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Is Object.GetHashCode() unique to a reference or a value?

Asked 2008-08-29T15:54:15.870
26

The MSDN documentation on Object.GetHashCode() describes 3 contradicting rules for how the method should work.

  1. If two objects of the same type represent the same value, the hash function must return the same constant value for either object.
  2. For the best performance, a hash function must generate a random distribution for all input.
  3. The hash function must return exactly the same value regardless of any changes that are made to the object.

Rules 1 & 3 are contradictory to me.

Does Object.GetHashCode() return a unique number based on the value of an object, or the reference to the object. If I override the method I can choose what to use, but I'd like to know what is used internally if anyone knows.

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2 Answers

5

Not sure what MSDN documentation you are referring to. Looking at the current documentation on Object.GetHashCode (http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx) provides the following "rules":

  • If two objects compare as equal, the GetHashCode method for each object must return the same value. However, if two objects do not compare as equal, the GetHashCode methods for the two object do not have to return different values.

  • The GetHashCode method for an object must consistently return the same hash code as long as there is no modification to the object state that determines the return value of the object's Equals method. Note that this is true only for the current execution of an application, and that a different hash code can be returned if the application is run again.

  • For the best performance, a hash function must generate a random distribution for all input.

If you are referring to the second bullet point, the key phrases here are "as long as there is no modification to the object state" and "true only for the current execution of an application".

Also from the documentation,

A hash function is used to quickly generate a number (hash code) that corresponds to the value of an object. Hash functions are usually specific to each Type and must use at least one of the instance fields as input. [Emphasis added is mine.]

As for the actual implementation, it clearly states that derived classes can defer to the Object.GetHashCode implementation if and only if that derived class defines value equality to be reference equality and the type is not a value type. In other words, the default implementation of Object.GetHashCode is going to be based on reference equalit

answered 2008-08-29T16:09:29.690
4

Rules 1 & 3 aren't really a contradiction.

For a reference type the hash code is derived from a reference to the object - change an object's property and the reference is the same.

For value types the hash code is derived from the value, change a property of a value type and you get a completely new instance of the value type.

answered 2008-08-29T16:11:24.240

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