How do you implement GetHashCode for structure with two string, when both strings are interchangeable

Question

I have a structure in C#:

public struct UserInfo
{
   public string str1
   {
     get;
     set;
   }

   public string str2
   {
     get;
     set;
   }           


        

Answer 1

Try out this one:

(((long)str1.GetHashCode()) + ((long)str2.GetHashCode())).GetHashCode()

Answer 2

Many possibilities. E.g.

return str1.GetHashCode() ^ str1.GetHashCode()

Answer 3

Sort them, then concatenate them:

return ((str1.CompareTo(str2) < 1) ? str1 + str2 : str2 + str1)
    .GetHashCode();

Answer 4

Going along the lines ReSharper is suggesting:

public int GetHashCode()
{
    unchecked
    {
        int hashCode;

        // String properties
        hashCode = (hashCode * 397) ^ (str1!= null ? str1.GetHashCode() : 0);
        hashCode = (hashCode * 397) ^ (str2!= null ? str1.GetHashCode() : 0);

        // int properties
        hashCode = (hashCode * 397) ^ intProperty;
        return hashCode;
    }
}

397 is a prime of sufficient size to cause the result variable to overflow and mix the bits of the hash somewhat, providing a better distribution of hash codes. Otherwise there's nothing special in 397 that distinguishes it from other primes of the same magnitude.

Answer 5

Perhaps something like str1.GetHashCode() + str2.GetHashCode()? or (str1.GetHashCode() + str2.GetHashCode()) / 2? This way it would be the same regardless of whether str1 and str2 are swapped....

Answer 6

1. As a general rule, a simple way to generate a hashcode for a class is to XOR all the data fields that can participate in generating the hash code (being careful to check for null as pointed out by others). This also meets the (artificial?) requirement that the hashcodes for UserInfo("AA", "BB") and UserInfo("BB", "AA") are the same.

2. If you can make assumptions about the use of your class, you can perhaps improve your hash function. For example, if it is common for str1 and str2 to be the same, XOR may not be a good choice. But if str1 and str2 represent, say, first and last name, XOR is probably a good choice.

Although this is clearly not meant to be a real-world example, it may be worth pointing out that: - This is probably a poor example of use of a struct: A struct should normally have value semantics, which doesn't seem to be the case here. - Using properties with setters to generate a hash code is also asking for trouble.