---
title: 'CHR0050: `[EventType]` and `[EventTypeGenerationFor<T>]` cannot be combined'
---

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

## Rule Description

A type is either the current event type (marked with `[EventType]`) or a previous generation of one (marked with `[EventTypeGenerationFor<T>]`), never both. Remove one of the two attributes.

## Severity

Error

## Example

### Violation

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

```csharp
using Cratis.Chronicle.Events;

// Error CHR0050: 'Chr0050ViolationUserRegistered' is marked with both [EventType] and
// [EventTypeGenerationFor<T>].
[EventType("user-registered")]
[EventTypeGenerationFor<Chr0050ViolationUserRegisteredV2>(1)]
public record Chr0050ViolationUserRegistered(string Name, string Email);

[EventType("user-registered", generation: 2)]
public record Chr0050ViolationUserRegisteredV2(string FirstName, string LastName, string Email);
```

</TabItem>
</Tabs>

### Fix

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

```csharp
using Cratis.Chronicle.Events;

// The previous generation carries only [EventTypeGenerationFor<T>].
[EventTypeGenerationFor<Chr0050FixUserRegisteredV2>(1)]
public record Chr0050FixUserRegisteredV1(string Name, string Email);

// The current generation carries only [EventType].
[EventType("user-registered", generation: 2)]
public record Chr0050FixUserRegisteredV2(string FirstName, string LastName, string Email);
```

</TabItem>
</Tabs>

## Why This Rule Exists

`[EventType]` declares a type as the current representation of an event, giving it an event type id. `[EventTypeGenerationFor<T>]` declares a type as a **previous generation** that should be migrated forward into the current shape. Combining them is contradictory: a type cannot simultaneously be the current generation and a previous one.

Without this check, a type marked with both attributes compiles cleanly but fails when the client connects to Chronicle, throwing `EventTypeGenerationForCannotBeCombinedWithEventType` at startup. This rule surfaces the same mistake at compile time instead.

The same analyzer class also reports **CHR0049** (`EventTypeGenerationForMustReferenceEventType`, Error) when `[EventTypeGenerationFor<T>]` references a type that itself has no `[EventType]` attribute — see [CHR0049](/chronicle/code-analysis/chr0049/) for that related check.

## Related Rules

- [CHR0049](/chronicle/code-analysis/chr0049/): `[EventTypeGenerationFor<T>]` must reference a type marked with `[EventType]` — validates that the referenced type is actually an event type.
- [CHR0037](/chronicle/code-analysis/chr0037/): Event type migration generations must belong to one event type — validates that a migration's two generations resolve to the same event type id.