Applying PII to ConceptAs types

The most robust way to declare that a value is personally identifiable is to mark the ConceptAs<T> type itself with [PII]. Doing so means the declaration lives exactly once — on the type — and every event property that uses that type is automatically encrypted, regardless of where or how many times it appears across your event model.

Tip

For background on the ConceptAs<T> pattern, see Concepts.

Why concept-level declaration is preferred

When you mark a property directly with [PII], you must remember to repeat the attribute on every event or read model that carries that value. If a new event is added months later and the developer forgets the annotation, a plaintext value is written to the event log with no warning.

A concept type solves this by making the protection part of the type's identity. You cannot use PersonName without the encryption — the two are inseparable.

// ❌ Property-level: requires repetition across every event
[EventType]
public record EmployeeRegistered([PII] string Name, string Department);

[EventType]
public record EmployeeNameChanged([PII] string NewName);  // must remember [PII] again

// ✅ Concept-level: declare once, apply everywhere automatically
[PII]
public record PersonName(string Value) : ConceptAs<string>(Value)
{
    public static readonly PersonName NotSet = new(string.Empty);
    public static implicit operator string(PersonName name) => name.Value;
    public static implicit operator PersonName(string value) => new(value);
}

[EventType]
public record EmployeeRegistered(PersonName Name, string Department);  // Name is encrypted

[EventType]
public record EmployeeNameChanged(PersonName NewName);  // also encrypted, no extra annotation needed

Defining a PII concept type

Follow the standard ConceptAs<T> pattern and add [PII] to the record declaration:

using Cratis.Chronicle.Compliance.GDPR;

[PII]
public record PersonName(string Value) : ConceptAs<string>(Value)
{
    public static readonly PersonName NotSet = new(string.Empty);

    public static implicit operator string(PersonName name) => name.Value;
    public static implicit operator PersonName(string value) => new(value);
}

For a Guid-backed concept:

[PII("National ID number — sensitive personal identifier")]
public record NationalIdNumber(string Value) : ConceptAs<string>(Value)
{
    public static readonly NationalIdNumber NotSet = new(string.Empty);

    public static implicit operator string(NationalIdNumber id) => id.Value;
    public static implicit operator NationalIdNumber(string value) => new(value);
}

Placement

Concept types belong in the feature folder they are primarily associated with, or at the feature root if they are shared across slices. Do not create a separate Concepts/ folder — keep concepts co-located with the code that uses them.

Features/
├── Employees/
│   ├── PersonName.cs          ← shared across Registration and Updates slices
│   ├── Registration/
│   │   ├── Registration.cs    ← uses PersonName
│   └── Updates/
│       ├── Updates.cs         ← also uses PersonName — encryption is automatic

Combining with details

Use the optional details parameter on [PII] to document the legal basis or purpose of the PII classification. This is stored in the event schema and can be surfaced by compliance tooling.

[PII("Collected under GDPR Art. 6(1)(b) — necessary for contract performance. Retention: contract duration + 7 years.")]
public record LegalName(string Value) : ConceptAs<string>(Value)
{
    public static readonly LegalName NotSet = new(string.Empty);

    public static implicit operator string(LegalName name) => name.Value;
    public static implicit operator LegalName(string value) => new(value);
}

What cannot be marked PII

EventSourceId types

Any type inheriting from EventSourceId or EventSourceId<T> cannot be marked with [PII]. Chronicle throws PIINotSupportedOnEventSourceId if you attempt this.

// ❌ Throws PIINotSupportedOnEventSourceId
[PII]
public record EmployeeId(Guid Value) : EventSourceId<Guid>(Value);

Use a non-sensitive surrogate key as the event source identifier and store sensitive identity values in a separate event property:

// ✅ Surrogate key as event source identifier
public record EmployeeId(Guid Value) : EventSourceId<Guid>(Value)
{
    public static EmployeeId New() => new(Guid.NewGuid());
}

// ✅ Sensitive value stored in a PII-marked concept type
[PII("Employee national ID — sensitive identifier")]
public record NationalIdNumber(string Value) : ConceptAs<string>(Value);

[EventType]
public record EmployeeRegistered(NationalIdNumber NationalId, PersonName Name);