---
title: 'CHR0057: An artifact must target one read model'
editUrl: https://github.com/Cratis/Chronicle/edit/main/Documentation/code-analysis/CHR0057.mdx
description: CHR0057 reports a type implementing multiple projection or reducer interfaces.
---


## Rule description

A .NET class must implement only one closed `IProjectionFor<TReadModel>` or only one closed `IReducerFor<TReadModel>` of each kind. Artifact identifiers belong to the class, not to each interface implementation, so one class cannot represent two projections or two reducers.

The rule also checks inherited interfaces. Two inheritance paths to the same closed interface count as one implementation.

## Severity

Warning

## How to fix

Split the class into separate projection or reducer classes, one per read model. Each class then has its own artifact identifier.

Default artifact discovery throws `TypeImplementsMultipleProjectionInterfaces` or `TypeMustImplementOnlyOneReducer` for an invalid class. Both name the class instead of silently skipping it, and the projection exception also lists its read models.

## Related topics

- [Declarative projections](/chronicle/projections/declarative/)
- [Reducers](/chronicle/reducers/)