How IEnumerable.Except() works?

Question

I'm trying to exclude entities to be added to database if they already exist there. So I decided newBillInstances.Except(dbContext.BillInstances) would be best approach for that. However it doesn't work at all (no entities are excluded) though for List<string> it works perfectly.

I read this discussion and actual decription of .Except() in MSDN. It states the class to be used in .Except() should implement IEqualityComparer<T> to use default comparer.

Actually the MSDN article doesn't fully describe process of comparison of two instances. I still don't get why both Equals() and GetHashObject() have to be overridden.

I have implemented IEqualityComparer<BillInstance> interface and put break points in boths methods, but while calling .Except(IEnumerable) it's not used. Only when I changed to .Except(IEnumerable, new BillInstanceComparer()) I've cough break in GetHashCode() but no breaks where in Equals().

Then I have implemented IEqualityComparer<BillInstance> right in BillInstance class and expected it would be used while using .Except(IEnumerable) but breaks weren't hit in both methods.

So I've got two questions:

1. What should be done to use .Except(IEnumerable)?
2. Why Equals() isn't used at all? Is it used only in case hash codes of two instances are same?

Answer 1

Because the Equals() is used only if two objects have the same GetHashCode(). If there are no objects that have the same GetHashCode() then there is no chance of using the Equals().

Internally the Except() uses a Set<> (you can see it here), that is an internal class that you should consider to be equivalent to HashSet<>. This class uses the hash of the object to "index" them, then uses the Equals() to check if two objects that have the same hash are the same or different-but-with-the-same-hash.

Link to other relevant answer: https://stackoverflow.com/a/371348/613130