Complex Objects as Hash Keys

  • Thread starter Thread starter Randolph Neall
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Randolph Neall

If I take a complex reference object and make it the key of a hash table,
can I be sure that the hash of that object will always remain the same
throughout the life of that object?

Every reference object has a reference pointer that is, I would guess, some
integer. If I'm right, that integer is what gets hashed when you make such
an object a key in a hash table. If I'm right, is that integer guaranteed to
be immutable throughout the life of the object, or does garbage collection
cause a change in this integer, such that the same object now has a new
reference number?

My concern is that if this reference integer does not remain constant,
entries in the Hashtable could become invalid.

Randy Neall
 
can I be sure that the hash of that object will always remain the same
throughout the life of that object?
Click to expand...

That depends on the implementation of GetHashCode(). For the default
implementation it's true.

Every reference object has a reference pointer that is, I would guess, some
integer.
Click to expand...

I don't quite understand what you mean here.

If I'm right, that integer is what gets hashed when you make such
an object a key in a hash table.
Click to expand...

Any data can be used to derive the hash. Again, it depends on the
GetHashCode() implementation.

or does garbage collection cause a change in this integer
Click to expand...

I don't think garbage collection could ever change the hash. But
making mutating changes to the object could.



Mattias
 
Thanks Mattias,

What I meant by "some integer" is the object's handle, presumably some
number.

So when I set the key of a hash table to a reference object, the hash value
that is used comes from that object's own GetHashKey method? And that hash
reflects both the "handle" of the object and the content of the object? Does
the "handle" always remain the same? Every object, I assume, has some handle
that should be independent of its particular values, fields, and properties.
What is it that gets hashed when you make a reference object the key of a
hash entry--the handle, the object's content (which can change), or both?
Under what conditions, if any, would the default hash of an object change
over its lifetime? What did you mean by "mutating changes?" Changes to
properties and fields?

Randy
 
Inline...

Mattias Sjögren said:
That depends on the implementation of GetHashCode(). For the default
implementation it's true.
Click to expand...

No it isn't -
http://msdn.microsoft.com/library/en-us/cpref/html/frlrfSystemObjectClassGetHashCodeTopic.asp.
Therefore you can't use the deault GetHashCode implementation for hashing.
I don't quite understand what you mean here.
Click to expand...

Not an integer but a pointer.
Any data can be used to derive the hash. Again, it depends on the
GetHashCode() implementation.
Click to expand...

Randolph is right and you can't use that for hashing. You have to implement
your own GetHashCode that will return the same value for a single object, no
matter what happens to that object.
I don't think garbage collection could ever change the hash. But
making mutating changes to the object could.
Click to expand...

Yes it does, for the default Object.GetHashCode implementation it does. Once
again, DO NOT USE IT in hash tables.
 
Hey Jerry,

Can't thank you enough for jumping in here. I've taken your message to
heart. We could have had some really vile bugs on our hands.

Thanks,

Randy Neall


Jerry Pisk said:
Inline...

Mattias Sjögren said:
That depends on the implementation of GetHashCode(). For the default
implementation it's true.
Click to expand...

No it isn't -
http://msdn.microsoft.com/library/en-us/cpref/html/frlrfSystemObjectClassGet
HashCodeTopic.asp.
Therefore you can't use the deault GetHashCode implementation for hashing.
I don't quite understand what you mean here.
Click to expand...

Not an integer but a pointer.
Any data can be used to derive the hash. Again, it depends on the
GetHashCode() implementation.
Click to expand...

Randolph is right and you can't use that for hashing. You have to implement
your own GetHashCode that will return the same value for a single object, no
matter what happens to that object.
I don't think garbage collection could ever change the hash. But
making mutating changes to the object could.
Click to expand...

Yes it does, for the default Object.GetHashCode implementation it does. Once
again, DO NOT USE IT in hash tables.
Click to expand...
 
No problem, I can't imagine the pain of trying to hunt down bugs caused by
the hash code changing...

Jerry

Randolph Neall said:
Hey Jerry,

Can't thank you enough for jumping in here. I've taken your message to
heart. We could have had some really vile bugs on our hands.

Thanks,

Randy Neall
Click to expand...
 
You might be right, I just went through the docs saying that the default
implementation isn't guaranteed to do that. That of course doesn't mean it
isn't, just that we shouldn't rely on it.

Jerry
 
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