Question

String.GetHashCode’s behavior is depend on the program architecture. So it will return one value in x86 and one value on x64. I have a test application which must run in x86 and it must predict the hash code output from an application which must run on x64.

Below is the disassembly of the String.GetHashCode implementation from mscorwks.

public override unsafe int GetHashCode()
{
      fixed (char* text1 = ((char*) this))
      {
            char* chPtr1 = text1;
            int num1 = 0x15051505;
            int num2 = num1;
            int* numPtr1 = (int*) chPtr1;
            for (int num3 = this.Length; num3 > 0; num3 -= 4)
            {
                  num1 = (((num1 << 5) + num1) + (num1 >≫ 0x1b)) ^ numPtr1[0];
                  if (num3 <= 2)
                  {
                        break;
                  }
                  num2 = (((num2 << 5) + num2) + (num2 >> 0x1b)) ^ numPtr1[1];
                  numPtr1 += 2;
            }
            return (num1 + (num2 * 0x5d588b65));
      }
}

Can anybody port this function to a safe implementation??

Answer 1

Hash codes are not intended to be repeatable across platforms, or even multiple runs of the same program on the same system. You are going the wrong way. If you don’t change course, your path will be difficult and one day it may end in tears.

What is the real problem you want to solve? Would it be possible to write your own hash function, either as an extension method or as the GetHashCode implementation of a wrapper class and use that one instead?