of things that can be done to make it work efficiently.

return 0; is actually a perfectly acceptable GetHashCode implementation. It conforms with all of the rules, it just has a 100% chance of causing collisions, so it will be extraordinarily inefficient and you shouldn’t ever actually do that.