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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

C#

using Cratis.Chronicle.Reactors;

// Missing [EventType] attribute
public record Chr0005CustomerRegistered(Guid CustomerId, string Email);

public class Chr0005CustomerReactor : IReactor
{
    // CHR0005: Type 'Chr0005CustomerRegistered' must be marked with [EventType] attribute
    public Task Registered(Chr0005CustomerRegistered @event) => Task.CompletedTask;
}

Fix

C#

using Cratis.Chronicle.Events;
using Cratis.Chronicle.Reactors;

[EventType]
public record Chr0005CustomerRegisteredFixed(Guid CustomerId, string Email);

public class Chr0005CustomerReactorFixed : IReactor
{
    // Now valid
    public Task Registered(Chr0005CustomerRegisteredFixed @event) => Task.CompletedTask;
}

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

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