hashCode understanding
in [CSharp]
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From: Tony Johansson on 11 Feb 2010 05:46 Hi! Assume I have 2 instances of a class called Test see below which will work as key in a hashtable. The Name property of instance1 is set to 'Paul', and the Name property of instance2 is set to 'Piet'. Both instances will generate different hashcode, and they're also different according to the Equals method.So far so good. When looking up Piet I will get the value object whatever it can be. Now, suppose that I change the Name of instance2 to 'Paul', then, according to my Equals method, both instances should be equal, and according to the hashcode and equals method will generate the same hashCode(same key) and that is not allowed to have in a hashtable.. What is the solution to this senario ? public class Test { string name; public string Name { get { return name; } set { name = value; } } public override GetHashCode() { return Name.GetHashcode(); } public override Equals(object other) { Test myTest = other as Test; if( myTest == null ) return false; return this.Name == myTest.Name; } }
From: Willem van Rumpt on 11 Feb 2010 07:09 Tony Johansson wrote: > Hi! > > Assume I have 2 instances of a class called Test see below which will work > as key in a hashtable. The Name property of instance1 is set to 'Paul', and > the Name property of instance2 is set to 'Piet'. Both instances will > generate different hashcode, and they're also different according to the > Equals method.So far so good. When looking up Piet I will get the value > object whatever it can be. > Now, suppose that I change the Name of instance2 to 'Paul', then, according > to my Equals method, both instances should be equal, and according to the > hashcode and equals method will generate the same hashCode(same key) and > that is not allowed > to have in a hashtable.. > What is the solution to this senario ? That would depend on the requirements for the classes themselves. If two classes are presumed to be the same when their Name properties are equal, then there is no solution. If two instances of your class can have the same value for the Name property, without reference equality, then your options are to either alter GetHashCode() or Equals() to account for this. -- Willem van Rumpt
From: Tony Johansson on 11 Feb 2010 07:45 "Willem van Rumpt" <nothing(a)nowhere.com> skrev i meddelandet news:OXGENMxqKHA.4636(a)TK2MSFTNGP06.phx.gbl... > Tony Johansson wrote: >> Hi! >> >> Assume I have 2 instances of a class called Test see below which will >> work as key in a hashtable. The Name property of instance1 is set to >> 'Paul', and the Name property of instance2 is set to 'Piet'. Both >> instances will generate different hashcode, and they're also different >> according to the Equals method.So far so good. When looking up Piet I >> will get the value object whatever it can be. >> Now, suppose that I change the Name of instance2 to 'Paul', then, >> according to my Equals method, both instances should be equal, and >> according to the hashcode and equals method will generate the same >> hashCode(same key) and that is not allowed >> to have in a hashtable.. >> What is the solution to this senario ? > > That would depend on the requirements for the classes themselves. > If two classes are presumed to be the same when their Name properties are > equal, then there is no solution. > > If two instances of your class can have the same value for the Name > property, without reference equality, then your options are to either > alter GetHashCode() or Equals() to account for this. > > -- > Willem van Rumpt H�! Assume I have a HashTable pupulated with element(key,value) but always keep the key unique can I then update all of these element and change both the key and value as I wish if I all the time keep the key unique ? //Tony
From: Willem van Rumpt on 11 Feb 2010 08:00 Tony Johansson wrote: > > H�! > > Assume I have a HashTable pupulated with element(key,value) but always keep > the key unique can I then > update all of these element and change both the key and value as I wish if I > all the time keep the key unique ? > I'm not sure if understand the question correctly. If you're asking if you can add an element to a hashtable, then alter (in what ever way) the hashcode (= key) for that element, while still being able to retrieve it from the hashtable it was previously added to, then, no. The value can be changed without consequences. -- Willem van Rumpt
From: Jeroen Mostert on 11 Feb 2010 10:41
On 2010-02-11 11:46, Tony Johansson wrote: > Assume I have 2 instances of a class called Test see below which will work > as key in a hashtable. The Name property of instance1 is set to 'Paul', and > the Name property of instance2 is set to 'Piet'. Both instances will > generate different hashcode, and they're also different according to the > Equals method.So far so good. When looking up Piet I will get the value > object whatever it can be. > Now, suppose that I change the Name of instance2 to 'Paul', then, according > to my Equals method, both instances should be equal, and according to the > hashcode and equals method will generate the same hashCode(same key) and > that is not allowed > to have in a hashtable.. > What is the solution to this senario ? > Don't change an object that's being used as a hashtable key in a way that changes its hash code. The best way to ensure this is to make the object immutable (making sure it doesn't change *ever*). You can achieve what you want by removing the old object, constructing a new one and adding that back to the table. If you must have a mutable object, remove it from any hashtables (or HashedSets or...) before changing it and add it back later. -- J. |