Controller query return types

Data return types

Arc’s default MVC GET handling wraps returned data in its nongeneric QueryResult. Unlike model-bound discovery, MVC does not require the action to live on the returned type.

Return value	Default Arc GET behavior
A model or custom summary	Wrapped as data
A list, array, or materialized enumerable	Wrapped collection; no built-in slicing
IQueryable<T>	Renderer applies requested paging/sorting before serialization
Task<T>	MVC awaits the task before Arc processes its data
Runtime ISubject<T> or IAsyncEnumerable<T>	Streaming adapter chooses handling from the request

A MongoDB Find(...) is fluent query construction; call ToList()/ToListAsync() to return materialized data. See dependency injection for a complete async action.

Query results

Do not return QueryResult from an ordinary wrapped controller GET to control the outer metadata. The action filter creates another QueryResult and treats the returned wrapper as data, producing a nested contract. A custom envelope is likewise ordinary data, not an instruction to replace Arc’s outer envelope.

Prefer IQueryable paging. If you need complete control over status and body, opt out using Cratis.Arc.AspNetResultAttribute and return a conventional MVC result. Opting out also skips Arc query rendering/interception; you own the entire response contract and must use a compatible client.

Nullable return types

A null response from a default Arc-wrapped MVC GET becomes a failed result with exceptionMessages containing "Null data returned"; it is not ordinary successful absence or an automatic 404. This differs from the model-bound pipeline, where null can remain a success.

Choose deliberately:

• An empty collection means a successful collection with no matches.
• A non-null application response model can represent absence explicitly inside its data contract.
• [AspNetResult] with NotFound() provides ordinary MVC 404 semantics. The route example shows the complete action.

Do not convert database exceptions into null/empty successful results.

Observable return types

Return ISubject<T> for a subject-backed query. An arbitrary Rx IObservable<T> is not sufficient: the controller adapter checks the runtime value for subject/async-enumerable support. Merely declaring IObservable<T> does not adapt a Select(...) result into a subject.

The return type does not establish a WebSocket; the request selects the transport. See controller observable queries for a complete GET action and subscription lifetime rules.