ARCCHR0001: Incorrect aggregate root event handler signature

Rule

Event handler methods (typically named On) on aggregate roots must accept an event parameter and optionally an EventContext parameter, and return void or Task.

Severity

Error

Allowed signatures

These are signature fragments. The analyzer first filters candidates to methods with one or two parameters and a known event as the first parameter. A three-parameter method is outside that filter, so it does not receive ARCCHR0001 even though it is not a supported aggregate event handler. Verify discovery/replay in specs rather than relying on silence.

void On(TEvent @event)
Task On(TEvent @event)
void On(TEvent @event, EventContext context)
Task On(TEvent @event, EventContext context)

Example

Violation

public class UserAggregateRoot : AggregateRoot
{
    // ARCCHR0001: Invalid return type
    public string OnUserCreated(UserCreated @event)
    {
        return "not allowed";
    }

    // ARCCHR0001: Task<T> is not allowed
    public Task<int> OnUserNameChanged(UserNameChanged @event)
    {
        return Task.FromResult(42);
    }

    // Not reported by ARCCHR0001: excluded before candidate validation
    public void OnUserUpdated(UserUpdated @event, EventContext context, string extra)
    {
    }
}

Fix

public class UserAggregateRoot : AggregateRoot
{
    public void OnUserCreated(UserCreated @event)
    {
        // Handle event.
    }

    public Task OnUserNameChanged(UserNameChanged @event)
    {
        return Task.CompletedTask;
    }

    public void OnUserUpdated(UserUpdated @event, EventContext context)
    {
        // Handle event with context.
    }
}

Why This Rule Exists

Arc’s aggregate integration discovers event handlers from their event parameter and supported signature; names such as On are conventions. Standardized signatures ensure:

Handlers are discovered consistently.
Event processing remains predictable.
Asynchronous handlers integrate cleanly with the runtime.

None