Comparing a nullable to null

Question

Basically, Nullable<T> is a structure, which explains things like calling .HasValue will never throw a NullReferenceException. I was wondering why - given a nullable which does not have a value - comparisons to null are always true, even when using Object.ReferenceEquals, which I thought would return false because it is a structure.

Is there special behaviour built into the CLR to make this possible? It would probably also explain why the generic struct constraint does not allow nullables.

Best Regards,
Oliver Hanappi

Answer 1

If you do:

int? x = null;
if (x == null)

that will use HasValue.

Using Object.ReferenceEquals will box the value first - and that will convert the null value into a null reference (which is indeed special CLR behaviour). When a nullable value type is boxed, the result is either a null reference or a box of the underlying value type. In other words, there's no such thing as a boxed value of a nullable value type.