CHR0016: Projection Define() must not contain imperative code

Rule Description

The Define() method of a class implementing IProjectionFor<T> must not contain imperative statements such as if/else, loops, variable declarations, or assignments. Projection definitions only declare the shape; they do not execute.

Severity

Error

Example

Violation

using Cratis.Chronicle.Projections;

public class UserReadModel { }

public class UserProjection : IProjectionFor<UserReadModel>
{
    public void Define(IProjectionBuilderFor<UserReadModel> builder)
    {
        if (someCondition)
        {
            builder.From<UserRegistered>(_ => _);
        }
    }
}

Fix

using Cratis.Chronicle.Projections;

public class UserReadModel { }

public class UserProjection : IProjectionFor<UserReadModel>
{
    public void Define(IProjectionBuilderFor<UserReadModel> builder) =>
        builder.From<UserRegistered>(_ => _);
}

Why This Rule Exists

The Define() method is called once at registration time to declare the projection shape. Chronicle serializes the resulting definition and sends it to the server for use during both live processing and replay. Any imperative code such as conditionals or loops creates non-determinism: different runs could produce different projection definitions, breaking replay consistency.

CHR0015: Projection must not have side effects
CHR0018: Constraint Define() must not contain imperative code