CHR0002: Declarative projection event type must have [EventType] attribute

Rule Description

Generic type arguments in declarative projections that reference event types must be marked with the [EventType] attribute.

Severity

Error

Example

Violation

// Missing [EventType] attribute
public record OrderCreated(Guid OrderId, decimal Amount);

public class OrderProjection : IProjectionFor<OrderReadModel>
{
    public void Define(IProjectionBuilderFor<OrderReadModel> builder)
    {
        // CHR0002: Type 'OrderCreated' must be marked with [EventType] attribute
        builder.From<OrderCreated>(e => e.OrderId)
            .Set(m => m.Id).To(e => e.OrderId)
            .Set(m => m.Amount).To(e => e.Amount);
    }
}

Fix

using Cratis.Chronicle.Concepts.Events;

[EventType]
public record OrderCreated(Guid OrderId, decimal Amount);

public class OrderProjection : IProjectionFor<OrderReadModel>
{
    public void Define(IProjectionBuilderFor<OrderReadModel> builder)
    {
        // Now valid
        builder.From<OrderCreated>(e => e.OrderId)
            .Set(m => m.Id).To(e => e.OrderId)
            .Set(m => m.Amount).To(e => e.Amount);
    }
}

Quick Fix

The analyzer provides a code fix that automatically adds the [EventType] attribute to the referenced type.

Why This Rule Exists

Declarative projections define how events are projected into read models. All event types used in these projections must be properly identified to ensure:

• Correct event routing and handling
• Proper schema tracking
• Type-safe projection definitions

Related Rules

• CHR0001: Event sequence append operations
• CHR0003: Model bound projection attributes