Emit the context check for the containing type map in CollectionMapper (fixes #4650)

[jbogard]

Fixes #4650.

CollectionMapper emits the OverMaxDepth check from the containing type map (memberMap.TypeMap) but decided whether to emit CheckContext from the element type map. With a containing map that has MaxDepth > 0 and a plain element map, the generated plan calls context.OverTypeDepth against the default context, which throws:

Context.Items are only available when using a Map overload that takes Action<IMappingOperationOptions>!

The message is misleading — ThrowInvalidMap is shared between Items and CheckDefault, and the real accessor is TypeDepth.

Because collection execution plans are cached per type pair (MapRequest equality ignores MemberMap), the first member map to compile a given collection pair wins the cache for every other call site. Once a map with MaxDepth compiles the plan, unrelated callers reaching it under the default context fail, so the failure depends on warmup order rather than on configuration. This became reachable for many more configurations in 0afaf1e, where self-referential maps started defaulting to MaxDepth = 64.

The fix emits the context check when either the element map or the containing type map requires it, so the guard and the depth check agree. When the containing map only sets PreserveReferences, the extra check is a no-op at runtime — its own plan has already replaced the default context.

Added CollectionMaxDepthContext, which maps the same List<Service> -> IList<ServiceDto> pair from a self-referential parent and a plain parent in both orders. The recursive-first case fails on main with the exception above and passes with this change; the plain-first case passes either way.

Full unit test suite passes (1219 tests, net10.0).

The underlying cache-key issue — member-specific state baked into a type-pair-keyed plan — is not addressed here and is worth a separate look.

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https://claude.ai/code/session_01Wm2m61jXrAs4FheXR5TufH

[Copilot]

🟢 Approval recommended

The change directly addresses the documented mismatch with a minimal, targeted fix and includes a regression test that covers the previously intermittent failure mode.

Pull request overview

Fixes an execution-plan generation bug in CollectionMapper where the OverMaxDepth depth check could run without the corresponding CheckContext guard when MaxDepth was required by the containing type map but not by the element type map—leading to intermittent runtime failures depending on execution plan cache warmup order.

Changes:

• Updated CollectionMapper to emit CheckContext when either the element type map or the containing memberMap.TypeMap requires context.
• Added a regression test that reproduces the cache-order-dependent failure and verifies both warmup orders succeed.

File summaries

File	Description
src/AutoMapper/Mappers/CollectionMapper.cs	Aligns CheckContext emission with the same containing type map used by OverMaxDepth, preventing default-context depth checks.
src/UnitTests/Bug/CollectionMaxDepthContext.cs	Adds a regression test covering both compilation/warmup orders for a shared collection type-pair plan.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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