GetHashCode issue

Question

Hi,

Can anyone help explain the following. I am having an issue with a Dictionary where ContainsKey evaluates to false while both Equals and GetHashCode for the objects are successful. Below is the output from the immediate window in Visual Studio:

?LocationToRackingGroup.Keys.ToArray()[23].Equals(location)
true
?LocationToRackingGroup.Keys.ToArray()[23] == (location)
true
?this.LocationToRackingGroup.ContainsKey(location)
false

Am I missing something? Any ideas are much appreciated.

Cheers,

James

Answer 1

Is location mutable? Because if it is, it might have changed since you put it into the dictionary.

Answer 2

Well, I'd want to look at a couple of things:

1: is GetHashCode correctly implemented:

?LocationToRackingGroup.Keys.ToArray()[23].GetHashCode() == location.GetHashCode()

2: if this is the generic dictionary, does the type also implement (explicitly) IEquatable<Location>

3: did you supply a custom IEqualityComparer<Location> to the dictionary in the constructor?

To rule out the last, perhaps look at:

?LocationToRackingGroup.Comparer.Equals(blah23, location); // should be true
?LocationToRackingGroup.Comparer.GetHashCode(blah23);  // should equal below
?LocationToRackingGroup.Comparer.GetHashCode(location);// should equal above

Answer 3

The rule is that the hashcode must be the same for every 'instance' of the same data. If it changes, then your hash function is broken.

IOW, the safest hash function is:

int GetHashcode()
{
  return 0;
}

Answer 4

OK, this is a long shot.

In the first two lines you refer to LocationToRackingGroup and in the last to this.LocationToRackingGroup, are they the same variable?