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

C#

using Cratis.Chronicle.Projections;

// Missing [EventType] attribute
public record Chr0002OrderCreated(Guid OrderId, decimal Amount);

public class Chr0002OrderReadModel
{
    public Guid Id { get; set; }
    public decimal Amount { get; set; }
}

public class Chr0002OrderProjection : IProjectionFor<Chr0002OrderReadModel>
{
    public void Define(IProjectionBuilderFor<Chr0002OrderReadModel> builder) =>
        // CHR0002: Type 'Chr0002OrderCreated' must be marked with [EventType] attribute
        builder.From<Chr0002OrderCreated>(_ => _
            .Set(m => m.Id).To(e => e.OrderId)
            .Set(m => m.Amount).To(e => e.Amount));
}

Fix

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Projections;

[EventType]
public record Chr0002OrderCreatedFixed(Guid OrderId, decimal Amount);

public class Chr0002OrderReadModelFixed
{
    public Guid Id { get; set; }
    public decimal Amount { get; set; }
}

public class Chr0002OrderProjectionFixed : IProjectionFor<Chr0002OrderReadModelFixed>
{
    public void Define(IProjectionBuilderFor<Chr0002OrderReadModelFixed> builder) =>
        // Now valid
        builder.From<Chr0002OrderCreatedFixed>(_ => _
            .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