Diagnostics

Every rule the generator and the analyzers enforce. VO0008 and VO0011 are warnings; every other one is an error. There is no VO0012.

Id	Meaning	Fix
VO0001	The type is not partial.	Add partial: the generated members are added to the same type.
VO0002	The type is not a readonly struct, or is a record.	Declare a readonly partial struct. A record struct is refused on purpose, because with and field-wise equality would bypass validation and the declared comparison; a class, because a value object is a value.
VO0003	Unsupported underlying type.	Use one of the supported types.
VO0004	A bound could not be parsed.	Write Minimum and Maximum as invariant-culture text: "0", "9.99", "2020-01-01".
VO0005	A closed value set declares no value.	Add [KnownValue] entries, or drop ValueSet = ValueSetKind.Closed: as declared, no value could be valid.
VO0006	A known value has an unusable name.	The first argument of [KnownValue] becomes a member: give it a valid, unique C# identifier.
VO0007	Arithmetic requested on a non-numeric type.	Remove Arithmetic = true, or change the underlying type.
VO0008	Length constraints on a non-string type (warning).	MinLength and MaxLength apply to string only; use Minimum and Maximum for a number.
VO0009	A containing type is not partial.	Every enclosing type must be partial, not only the value object.
VO0010	An uninitialized value object: default(T) or new T().	Construct through Create, TryCreate or Parse, and express absence as T?. If the zero state is genuinely meaningful, set AllowDefault = true on the type.
VO0011	A rule written without its hook interface (warning).	Declare the interface — IValueObjectNormalizer<T>, IValueObjectValidator<T>, IValueObjectFormatter<T>, IValueObjectStringFormatter<T>. Until then the rule never runs.
VO0013	A known value could not be converted.	A Guid, a decimal or a DateOnly is written as invariant-culture text and converted at compile time; fix the text.
VO0014	An invalid regular expression.	Fix Pattern. A verbatim string (@"^\d+$") keeps escapes intact.
VO0015	A malformed entity identifier prefix.	One or more lower-case segments separated by _, each starting with a letter: "acc", "sk_live".
VO0016	Two types claim the same prefix.	Give each identifier type its own prefix, or one kind of identifier would parse as another.
VO0017	A normalization hook on an entity identifier.	[EntityId] normalizes its own format and would never call the hook: remove it.
VO0018	Both [EntityId] and [ValueObject<T>] on one type.	Each generates a whole implementation; keep the one that describes the type.

VO0011 deserves its warning more than most. The code it reports compiles and looks right, and in a project without TreatWarningsAsErrors it ships with the rule silently absent. It also recognizes the names hooks had before they became interfaces — NormalizeCore, ValidateCore, TryFormatCore, FormatCore — which should be renamed to NormalizeValue, ValidateValue, TryFormatValue and FormatValue.

Suppressing VO0010 in a test

A test that must build an uninitialized instance disables the diagnostic on the spot, with the reason:

#pragma warning disable VO0010 // The guard under test must reject an uninitialized instance.
var missing = default(Iban);
#pragma warning restore VO0010