Wednesday, January 1, 2014
Hashing for collection's keys
Key objects must be immutable as long as they are used as keys in the Hashtable.
When an element is added to the Hashtable, the element is placed into a bucket based on the hash code of the key. Subsequent lookups of the key use the hash code of the key to search in only one particular bucket, thus substantially reducing the number of key comparisons required to find an element.
Any object that creates a hashvalue should change the hashvalue, when the object changes, but it must not - absolutely must not - allow any changes to itself, when it is used inside a Hashtable (or any other Hash-using object, of course).
So, just make GetHashCode result change, when the object data changes, and if the use of the object inside of hash using lists or objects is intended (or just possible) then make the object either immutable or create a readonly flag to use for the lifetime of a hashed list containing the object.
(By the way: All of this is not C# oder .NET specific - it is in the nature of all hashtable implementations, or more generally of any indexed list, that identifying data of objects should never change, while the object is in the list. Unexpected and unpredictable behaviour will occur, if this rule is broken. Somewhere, there may be list implementations, that do monitor all elements inside the list and do automatic reindexing the list - but the performance of those will surely be gruesome at best.)
What is GetHashCode used for?
It is by design useful for only one thing: putting an object in a hash table. Hence the name.
Remember, objects can be put into hash tables in ways that you didn't expect. A lot of the LINQ sequence operators use hash tables internally. Don't go dangerously mutating objects while enumerating a LINQ query that returns them!
GetHashCode may returns different values in time, on different processes and in different .NET versions.
GetHashCode is designed to do only one thing: balance a hash table. Do not use it for anything else.
See all thoughts at the Guidelines and rules for GetHashCode
GetHashCode Guidelines in C#
Guidelines and rules for GetHashCode