CHR0005: Reactor event parameter must have [EventType] attribute

Rule Description

Event parameters in reactor methods must be types marked with the [EventType] attribute. This ensures that reactors only handle properly identified event types.

Severity

Error

Example

Violation

// Missing [EventType] attribute
public record CustomerRegistered(Guid CustomerId, string Email);

public class CustomerReactor : IReactor
{
    // CHR0005: Type 'CustomerRegistered' must be marked with [EventType] attribute
    public async Task Registered(CustomerRegistered @event)
    {
        // Send welcome email
        await SendWelcomeEmail(@event.Email);
    }
}

Fix

using Cratis.Chronicle.Concepts.Events;

[EventType]
public record CustomerRegistered(Guid CustomerId, string Email);

public class CustomerReactor : IReactor
{
    // Now valid
    public async Task Registered(CustomerRegistered @event)
    {
        // Send welcome email
        await SendWelcomeEmail(@event.Email);
    }
}

Quick Fix

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

Why This Rule Exists

Reactors handle side effects when events occur. Ensuring all event types are properly marked:

• Guarantees proper event routing to reactors
• Enables type-safe event handling
• Ensures events are correctly serialized/deserialized
• Maintains schema tracking for events

Related Rules

• CHR0001: Event sequence append operations
• CHR0004: Reactor method signatures
• CHR0007: Reducer event parameters