CHR0017: Constraint must not have side effects

Rule Description

A class implementing IConstraint must not inject ICommandPipeline or IEventLog via its constructor. Constraints are pure rule builders and must not produce side effects.

Severity

Error

Example

Violation

using Cratis.Chronicle.EventSequences;
using Cratis.Chronicle.Events.Constraints;

public class UniqueEmailConstraint : IConstraint
{
    readonly IEventLog _eventLog;

    public UniqueEmailConstraint(IEventLog eventLog)
    {
        _eventLog = eventLog;
    }

    public void Define(IConstraintBuilder builder) =>
        builder.Unique<UserRegistered>();
}

Fix

using Cratis.Chronicle.Events.Constraints;

public class UniqueEmailConstraint : IConstraint
{
    public void Define(IConstraintBuilder builder) =>
        builder.Unique<UserRegistered>();
}

Why This Rule Exists

Constraints declare rules about what combinations of events are valid. They are evaluated by the server in a pure, stateless fashion. Injecting IEventLog or ICommandPipeline implies side effects that cannot be guaranteed to execute at constraint-check time, leading to unpredictable behaviour.

Related Rules

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