---
title: 'CHR0041: Event filter attribute on a projection has no effect'
---

import { Tabs, TabItem } from '@astrojs/starlight/components';

## Rule Description

An event-metadata filter attribute — `[EventStreamType]`, `[EventSourceType]`, or `[FilterEventsByTag]` — is applied to a projection, where it does nothing. A projection observes *every* event of the types its definition declares; there is no field for a metadata filter anywhere in a projection's definition, so no client can express one and no kernel could honor it.

Filtering observed events on metadata is what reactors and reducers do — see [appended event metadata filters](/chronicle/events/filtering/). To narrow a projection, narrow the event types it declares, or pair it with a reactor or reducer that owns the filtered subset.

Both projection spellings are covered: a model-bound `[ReadModel]` carrying projection attributes, and a fluent `IProjectionFor<T>`.

## Severity

Warning

## Example

<Tabs syncKey="chronicle-client">
<TabItem label="C#">

```csharp
using Cratis.Chronicle;
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Keys;
using Cratis.Chronicle.Projections.ModelBound;

[EventType]
public record Chr0041TemperatureRead(double Degrees);

// Warning CHR0041: '[FilterEventsByTag]' on projection 'Chr0041LatestTemperature' has no
// effect - a projection observes every event of the types it declares and cannot filter
// on event metadata. Readings from every sensor land in this read model, not just sensor-a's.
[FilterEventsByTag("sensor-a")]
[FromEvent<Chr0041TemperatureRead>]
public record Chr0041LatestTemperature(
    [Key] Guid Id,
    double Degrees);
```

</TabItem>
</Tabs>

## Why This Rule Exists

The filter attributes' own documentation used to say a projection could filter, and that is what an IDE tooltip shows at the moment of authoring. A developer who trusts it designs a filtered projection that silently observes everything — no build error, no startup failure, no runtime signal, just a read model carrying values from streams it was never meant to see. The natural way to debug that — re-reading the attribute to confirm it is present and spelled right — confirms the wrong conclusion.

Correcting the documentation helps the reader who reads it; this rule helps the one who does not.

## Related Rules

- [CHR0045: Event stream metadata attribute on an event type has no effect](/chronicle/code-analysis/chr0045/) — the same attributes on the other placement that reads as stream identity and is not one.
- [CHR0024: Read model property has no mapping source](/chronicle/code-analysis/chr0024/) — another declaration that looks wired but silently does nothing.
- [CHR0028: Redundant .AutoMap() call](/chronicle/code-analysis/chr0028/) — another call with no effect, though there the surprise is harmless.