Setting Constant Values

The SetValue attribute sets a property to a compile-time constant whenever an event of a specified type occurs. Use it when a property should take a fixed value in response to an event — for example, setting a status flag, assigning a default category, or recording a fixed version number.

Basic Usage

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;

[EventType]
public record OrderPlaced(string CustomerName);

[EventType]
public record OrderCanceled;

public record Order(
    [Key]
    Guid Id,

    [SetFrom<OrderPlaced>(nameof(OrderPlaced.CustomerName))]
    string CustomerName,

    [SetValue<OrderPlaced>("active")]
    [SetValue<OrderCanceled>("canceled")]
    string Status);

When an OrderPlaced event occurs, Status is set to "active". When an OrderCanceled event occurs, Status is set to "canceled".

Supported Value Types

SetValue accepts any compile-time constant: strings, integers, longs, doubles, booleans, and enum values.

[EventType]
public record ThingHappened;

public record Thing(
    [Key]
    Guid Id,

    [SetValue<ThingHappened>("pending")]
    string StatusLabel,

    [SetValue<ThingHappened>(42)]
    int Priority,

    [SetValue<ThingHappened>(true)]
    bool IsActive,

    [SetValue<ThingHappened>(3.14)]
    double Score);

Multiple Events

Apply the attribute more than once to respond to multiple event types:

[EventType]
public record SubscriptionStarted;

[EventType]
public record SubscriptionPaused;

[EventType]
public record SubscriptionCanceled;

public record Subscription(
    [Key]
    Guid Id,

    [SetValue<SubscriptionStarted>("active")]
    [SetValue<SubscriptionPaused>("paused")]
    [SetValue<SubscriptionCanceled>("canceled")]
    string State);

SetValue vs SetFrom

Attribute	Source	When to use
SetValue<TEvent>(value)	Compile-time constant	The value does not come from the event payload
SetFrom<TEvent>(property)	Event property	The value comes from a property on the event

Combine both on the same read model when different properties come from different sources:

public record Invoice(
    [Key]
    Guid Id,

    [SetFrom<InvoiceIssued>(nameof(InvoiceIssued.Amount))]
    decimal Amount,

    [SetValue<InvoiceIssued>("issued")]
    [SetValue<InvoicePaid>("paid")]
    string Status);