32-bit GetHashCode() on a 64-bit .NET 4.5 CLR

Question

On a 64-bit .NET 4.5 platform, is there anyway to compute what would be the results of a string's GetHashCode() method on a 32-bit .NET 4.5 platform?

It is obvious this is not a good idea. That has been asked and explained here and here. Only an answer that says how to do it will be accepted.

Why do you want to know how to do it? `GetHashCode()` is only deterministic over one run of the program (in a single app domain). I don't see how your question is any different than the two you linked, the answer is "You can't". — Scott Chamberlain, Mar 31 '14 at 23:24
The database the application is running over requires it. It was a bad design decision by the original developer. Knowing the answer to this question will be the easiest way to fix it. — Joshcodes, Mar 31 '14 at 23:28
Please do not edit in answers to the question. This question is getting votes to reopen. I'm looking to vote that way myself, perhaps. — Andrew Barber, Apr 01 '14 at 15:35
After reading the question this was marked duplicate of, I'm having a difficult time seeing how the accepted answer there does not answer this question, with simply the additional information that there is no difference based on whether it's running 64/32 bit. — Andrew Barber, Apr 01 '14 at 15:37
Agreed, one of the "duplication question"'s answers does answer this question. However, it is a different question and the proof is in the fact that there are potential answers to this question that do not answer the other question and vice-versa. Imagine there was a set of build flags that would cause 64-bit to function like 32 bit. That would answer this questions but not the other one. An answer for how GetHashCode functions in .NET 5 or 7 would answer the other question but not answer this question. — Joshcodes, Apr 01 '14 at 15:55

score 3 · Answer 1 · answered Mar 31 '14 at 23:35

GetHashCode() is not 32 bit nor 64 bit specific so calling it on a 32 bit system is the same as calling it on a 64 bit system.

It appears you are trying to reuse a hashcode for some kind of serialization purposes. You can't reliably do that. If it makes it easier, think of string's GetHashCode() function as the following.

static Dictionary <string, int> HashCodes = new Dictionary<string, int>();
static Random Rand = new Random();

static int PsudoGetHashCode(string stringInQuestion)
{
    lock(HashCodes)
    {
        int result;
        if(!HashCodes.TryGetValue(stringInQuestion, out result)
        {
            result = Rand.Next();
            HashCodes[stringInQuestion] = result;
        }

        return result;
    }
}

You are only guaranteed to get the same GetHashCode() value for the same string per run of the program, if you close the program and re-open it you can very possibly get a new value.

Your only solution that will give you reliable results is fix the design of your database to not store the built in GetHashCode() from .NET

32-bit GetHashCode() on a 64-bit .NET 4.5 CLR

1 Answers1