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

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

C#

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);

Fix

C#

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);

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 for that related check.

Related Rules

• CHR0049: [EventTypeGenerationFor<T>] must reference a type marked with [EventType] — validates that the referenced type is actually an event type.
• 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.