One of the devs on the BCL team just added a bit to my recent post on hashcodes  Enjoy!


Brad's comment above applies to Object's GetHashCode implementation, which most interesting classes override, providing their own hash function. We believe GetHashCode should be used as a hash function that returns a seemingly-random value that could be negative or duplicated for multiple values. In V1, Object's GetHashCode unfortunately gave some stronger guarantees than this that a few people wanted to depend on, but that wasn't in the contract of the method. Their code is already broken on version 2 (we think the only people that depended on this were internal the company, and they would have long since found their bug & corrected it).

Note that we also don't want user code taking a dependency on our existing hash function implementations for any type - ideally we could change them every time we build the product. To elaborate on that, let's look at String.

String uses a different hash function that looks at each character, XOR'ing in the new character with a (presumably prime) number. We'll change String's hash function in a future version so it both executes faster and produces a better distribution. This will improve lookups in hash tables when using strings as keys. But because we'll change the hash function, it is also important to not depend on one particular version's implementation of GetHashCode. IE, never write the values you get back from GetHashCode to disk and read them back later, or sort values based on their hash function then persist that data to a file or send it over a network.

Brian Grunkemeyer
MS CLR Base Class Library team