c# - 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 of code.

When I step through my code, the code goes through my GetHashCode implementation but not the Equals? I do not really understand the GetHashCode method, despite reading around on the internet and what exactly it is doing.

List<FactorPayoffs> missingfactorPayoffList = 
    factorPayoffList.Except(
        factorPayoffListOrg,
        new FactorPayoffs.Comparer()).ToList();

List<FactorPayoffs> missingfactorPayoffListOrg =
    factorPayoffListOrg.Except(
        factorPayoffList,
        new FactorPayoffs.Comparer()).ToList();

So in the two lines of code above the two lists return me every item, telling me that the two lists do not contain any items that are the same. This is not true, there is only row that is different. I'm guessing this is happening because the Equals method is not getting called which in turns makes me wonder if my GetHashCode method is working as its supposed to?

class FactorPayoffs
    {
        public string FactorGroup { get; set; }
        public string Factor { get; set; }
        public DateTime dtPrice { get; set; }
        public DateTime dtPrice_e { get; set; }
        public double Ret_USD { get; set; }

        public class Comparer : IEqualityComparer<FactorPayoffs>
        {
            public bool Equals(FactorPayoffs x, FactorPayoffs y)
            {                    
                return x.dtPrice == y.dtPrice && 
                    x.dtPrice_e == y.dtPrice_e && 
                    x.Factor == y.Factor && 
                    x.FactorGroup == y.FactorGroup;
            }

            public int GetHashCode(FactorPayoffs obj)
            {
                int hash = 17;
                hash = hash * 23 + (obj.dtPrice).GetHashCode();
                hash = hash * 23 + (obj.dtPrice_e).GetHashCode();
                hash = hash * 23 + (obj.Factor ?? "").GetHashCode();
                hash = hash * 23 + (obj.FactorGroup ?? "").GetHashCode();
                hash = hash * 23 + (obj.Ret_USD).GetHashCode();
                return hash;
            }
        }
    }

Answer 1

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).