How to get same hash value also with different instances [duplicate]

Question

I'm setting ID of database entry's by creating and using a hash value. The problem is that when I start the application again, the hash value of the same origin values are different and I get a doubling (same values, different ID). Below you find my example code. Start the CLI, remember the given hash value, start again --> different value.

How can I reproduce the same hash value with each instance?

static void Main(string[] args)
{
    int drid = 3081;
    DateTime dt = DateTime.ParseExact("2019-04-11 00:23:10", "yyyy-MM-dd HH:mm:ss", null);
    string idAsString = drid.ToString() + dt.ToString();
    Console.WriteLine(idAsString.GetHashCode().ToString());
    Console.ReadKey();
}

I too always have the same value 745858392. Howevever, I suggest you change dt.ToString() to something like dt.ToString("s") or another culture invariant date format — Justin Lessard
– Justin Lessard, Commented Apr 11, 2019 at 17:09
@CamiloTerevinto It is specifically documented as not being consistent across different executions of the application. — Servy
– Servy, Commented Apr 11, 2019 at 17:11
This blog post might help: Why is string.GetHashCode() different each time I run my program in .NET Core? — IronGeek
– IronGeek, Commented Apr 11, 2019 at 17:28
This question is a duplicate of I think my answer I found there: stackoverflow.com/questions/5154970/… — Frank Mehlhop
– Frank Mehlhop, Commented Apr 12, 2019 at 6:42

Zazaeil · Accepted Answer · 2019-04-11 17:12:33Z

0

Do not do this. Never ever ever. Hash codes are far not unique and semantically different objects at some point will result to a collision - i.e. both will produce exactly the same hashcode, which, in turn, will break down your database.

Next thing to notice is that hashing objects is complicated. It is good to keep hashcode the same as long as your object lives, which implies that you compute it once - when the object is created - or compute it on the fly ignoring mutable fields (since generally each mutation will change the hashcode).

Even more than that: it isn't easy to come up with good enough hashing algorithm: the one which is unpredictable enought and yet collision-rare.

edited Apr 11, 2019 at 17:12

answered Apr 11, 2019 at 17:09

Zazaeil

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7 Comments

Camilo Terevinto Over a year ago

While this is correct, using something like SHA2-256 or SHA2-512 (.NET does't have SHA3 yet) would make collisions far less likely

Zazaeil Over a year ago

@CamiloTerevinto, it does not change the thing. Id is unique, hashes are not.

Servy Over a year ago

"Even more than that: it isn't easy to come up with good enough hashing algorithm: the one which is unpredictable enought and yet collision-rare." It's not that hard. Avoiding collisions mostly comes down to just increasing the size of the hash, as long as the hashing algorithm isn't truly bad. Hashing to an int is never going to have a low collision rate for any non-trivial collection size, but it doesn't need to be that big for the odds of a collision to be low enough that the sun is more likely to explode before a collision happens, which tends to be good enough for most purposes.

Zazaeil Over a year ago

@Servy, we are not discussing hashing algorithm itself, rathether the way you implement GetHashCode() on a particular (complicated?) object.

Servy Over a year ago

@SerejaBogolubov No, you do not in fact need to inevitable call GetHashCode to compute a hash of a string. You can absolutely write code to compute a hash without using it. That you think it's impossible to write any hashing algorithm besides string.GetHashCode makes no sense. Why do you think that's the only possible way to hash anything?

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