NUnit's default FixtureLifeCycle is SingleInstance — every test in a
class shares one fixture instance, so [SetUp]-initialised fields like
_master / _viewer / _service are reset on every test against the same
object. Under serial execution that's fine; under parallel execution
concurrent SetUps wipe each other's Mock setups and the service code
NREs trying to dereference unconfigured stubs.
Compounding it, NewInMemoryDb was being called with nameof(SetUp) which
is the literal string "SetUp", so every test in the fixture also shared
the same EF InMemory database (the provider keys stores by name).
Two fixes:
- [FixtureLifeCycle(LifeCycle.InstancePerTestCase)] on StoryServiceTests
so each test gets its own instance with its own Mocks.
- Suffix the InMemoryDb name with a Guid so concurrent callers never
share a store.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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