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

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

public class UserService
{
    private readonly IEventSequence _eventSequence;

    public async Task CreateUser(string username, string email)
    {
        // CHR0001: Type 'UserCreated' must be marked with [EventType] attribute
        await _eventSequence.Append(
            eventSourceId: userId,
            @event: new UserCreated(username, email));
    }
}

Fix

using Cratis.Chronicle.Concepts.Events;

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

public class UserService
{
    private readonly IEventSequence _eventSequence;

    public async Task CreateUser(string username, string email)
    {
        // Now valid
        await _eventSequence.Append(
            eventSourceId: userId,
            @event: new UserCreated(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