That should like it is a bug in how they are being put out

I tested but found that Intersect() first test for hash equality before calling Equal().

This is a reasonable implementation. What's your opinion?

The title of this post is very misleading.  It suggests that Equals never gets called when using Enumerable.Intersect(IEnumerable<T>, IEnumerable<T>, IEqualityComparer<T>).

My tests show that it does.  It calls IEqualityComparer.GetHashCode first, then T.GetHashCode() then eventually calls IEqualityComparer.Equals.

As the post above pointed out equals does eventual get called assuming you have correctly implemented GetHashCode.  i.e. so that the has codes for two equal objects are the same.  So you shouldn't get any false positives even with very large sets.  Presumably GetHashCode gets called because the set extension methods use a dictionary.  If you are using a custom equals method you have to create a custom GetHashCode that 'matches' for dictionary type collections to work properly.

Hmm "take note that it's the objects' GetHashCode() method that's determining uniqueness, not Equals" is not true in my point of view.

GetHashCode() is called first and only if 2 items (x,y)  return the same HashCode then Equals() is called. GetHashCode gives the comparer an indication whether its worth really comparing 2 objects by calling the Equals() method.

This makes a lot sense for large objects. Guess you want compare a hundred strings each 10 Megs.

Comparing each string by comparing their content (Equals) would be extremly slow.

So all of that blah, blah, blah for a wrong assertion.  Equals must be consistent with GetHashCode so the optimal approach is to call GetHashCode first.  All the blather about the odds of a duplicate is irrelevant, too. Nobody assumes GetHashCode is unique. In fact, if you like you can return 666 on every object, every time. That would be very, very silly (you've lost the speed benefits) but it's fully functional.

Why doesn t iequalitycomparer use equals in linq s intersect including a stroll down the lane of gethashcode and the birthday paradox.. Corking :)