HashSet.IntersectWith() returning 0 results

Question

I am trying to grab common values between two HashSets of the same type with identical lists, yet I get 0 values returned after the IntersectWith command. I am using the same list to begin with as a troubleshooting test, but eventually the value of returnlist will change as int y iterates through the sequence.

Debugging shows that comparelist.IntersectWith(returnlist); changes comparelist to 0 items. Just to clarify, returnlist and comparelist contain the same items in the same order.

CfgPersonQuery firstquery = new CfgPersonQuery();
firstquery.Filter.Add("skill_dbid", skills.First());
comparelist = new HashSet<CfgPerson>(confService.RetrieveMultipleObjects<CfgPerson>(firstquery));
foreach (int y in skills.Skip(1))
{
    try
    {
        CfgPersonQuery query = new CfgPersonQuery();
        query.Filter.Add("skill_dbid", skills.First());
        HashSet<CfgPerson> returnlist = new HashSet<CfgPerson>(
            confService.RetrieveMultipleObjects<CfgPerson>(query));
        comparelist.IntersectWith(returnlist);
    }
    catch
    {                    
        return null;
    }
}

Answer 1

Since you do not own CfgPerson class and you cannot implement Equals method you should tell HashSet how to determine equality for that type. You can create HashSet using constructor which takes IEqualityComparer<T> as parameter.

So:

1) create class CfgPersonEqualityComparer : IEqualityComparer<CfgPerson> (need to read documentation how to do it)
2) var comparer = new CfgPersonEqualityComparer()
3) var hashSet1 = new HashSet(collection1, comparer)
4) var hashSet2 = new HashSet(collection2, comparer)
5) var result = hashSet1.IntersectWith(hashSet2) (it will automatically detect that both HashSets use the same equality comparers)

Or just create wrapper around CfgPerson which implements GetHasCode, Equals and IEquatable<CfgPerson>.