ARCCHR0002: Ambiguous command identity

Rule

Warning. A [Command] has two or more public instance properties that can supply the command’s event source id, does not implement ICanProvideEventSourceId, and does not return events that carry their own event source id.

A property is a candidate when it is any of these:

Candidate	Example
A type that is or derives from EventSourceId	Isbn Isbn where Isbn : EventSourceId
A type with an implicit conversion to EventSourceId	A ConceptAs<Guid> with an implicit conversion
A property marked with Chronicle’s [Key]	[Key] Guid OrderId
A positional parameter marked with [Key] and the property it creates	Counted once

Without a decision, Arc uses the first matching property. That choice also decides which entity the command appends to and which key a read model parameter reads by default, so a silent wrong choice changes the rule.

Resolve the diagnostic

Situation	Fix
Only one of the properties is the command’s subject	Make the others ConceptAs<T> values, not EventSourceId
The subject is computed or fixed	Implement ICanProvideEventSourceId and return the id
The command appends to both entities	Return EventForEventSourceId for each event. Each event then carries its own id and the rule does not apply

The analyzer also recognizes a single wrapper or a sequence of wrappers in the success branch of Result<TEvent, TError>, including Task<Result<TEvent, TError>>. No ICanProvideEventSourceId is needed for those return values. An error branch carrying a wrapper does not give an untargeted success event a destination.

using Cratis.Arc.Commands.ModelBound;
using Cratis.Chronicle.Events;

public record Isbn(string Value) : EventSourceId(Value);

public record MemberId(string Value) : EventSourceId(Value);

[EventType]
public record BookBorrowed(MemberId Member);

[Command]
public record BorrowBook(Isbn Isbn, MemberId Member) : ICanProvideEventSourceId
{
    public EventSourceId GetEventSourceId() => Isbn;

    public BookBorrowed Handle() => new(Member);
}

[Key] on one of the properties does not resolve the ambiguity. It marks a candidate. See Resolving EventSourceId for the rules Arc applies at run time.

Boundaries

This analyzer’s candidate rule includes implicit conversions, which run-time discovery does not generally do. A conversion-only ConceptAs<Guid> is not a safe run-time identity declaration. Use EventSourceId<Guid> ancestry or a selected key.

Neither a clean analyzer result nor a recognized return type proves that dependencies resolved before Handle() (an aggregate, a read model parameter) use the id you intend. Under read protection On, name the key of a read model parameter with [LookUpBy]. No code fix is offered.