---
title: 'CHR0026: [Key] or [Subject] on an EventSourceId<T> is redundant'
---

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

## Rule Description

A property or record parameter whose type derives from `EventSourceId<T>` already is both the key (stream identity) and the compliance subject. `[Key]` and `[Subject]` exist for non-`EventSourceId<T>` values; on an `EventSourceId<T>` they are at best redundant and at worst mask an ambiguous multi-identity design that should be split into distinct types rather than annotated around.

Remove the attribute.

## Severity

Warning

## Example

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

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

public record Chr0026AccountId(Guid Value) : EventSourceId<Guid>(Value);

public record Chr0026Account(
    // Warning CHR0026: Chr0026AccountId derives from EventSourceId<T>, so it already is the
    // key/stream identity and compliance subject — the [Key] attribute is redundant.
    [Key] Chr0026AccountId Id,
    string Name);
```

</TabItem>
</Tabs>

## Why This Rule Exists

`EventSourceId<T>` is the strongly-typed event-source identity. Being that identity, it is automatically the key Chronicle resolves the stream by, and it is automatically the compliance subject a value's PII is encrypted under. `[Key]` and `[Subject]` exist precisely to designate those roles on values that would not otherwise carry them — a plain `Guid`, a string, another concept. On an `EventSourceId<T>` there is nothing to designate; the type already says it.

At best the attribute is noise. At worst it is a signal that the design is trying to carry more than one identity on a single type — for example annotating a second `EventSourceId<T>`-derived property as the `[Key]` to disambiguate. That ambiguity should be resolved by splitting the identities into distinct types, not papered over with an attribute. Removing the attribute keeps the identity's meaning where it belongs: in the type.

## Related Rules

- [Event source](/chronicle/concepts/event-source/) — how `EventSourceId<T>` identifies a stream.
- [Subject](/chronicle/concepts/subject/) — how the compliance subject is resolved.