c# - Is it possible to combine hash codes for private members to generate a new hash code?

Question

I have an object for which I want to generate a unique hash (override GetHashCode()) but I want to avoid overflows or something unpredictable.

The code should be the result of combining the hash codes of a small collection of strings.

The hash codes will be part of generating a cache key, so ideally they should be unique however the number of possible values that are being hashed is small so I THINK probability is in my favour here.

Would something like this be sufficient AND is there a better way of doing this?

int hash = 0;
foreach(string item in collection){
    hash += (item.GetHashCode() / collection.Count)
}
return hash;

EDIT: Thanks for answers so far. @Jon Skeet: No, order is not important

I guess this is almost a another question but since I am using the result to generate a cache key (string) would it make sense to use a crytographic hash function like MD5 or just use the string representation of this int?

Answer 1

Hashes aren't meant to be unique - they're just meant to be well distributed in most situations. They're just meant to be consistent. Note that overflows shouldn't be a problem.

Just adding isn't generally a good idea, and dividing certainly isn't. Here's the approach I usually use:

int result = 17;
foreach (string item in collection)
{
    result = result * 31 + item.GetHashCode();
}
return result;

If you're otherwise in a checked context, you might want to deliberately make it unchecked.

Note that this assumes that order is important, i.e. that { "a", "b" } should be different from { "b", "a" }. Please let us know if that's not the case.