---
title: 'CHR0019: Projection expression lambda must only access members'
---

## Rule Description

Expression lambdas passed to projection builder methods (e.g. `Set(x => ...)`, `To(e => ...)`, `IdentifiedBy(x => ...)`, `UsingKey(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.Projections;

public class UserReadModel { public string Name { get; set; } }
public class UserRegistered { public string Name { get; set; } }

public class UserProjection : IProjectionFor<UserReadModel>
{
    public void Define(IProjectionBuilderFor<UserReadModel> builder) =>
        builder.From<UserRegistered>(evt => evt
            .Set(x => x.Name)
            .To(e => e.Name.ToUpper()));  // ❌ method call — never executed
}
```

### Fix

```csharp
using Cratis.Chronicle.Projections;

public class UserReadModel { public string Name { get; set; } }
public class UserRegistered { public string Name { get; set; } }

public class UserProjection : IProjectionFor<UserReadModel>
{
    public void Define(IProjectionBuilderFor<UserReadModel> builder) =>
        builder.From<UserRegistered>(evt => evt
            .Set(x => x.Name)
            .To(e => e.Name));  // ✅ simple member access
}
```

## Why This Rule Exists

In Chronicle's projection builder, any parameter typed as `Expression<Func<...>>` is **parsed at startup** to extract a property path for the read-model mapping—the lambda body is **never executed at runtime**. Writing method calls, arithmetic (`e.Count + 1`), conditional expressions (`e.Flag ? e.A : e.B`), or similar executable code in such a lambda will be silently ignored, producing an incorrect or incomplete projection definition that is very difficult to debug.

The only valid lambda body is a direct property access chain, for example `x => x.Property` or `x => x.Parent.Child`.

## Related Rules

- [CHR0016](/chronicle/code-analysis/chr0016/): Projection Define() must not contain imperative code
- [CHR0020](/chronicle/code-analysis/chr0020/): Constraint expression lambda must only access members