xUnit 4 ParallelMode.All: Protect Shared State from Test Races

xUnit 4.0.0 makes full test-case parallelization an explicit option. That is useful, but xUnit 4 ParallelMode.All changes a quiet assumption in many suites: tests in the same class, including separate rows of one theory, may now overlap. A static fake, shared fixture, temporary file, or database record that was safe under collection-level parallelism can become a race. I treat this as an isolation change, not a speed switch. Before enabling it across a suite, I want a deterministic failure that proves the risk and a deterministic check for each guardrail. What xUnit 4 ParallelMode.All changes The xUnit.net v3 4.0.0 release notes describe full test-case parallelization as a new feature. The default is still ParallelMode.Collections , so upgrading does not silently enable the broader mode. I have to opt in at the assembly level: using Xunit.Sdk ; using Xunit.v3 ; [ assembly : Parallelization ( Mode = ParallelMode . All , MaxThreads = 2 , Algorithm = ParallelAlgorithm . Conservative )] With Collections , tests within a collection are serialized. With All , every test case is eligible to run beside every other test case. That includes two cases from the same class and two pre-enumerated rows from the same theory. The official parallel test execution guide documents the modes, algorithms, and available opt-out scopes. I set MaxThreads = 2 in the sample so the scheduling condition is easy to inspect. It is a demonstration setting, not a recommendation for CI. The right value depends on available CPU, memory, and the external systems touched by the tests. Before changing the mode, I scan for mutable static fields, IClassFixture and ICollectionFixture implementations, fixed file names, environment-variable changes, test servers bound to fixed ports, and records addressed by shared IDs. I also check theory data sources for objects that rows can mutate. That inventory tells me whether the resource should become concurrency-safe, receive a unique per-test identity, or stay beh