C# Enums: Nullable or 'Unknown' Value?

Solution 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.

Solution 2:

I agree with archimed7592 that there is a real difference between absent value and "Unknown" value.

Example:

"what is your blood type?"

• Null = absent value --> question hasn't been answered --> ask the question

• "Unknown" --> patient indicates he doesn't know --> order lab test for the blood type