---
title: 'CHR0020: Constraint expression lambda must only access members'
---

## Rule Description

Expression lambdas passed to constraint builder methods (e.g. `On<TEvent>(e => ...)`) must only contain simple member access. They cannot contain method calls, arithmetic, conditional expressions, or any other executable code.

## Severity

Error

## Example

### Violation

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

public class OrderPlaced { public string OrderId { get; set; } }

public class OrderConstraint : IConstraint
{
    public void Define(IConstraintBuilder builder) =>
        builder.Unique(u => u.On<OrderPlaced>(e => e.OrderId.ToLower()));  // ❌ method call — never executed
}
```

### Fix

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

public class OrderPlaced { public string OrderId { get; set; } }

public class OrderConstraint : IConstraint
{
    public void Define(IConstraintBuilder builder) =>
        builder.Unique(u => u.On<OrderPlaced>(e => e.OrderId));  // ✅ simple member access
}
```

## Why This Rule Exists

In Chronicle's constraint builder, any parameter typed as `Expression<Func<...>>` is **parsed at startup** to extract a property name for the constraint rule—the lambda body is **never executed at runtime**. Writing method calls, arithmetic, conditional expressions, or similar executable code in such a lambda will be silently ignored, producing an incorrect or incomplete constraint definition.

The only valid lambda body is a direct property access, for example `e => e.Email` or `e => e.OrderId`.

If case-insensitive comparison is needed, use `builder.Unique(u => u.On<OrderPlaced>(e => e.Email).IgnoreCasing())` instead of transforming the value in the lambda.

## Related Rules

- [CHR0018](/chronicle/code-analysis/chr0018/): Constraint Define() must not contain imperative code
- [CHR0019](/chronicle/code-analysis/chr0019/): Projection expression lambda must only access members