C# Enums: Nullable or 'Unknown' Value?

Question

If I have a class with an enum member, and I want to be able to represent situations where this member is not defied, which is it better:

A) Declare the member as nullable in the class using nullable types. E.g.:

public SomeEnum? myEnum;

B) Add a default, 'unknown' value to the enumeration. E.g.:

public enum SomeEnum {
    Unknown,
    SomeValueA,
    SomeValueB,
    SomeValueC,
}

I can't really see any major pros/cons either way. But perhaps one is preferrable over the other?

Answer 1

Definitely use a nullable value type - that's what they're for. It explicitly states your intention. It also means you can use Enum.IsDefined (or the equivalent from Unconstrained Melody if you want generic type safety) to easily determine whether a particular value is a real value without worrying about the "fake" one too.

Answer 2

If it's nullable, you'd get it as a default value, you'd get exceptions if you tried to use it when null (which is a good thing!)

Answer 3

You just have to decide whether you need a value to represent unknown values, or you do need a way to represent absence of any value.

In the case of a need to represent unknown values, an extra enum-member sounds good as a solution.

In case of a need to represent absence of any value, make it nullable.

Have in mind, that there is nothing wrong with having both "unknown" enum-member and the enum itself being nullable at the same time.

HTH.