CHR0001: Event type must have [EventType] attribute

Rule Description

Types that are appended to event sequences must be marked with the [EventType] attribute. This ensures that all events in the event store are properly identified and can be correctly deserialized.

Severity

Error

Example

Violation

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.EventSequences;

// Missing [EventType] attribute
public record Chr0001UserCreated(string Username, string Email);

public class Chr0001UserService(IEventSequence eventSequence)
{
    public Task CreateUser(EventSourceId userId, string username, string email) =>
        // CHR0001: Type 'Chr0001UserCreated' must be marked with [EventType] attribute
        eventSequence.Append(userId, new Chr0001UserCreated(username, email));
}

Fix

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.EventSequences;

[EventType]
public record Chr0001UserCreatedFixed(string Username, string Email);

public class Chr0001UserServiceFixed(IEventSequence eventSequence)
{
    public Task CreateUser(EventSourceId userId, string username, string email) =>
        // Now valid
        eventSequence.Append(userId, new Chr0001UserCreatedFixed(username, email));
}

Quick Fix

The analyzer provides a code fix that automatically:

1. Adds the [EventType] attribute to the event type
2. Adds the required using directive if not already present

Why This Rule Exists

The [EventType] attribute is crucial for Chronicle’s event handling:

• It uniquely identifies the event type in the event store
• It enables proper event serialization and deserialization
• It allows Chronicle to track event type generations and schemas
• It ensures type safety when working with events

Related Rules

• CHR0003: Model bound projection attributes
• CHR0005: Reactor event parameters
• CHR0007: Reducer event parameters