IEqualityComparer GetHashCode being called but Equals not

Question

I have two lists that I am trying to compare. So I have created a class that implements the IEqualityComparer interface, please see below in the bottom section

Answer 1

If you want to force the execution of the Equals you can implement it as follows

public int GetHashCode(FactorPayoffs obj) {
        return 1;
    }

Answer 2

GetHashCode is intended as a fast but rough estimate of equality, so many operations potentially involving large numbers of comparisons start by checking this result instead of Equals, and only use Equals when necessary. In particular, if x.GetHashCode()!=y.GetHashCode(), then we already know x.Equals(y) is false, so there is no reason to call Equals. Had x.GetHashCode()==y.GetHashCode(), then x might equal y, but only a call to Equals will give a definite answer.

If you implement GetHashCode in a way that causes GetHashCode to be different for two objects where Equals returns true, then you have a bug in your code and many collection classes and algorithms relying on these methods will silently fail.

Answer 3

Your Equals and GetHashCode implementations should involve the exact same set of properties; they do not.

In more formal terms, GetHashCode must always return the same value for two objects that compare equal. With your current code, two objects that differ only in the Ret_USD value will always compare equal but are not guaranteed to have the same hash code.

So what happens is that LINQ calls GetHashCode on two objects you consider equal, gets back different values, concludes that since the values were different the objects cannot be equal so there's no point at all in calling Equals and moves on.

To fix the problem, either remove the Ret_USD factor from GetHashCode or introduce it also inside Equals (whatever makes sense for your semantics of equality).

Answer 4

Rewrite you GetHashCode implementation like this, to match the semantics of your Equals implementation.

public int GetHashCode(FactorPayoffs obj)
{
    unchecked
    {
            int hash = 17;
            hash = hash * 23 + obj.dtPrice.GetHashCode();
            hash = hash * 23 + obj.dtPrice_e.GetHashCode();
            if (obj.Factor != null)
            {
                hash = hash * 23 + obj.Factor.GetHashCode();
            }

            if (obj.FactorGroup != null)
            {
                hash = hash * 23 + obj.FactorGroup.GetHashCode();
            }

            return hash;
    }
}

Note, you should use unchecked because you don't care about overflows. Additionaly, coalescing to string.Empty is pointlessy wasteful, just exclude from the hash.

See here for the best generic answer I know,