Should the hash code of null always be zero, in .NET

Question

Given that collections like System.Collections.Generic.HashSet<> accept null as a set member, one can ask what the hash code of null

Answer 1

Bear in mind that the hash code is used as a first-step in determining equality only, and [is/should]never (be) used as a de-facto determination as to whether two objects are equal.

If two objects' hash codes are not equal then they are treated as not equal (because we assume that the unerlying implementation is correct - i.e. we don't second-guess that). If they have the same hash code, then they should then be checked for actual equality which, in your case, the null and the enum value will fail.

As a result - using zero is as good as any other value in the general case.

Sure, there will be situations, like your enum, where this zero is shared with a real value's hash code. The question is whether, for you, the miniscule overhead of an additional comparison causes problems.

If so, then define your own comparer for the case of the nullable for your particular type, and ensure that a null value always yields a hash code that is always the same (of course!) and a value that cannot be yielded by the underlying type's own hash code algorithm. For your own types, this is do-able. For others - good luck :)