---
title: 'CHR0006: Reducer method signature must match allowed signatures'
---

## Rule Description

Methods in reducer classes must follow one of the allowed signatures for event handling. This ensures proper integration with Chronicle's reducer infrastructure.

## Severity

Warning

## Allowed Signatures

Event handler methods in reducers must have one of the following signatures:

```csharp
// Async with event only
Task MethodName(TEvent @event)

// Async with event and context
Task MethodName(TEvent @event, EventContext context)

// Synchronous with event only
void MethodName(TEvent @event)

// Synchronous with event and context
void MethodName(TEvent @event, EventContext context)
```

Where `TEvent` is a type marked with `[EventType]`.

## Example

### Violation

```csharp
public class ShoppingCartReducer : IReducerFor<ShoppingCart>
{
    // CHR0006: Invalid signature - returns Task<int> instead of Task
    public async Task<int> ItemAdded(ItemAdded @event)
    {
        // Update cart
        return 1;
    }

    // CHR0006: Invalid signature - too many parameters
    public async Task ItemRemoved(ItemRemoved @event, EventContext context, bool validate)
    {
        // Update cart
    }
}
```

### Fix

```csharp
public class ShoppingCartReducer : IReducerFor<ShoppingCart>
{
    // Valid signature
    public async Task ItemAdded(ItemAdded @event)
    {
        // Update cart
    }

    // Valid signature with context
    public async Task ItemRemoved(ItemRemoved @event, EventContext context)
    {
        // Update cart
    }

    // Valid synchronous signature
    public void CartCleared(CartCleared @event)
    {
        // Clear cart
    }
}
```

## Why This Rule Exists

Reducers maintain state by processing events in sequence. Standardized method signatures ensure:
- Predictable reducer behavior
- Proper infrastructure integration
- Clear code patterns across the codebase
- Reliable state management

## Related Rules

- [CHR0004](/chronicle/code-analysis/chr0004/): Reactor method signatures
- [CHR0007](/chronicle/code-analysis/chr0007/): Reducer event parameter must have [EventType] attribute