CODEWITHSUNDEEP
c#.netequalshashcode
James
James

Reputation: 3625

GetHashCode issue

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.

Upvotes: 2

Views: 289

Answers (4)

heijp06
heijp06

Reputation: 11808

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?

Upvotes: 1

leppie
leppie

Reputation: 117320

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;
}

Upvotes: 0

Marc Gravell
Marc Gravell

Reputation: 1063864

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

Upvotes: 3

Robert Giesecke
Robert Giesecke

Reputation: 4314

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

Upvotes: 6

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