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Overriding GetHashCode for mutable objects?

Asked 2009-05-17T00:53:35.423
63

I've read about 10 different questions on when and how to override GetHashCode but there's still something I don't quite get. Most implementations of GetHashCode are based on the hash codes of the fields of the object, but it's been cited that the value of GetHashCode should never change over the lifetime of the object. How does that work if the fields that it's based on are mutable? Also what if I do want dictionary lookups etc to be based on reference equality not my overridden Equals?

I'm primarily overriding Equals for the ease of unit testing my serialization code which I assume serializing and deserializing (to XML in my case) kills the reference equality so I want to make sure at least it's correct by value equality. Is this bad practice to override Equals in this case? Basically in most of the executing code I want reference equality and I always use == and I'm not overriding that. Should I just create a new method ValueEquals or something instead of overriding Equals? I used to assume that the framework always uses == and not Equals to compare things and so I thought it was safe to override Equals since it seemed to me like its purpose was for if you want to have a 2nd definition of equality that's different from the == operator. From reading several other questions though it seems that's not the case.

EDIT:

It seems my intentions were unclear, what I mean is that 99% of the time I want plain old reference equality, default behavior, no surprises. For very rare cases I want to have value equality, and I want to explicitly request value equality by using .Equals instead of ==.

When I do this the compiler recommends I override GetHashCode as well, and that's how this question came up. It se

c# .net overriding equals gethashcode
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2 Answers

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Wow, that's actually several questions in one :-). So one after the other:

it's been cited that the value of GetHashCode should never change over the lifetime of the object. How does that work if the fields that it's based on are mutable?

This common advice is meant for the case where you want to use your object as a key in a HashTable/dictionary etc. . HashTables usually require the hash not to change, because they use it to decide how to store & retrieve the key. If the hash changes, the HashTable will probably no longer find your object.

To cite the docs of Java's Map interface:

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map.

In general it's a bad idea to use any kind of mutable object as a key in a hash table: It's not even clear what should happen if a key changes after it's been added to the hash table. Should the hash table return the stored object via the old key, or via the new key, or via both?

So the real advice is: Only use immutable objects as keys, and make sure their hashcode never changes either (which is usually automatic if the object is immutable).

Also what if I do want dictionary lookups etc to be based on reference equality not my overridden Equals?

Well, find a dictionary implementation that works like that. But the standard library dictionaries use the hashcode&Equals, and there's no way to change that.

I'm primarily overriding Equals for the ease of unit testing my serialization code which I assume serializing and deserializing (to XML in my case) kills the reference equality so I want to make sure at least it's correct by value equality. Is this bad practice to override Equal

answered 2009-05-17T01:13:39.463
2

I don't know about C#, being a relative noob to it, but in Java, if you override equals() you need to also override hashCode() to maintain the contract between them (and vice-versa)... And java also has the same catch 22; basically forcing you use immutable fields... But this is an issue only for classes which are used as a hash-key, and Java has alternate implementations for all hash-based collections... which maybe not as fast, but they do effecitely allow you to use a mutable object as a key... it's just (usually) frowned up as a "poor design".

And I feel the urge to point out that this fundamental problem is timeless... It's been around since Adam was a lad.

I've worked on fortran code which is older than I am (I'm 36) which breaks when a username is changed (like when a girl gets married, or divorced ;-) ... Thus is engineering, The adopted solution was: The GetHashCode "method" remembers the previously calculated hashCode, recalculates the hashCode (i.e. a virtual isDirty marker) and if the keyfields have changed it returns null. This causes the cache to delete the "dirty" user (by calling another GetPreviousHashCode) and then the cache returns null, causing the user to re-read from the database. An interesting and worthwhile hack; even if I do say so myself ;-)

I'll trade-off mutability (only desirable in corner cases) for O(1) access (desirable in all cases). Welcome to engineering; the land of the informed compromise.

Cheers. Keith.

answered 2009-05-17T06:57:17.377

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