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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

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);
    }

    // ARCCHR0001: Too many parameters
    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

Chronicle discovers event handler methods based on naming and signature conventions. Standardized signatures ensure:

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

None