When Implementing IEqualityComparer Should GetHashCode check for null?
The documentation says that null values should never be hashable, and that attempting to do so should always result in an exception.
Of course, you're free to do whatever you want. If you want to create a hash based structure for which null keys are valid, you're free to do so, in this case you should simply ignore this warning.
ReSharper is wrong.
Obviously code you write can call that particular GetHashCode method and pass in a null value. All known methods might ensure this will never happen, but obviously ReSharper can only take existing code (patterns) into account.
So in this case, check for null and do the "right thing".
Corollary: If the method in question was private, then ReSharper might analyze (though I'm not sure it does) the public code and verify that there is indeed no way that this particular private method will be called with a null reference, but since it is a public method, and one available through an interface, then
ReSharper is wrong.