Where a default instance can come from at run time

A non-default instance is normalized and valid by construction, and VO0010 keeps the default out of source: it refuses default(T) and new T() where they are written, and VO0032 does the same in the code Riok.Mapperly and the configuration binding generator write. Neither can see what other code does at run time. A reflection-based serializer that finds no settable member, a binder that falls back to the type's default, a source generator that cannot see the generated members: each can build a value object without going through Create. Some hand the application a default instance, IsDefault true. Others hand it an instance that looks initialized and breaks its own rules.

Each source below was observed in a probe against packages built from this repository's sources, with the version tested, unless its row says otherwise. Several are another library's default, stated as it stands, and a later version of that library may change it. What this library's own writers do with such an instance is in A value refused on write: they refuse to write a value its type rejects.

Sources of a default instance

Source	What reaches the application	Fix
An array element, or a field or a property never assigned	default(T). VO0010 sees only a default or a new written in source.	Express absence as T?, and guard at the edge.
A JSON property missing from the payload, under System.Text.Json	default(T), which no rule checks: {"OrderId":"3f2504e0-…"} read into a record of three value objects leaves the other two default.	RespectRequiredConstructorParameters = true for a positional record, which turns the missing members into a JsonException; [JsonRequired] or C# required for a settable property. JSON has the recommendation.
Riok.Mapperly 4.3.1, mapper in the value objects' own project	var target = new global::Iban(); return target; for every mapping into a value object.	VO0032 fails the build. Give Mapperly static mapping methods that call Create.
Mapperly, value objects in a referenced project	Mappings into a value object validate, through Create or Parse, but OrderId to Guid maps to new Guid(), Guid.Empty.	Static mapping methods out of the value objects too, returning .Value.
The configuration binding generator, which PublishAot turns on	new global::Domain.Iban(): every value-object property of the options is default, even with valid input.	VO0032 fails the build. Keep the underlying types in the options class.
The reflection-based configuration binder, a missing key	A default instance, silently: Iban=''.	Declare the property T? and check it, or validate the options, with FluentValidation's NotDefault for one.
Bogus Faker<T> with no rule for a property	Every value-object property left without a rule is default.	StrictMode(true), which refuses a Faker<T> that leaves a property without a rule, and RuleForValueObjects() from AdCodicem.ValueObjects.Bogus, which gives each a value its rules accept.
MongoDB.Driver 3.12, no serializer registered	Written as {}, read back as default, and x.Iban == iban matches every document.	AdCodicem.ValueObjects.MongoDB: a value object is stored as its underlying value, and a default its type rejects is refused on write.
MongoDB.Driver, a field missing from a stored document	The class map leaves the member as it was: a default instance, which no rule checks.	[BsonRequired] on the member, which makes the driver refuse the document: MongoDB. The validator built from the rules lists the member as required, so the server refuses to store such a document in the first place.
MongoDB.EntityFrameworkCore 10.0.4 without ConfigureValueObjects	Written as {}, read back as default.	Call ConfigureValueObjects. It is mandatory with this provider, while a relational provider refuses to build a model without it.
Microsoft.Extensions.AI 10.10, a tool argument sent as JSON null, which the OpenAI adapter hands over as a C# null	AIFunctionFactory's binding passes the null on to a value-object parameter that cannot be null, which then holds a default instance, without calling the converter.	WithValueObjectValidation() from AdCodicem.ValueObjects.AI, which answers it value_object.required, as a result the model reads.
SignalR, MessagePack hub protocol	{} on the wire: the hub receives Sku='' Quantity=0.	UseValueObjects() in AddMessagePackProtocol, on the server and on the .NET client: the values travel bare and are read through their rules.
MessagePack, ContractlessStandardResolver	{"Sku":{}}, read back as default.	WithValueObjects() on the options: a value object is written as its underlying value, and read through its rules.
XmlSerializer, and the MVC XML formatters built on it, in an assembly that does not opt in	An empty element on write; a default on read, even from an element with content.	[assembly: ValueObjectXmlSerialization]: the value objects implement IXmlSerializable, XML.
DataContractSerializer, CoreWCF, Dapr actor remoting, in an assembly that does not opt in	As XmlSerializer.	As XmlSerializer; for Dapr actors, also the JSON serialization Dapr documents for actor remoting (not run here): Dapr.
XmlSerializer or DataContractSerializer, an element missing from the document	Nothing reads the missing element: the member keeps the default instance its container left.	[DataMember(IsRequired = true)] with DataContractSerializer; with XmlSerializer, check the member after reading. XML.
CsvHelper 33.1.0	A Value column per value object on write; a default on read. The probe also wrote an IsDefault column, a public property then, and no longer is (inferred, not re-run).	A DefaultTypeConverter of your own over TryParse.
YamlDotNet 18.1.0, static builder	Iban: {}, and one YDNG001 warning per value object.	A typed IYamlTypeConverter of your own.
ServiceStack.Text 10.3.0	A value the type rejects becomes a default: {"Quantity":500} reads Quantity=0.	JsConfig.ThrowOnError = true.
LiteDB	{"Value":"FR76…"} written, a default read. The probe also wrote "IsDefault":false, a public property then, and no longer is (inferred, not re-run).	BsonMapper.RegisterType, writing .Value and reading through a factory that calls Create.
Azure.Data.Tables	Reading into a class leaves every value-object property default; writing one throws.	Read into an entity of underlying types, then TryCreate.
MassTransit 8.5.11, message initializer fed raw values	The initializer drops what it cannot convert, and the message holds defaults. Under Newtonsoft.Json without the converter, it travels as {} and is consumed as defaults. Under System.Text.Json it travelled as {"orderId":"00000000-…","customer":"","quantity":0}; the converter refuses to write a default its type rejects, so such a message now fails to publish with a JsonException, and only a value object accepting its zero travels as zero (inferred from the converter, not run).	Pass value objects to the initializer, or publish concrete message types: MassTransit.
Wolverine.Http 6.45.0, query string	A missing value binds the default; so does a rejected one under the default RejectUnparseableQueryValues = false.	RejectUnparseableQueryValues = true, the default from Wolverine 7, and T? for an optional parameter: Wolverine.
The Request Delegate Generator 10.0.12, minimal API binding	Empty query text, ?quantity=, binds the default to a parameter that cannot be null, or to an array element, and the handler runs: the RDG refuses no empty query text, where the reflection-based binding answers 400.	Declare the parameter T?, which it binds to null, and check it: the Request Delegate Generator.
Spectre.Console.Cli, an option left out	A non-nullable value-object option is default.	Declare the option T?, or make the value a required <arg>: Spectre.Console.Cli.
Blazor @bind	Emptying the input calls the setter with default(Iban); an invalid input is ignored, with no message.	Bind to a string, and parse it.
Quartz 4.3.0, job data	A value the type rejects leaves the property default and the job runs, under the default PropertyMismatchBehavior.Ignore; a null entry gives a default too.	PropertyMismatchBehavior.Throw, or IJob<TInput> with required members: Quartz.
JSON Patch remove, System.Text.Json and Newtonsoft.Json versions, 10.0.12	A non-nullable value-object property is set to its default.	Declare the property T?, or check the operations before ApplyTo.
ASP.NET Core OData 9.5.0	Every value object written as {}.	Not supported: project to classes of underlying types.
SQL Server dynamic data masking, an integer column	A masked value reads as 0, a default Quantity.	Strict reads, or no value object on a masked column.

Sources of an initialized but invalid instance

IsDefault is false for these, so a check for the default does not catch them:

Source	What reaches the application	Fix
SQL Server dynamic data masking, a string column	DEXXXXXXXX3000, through the default read, which trusts the column: an Iban its rules refuse, IsDefault false.	Strict reads, ConfigureValueObjects(strict: true, …).
MassTransit.MessagePack	Each value object written as a wrapper holding Value and the private _value, and read back without validation.	Not covered: use the System.Text.Json serializer.

Closed by the library

These were sources too, and no longer are:

Source	What happens now
AutoMapper 16, no explicit map	The mapping throws AutoMapperMappingException, "Missing type map configuration or unsupported mapping.", since no constructor takes the value alone. Map each type: CreateMap<string, Iban>().ConvertUsing(s => Iban.Create(s)).
Entity Framework Core, writing a default	Refused: on a property of the value object's type, SaveChanges throws and writes nothing; an optional property stores NULL. No later read finds an empty Iban to trust. EFCore.BulkExtensions writes through the same converter (inferred, not run).
Dapper, a default as a parameter	Refused with a DataException before the command runs, from an Iban and an Iban? alike.
MongoDB.Driver with AdCodicem.ValueObjects.MongoDB, writing a default	Refused with a BsonSerializationException before anything is sent, from an Iban and an Iban? alike, in a document, an update or a query constant.
MessagePack and SignalR with AdCodicem.ValueObjects.MessagePack, writing a default	Refused with a MessagePackSerializationException, from an Iban, a dictionary key and an Iban? holding one alike: a .NET client's invocation throws before it is sent, and a hub method's result fails to be written, which closes the connection.
System.Text.Json and Newtonsoft.Json with its converter, writing a default	Refused with a JsonException or a JsonSerializationException, so a service reading the payload never receives it.

A type that accepts its zero, an Amount with a minimum of 0 or an unconstrained Guid, is still written as zero: over a value type, nothing tells the default from a constructed zero.

Guards

• FluentValidation's NotDefault rule checks IsDefault at the edge. Over a value type, IsDefault is true for a valid zero as well, Amount.Create(0m), which it refuses too.
• DataAnnotations [Required] passes on a default value object: a struct is never null. CommunityToolkit.Mvvm's ObservableValidator showed it.
• A strict Entity Framework Core read refuses what the default read lets through.