[rvdginste]

I use the convention that an 'null' value is a value that was not explicitly set by the user.

So in C#, I would always use a nullable boolean in this case (choices yes and no). If the field is a required value, I would annotate the field as such.

When using the field in an ORM, the required annotation would lead to a not-null field in the database.

When using the field in a dto, the required annotation could be used for validation of incoming input.

If the user should have the choice "yes", "no", "unknown", I would use a nullable enum to express this. An 'null' value means that the value was not explicitly set. The enum value "unknown" indicates that the user explicitly chose the "unknown" value.

So, in both cases the 'null' is not a separate state, it simply indicates that no explicit value was given to the field.