Generate hash code based on object identity in .net

Question

How can I generate a hash code for an object based on its identity.

What I mean is that:

if object.ReferenceEquals(a, b) == true, then a and b will get the same hash code.
if object.ReferenceEquals(a, b) == false, then a and b should have a decent chance to get different hash codes even if they are memberwise equal.

What I have is:

class SomeClassThatMakesSenseToCompareByReferenceAndByValue { override Equals(object o) { return MemberwiseEquals(o); } override GetHashCode() { return MemberwiseGetHashCode(); } } class SomeClassThatNeedsReferenceComparison { SomeClassThatMakesSenseToCompareByReferenceAndByValue obj; override Equals(object o) { return o is SomeClassThatNeedsReferenceComparison && object.ReferenceEquals(this.obj, (o as SomeClassThatNeedsReferenceComparison).obj); } override GetHashCode() { return ????? } }

It may help to explain in a bit more detail your use cases for this. — Rowland Shaw
– Rowland Shaw, Commented Aug 23, 2010 at 11:15
"[...] get different hash codes even if they are memberwise equal" - that sounds a bit odd. Consider the following text from the GetHashCode documentation (under "Notes to implementers"): "If two objects compare as equal, the GetHashCode method for each object must return the same value". I may miss something though. — Fredrik Mörk
– Fredrik Mörk, Commented Aug 23, 2010 at 11:40
That's why my SomeClassThatMakesSenseToCompareByReferenceAndByValue class returns a consistent hash code. But in my other class, I want to use GetHashCode AND Equals based on identity. — erikkallen
– erikkallen, Commented Aug 23, 2010 at 12:01

Miserable Variable · Accepted Answer · 2010-08-23 11:14:25Z

3

You are probably looking for RuntimeHelpers.GetHashCode

answered Aug 23, 2010 at 11:14

Miserable Variable

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2 Comments

Andrey Over a year ago

it is complete equivalent for object.GetHashCode

Miserable Variable Over a year ago

@Andrey it is equivalent only if the dynamic type of object does not override GetHashCode. @erikkallen you are welcome.

Andrey · Accepted Answer · 2010-08-23 11:07:31Z

2

if you don't override GetHashCode it will return that identic hash code.

answered Aug 23, 2010 at 11:07

Andrey

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4 Comments

erikkallen Over a year ago

Yes, but I need access to that functionality in another way.

Martin Harris Over a year ago

@erikkallen In what other way?

Fredrik Mörk Over a year ago

given that object.ReferenceEquals(a, b) == true I would say that hardly any implementation of GetHashCode would yield a different result for a and b (it would basically require involving random elements or time).

Andrey Over a year ago

@Fredrik Mörk that's true. i wanted to point to that default implementation will return different hash codes for memberwise equal objects.

Martin Harris · Accepted Answer · 2010-08-23 11:09:16Z

Don't do anything - since both objects point to the same instance the same HashCode will always be generated for both objects using the default implementation.

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