Is Nullable DateTime work correct?

Question

Please check following code

DateTime? tmp = new DateTime();
tmp = null;
return tmp.ToString();

It returns String.Empty.

Is it correct?

May be it will be better to rise exception in second line of code

Spahi

Answer 1

Yes, it's correct. From the documentation

The text representation of the value of the current Nullable<T> object if the HasValue property is true, or an empty string ("") if the HasValue property is false.

Note also that Nullable<T>.Equals and Nullable<T>.GetHashCode do not throw in this case but that Nullable<T>.GetType does throw. This is because Object.Equals, Object.GetHashCode and Object.ToString are overridden for Nullable<T> but that Object.GetType is not (because it can not be as it is not marked as virtual).

Edmondson

ok, thanks for you answer. but how you think is it correct way? because if you will use String tmp = null; tmp.ToString() it will rise exception – Spahi

Because a nullable type with HasValue as false is not a null reference. From a conceptual perspective, a nullable type represents a value type with the possibility of the value being "missing." We use null to represent when the value is missing, but this is not the same as a null reference. Note that tmp.Value will throw in the case of tmp being an instance of a nullable type with HasValue as false. The value is missing so trying to obtain said value should and does throw accordingly. – Edmondson