GetHashCode() problem using xor

Question

My understanding is that you're typically supposed to use xor with GetHashCode() to produce an int to identify your data by its value (as opposed to by its reference). Here's a simple example:

class Foo
{
    int m_a;
    int m_b;

    public int A
    {
        get { return m_a; }
        set { m_a = value; }
    }

    public int B
    {
        get { return m_b; }
        set { m_b = value; }
    }

    public Foo(int a, int b)
    {
        m_a = a;
        m_b = b;
    }

    public override int GetHashCode()
    {
        return A ^ B;
    }

    public override bool Equals(object obj)
    {
        return this.GetHashCode() == obj.GetHashCode();
    }
}

The idea being, I want to compare one instance of Foo to another based on the value of properties A and B. If Foo1.A == Foo2.A and Foo1.B == Foo2.B, then we have equality.

Here's the problem:

Foo one = new Foo(1, 2);
Foo two = new Foo(2, 1);

if (one.Equals(two)) { ... }  // This is true!

These both produce a value of 3 for GetHashCode(), causing Equals() to return true. Obviously, this is a trivial example, and with only two properties I could simply compare the individual properties in the Equals() method. However, with a more complex class this would get out of hand quickly.

I know that sometimes it makes good sense to set the hash code only once, and always return the same value. However, for mutable objects where an evaluation of equality is necessary, I don't think this is reasonable.

What's the best way to handle property values that could easily be interchanged when implementing GetHashCode()?

See Also

What is the best algorithm for an overridden System.Object.GetHashCode?

Answer 1

Hashing always involves collisions and you have to deal with it (f.e., compare hash values and if they are equal, exactly compare the values inside the classes to be sure the classes are equal).

Using a simple XOR, you'll get many collisions. If you want less, use some mathematical functions that distribute values across different bits (bit shifts, multiplying with primes etc.).