ARC0001: Incorrect query method signature on ReadModel

Rule

Reports public static non-void methods on [ReadModel] types whose return type is not the read model or an accepted wrapper. Methods accepting their own read-model type, nullable or not, are excluded from query discovery and this diagnostic, regardless of parameter name or position. Default severity: Error. Move unrelated utility methods off the read model rather than exposing them as queries, or use explicit endpoint selection.

Accepted return shapes

For read model T, the analyzer accepts:

• T and T[]
• IEnumerable<T> and types implementing it, including List<T> and IQueryable<T>
• Task<T> and Task<C> where C is an accepted collection of T
• IAsyncEnumerable<T>
• ISubject<T> and ISubject<C> where C is an accepted collection of T

These are type shapes, not declarations to paste into a file. A nongeneric Task and ValueTask<T> are not accepted by this analyzer. For transport behavior, see query return types.

Diagnostic example

Compile each block separately with Cratis.Arc.Core. The first intentionally produces ARC0001; it is not a runnable application.

using System;
using Cratis.Arc.Queries.ModelBound;

[ReadModel]
public record User(Guid Id, string Name)
{
    public static string GetName(Guid id) => "Ada";
}

Return the read model instead:

using System;
using Cratis.Arc.Queries.ModelBound;

[ReadModel]
public record User(Guid Id, string Name)
{
    public static User GetById(Guid id) => new(id, "Ada");
}

This fixed declaration returns sample data; it does not demonstrate database persistence or host setup. Both declarations use records so ARC0008 does not distract from the return-type error.

Related rules

• ARC0014: Generic query methods
• ARC0002: Missing command attribute