---
title: 'CHR0012: Event types should avoid nullable properties'
---

## Rule Description

Event types can contain nullable properties, but this is often an anti-pattern in event sourcing. A nullable field can hide multiple meanings in one event. Prefer expressing optional facts with a dedicated event type when possible.

## Severity

Warning

## Example

### Warning

```csharp
using Cratis.Chronicle.Events;

[EventType]
public record MissionAccepted(
    string MissionId,
    DateOnly? StartDate);
```

### Preferred

```csharp
using Cratis.Chronicle.Events;

[EventType]
public record MissionAccepted(
    string MissionId,
    DateOnly StartDate);

[EventType]
public record MissionAcceptedWithoutStartDate(
    string MissionId);
```

## Why This Rule Exists

Events represent immutable facts. Nullable properties can blur the meaning of those facts and force consumers to branch on missing values.

Keeping event types explicit:

- improves readability
- reduces ambiguity for projections, reducers, and reactors
- keeps event history easier to understand

## Related Rules

- [CHR0001](/chronicle/code-analysis/chr0001/): Event sequence append operations