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CHR0004: Reactor method has a parameter that cannot be resolved

Rule Description

A reactor handler method takes the event it reacts to as its first parameter. Any further parameters are resolved as dependencies when the method is invoked:

The EventContext for the triggering event.
A read model — a type that has a reducer or projection. It is materialized strongly consistent from that reducer or projection.
A service resolved from the service provider.

A parameter that is a primitive, value type, or string can never be a read model and is almost never a service, so it is flagged: at runtime it would fail to resolve and the partition would fail.

Severity

Warning

Allowed Signatures

C#

using Cratis.Chronicle.Events;

public interface ValidReactorMethodSignatures<TEvent, TReadModel, IService>
{
    void MethodName(TEvent @event);
    Task MethodNameAsync(TEvent @event);

    void MethodName(TEvent @event, EventContext context);
    Task MethodNameAsync(TEvent @event, EventContext context);

    Task MethodNameAsync(TEvent @event, EventContext context, TReadModel readModel, IService service);
    Task MethodNameAsync(TEvent @event, TReadModel readModel);
}

Where TEvent is a type marked with [EventType], and TReadModel is a type that has a reducer or projection.

Example

Violation

C#

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

[EventType]
public record Chr0004OrderUpdated;

public class InvalidOrderReactor : IReactor
{
    public Task OrderUpdated(Chr0004OrderUpdated @event, int count) =>
        Task.CompletedTask;
}

Fix

C#

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

[EventType]
public record Chr0004FixedOrderUpdated;

public record Chr0004Order(int Count);

public class ValidOrderReactor : IReactor
{
    public Task OrderUpdated(Chr0004FixedOrderUpdated @event, Chr0004Order order) =>
        Task.CompletedTask;
}

Why This Rule Exists

Reactors take dependencies on their handler methods. Read models are resolved directly by Chronicle, and other dependencies come from the service provider. A primitive, value type, or string is neither, so flagging it early prevents a runtime resolution failure that would pause the event source partition.

CHR0005: Reactor event parameter must have [EventType] attribute
CHR0006: Reducer method signatures