order-unit-tests: clarify ordering vs. serial execution

[Copilot]

The article explained how to control test order but never addressed that tests can still run concurrently — leaving readers who want strictly serial execution without actionable guidance.

Changes

• Top-level NOTE: ordering and parallelization are orthogonal; disabling parallelization is required to guarantee serial execution
• MSTest: sequential within a class by default; parallelism requires explicit <Parallelize> in .runsettings
• xUnit: classes run in parallel by default; ITestCaseOrderer serializes only within a single class; add [assembly: CollectionBehavior(DisableTestParallelization = true)] to disable globally
• NUnit: sequential by default; [Order] is sufficient unless [Parallelizable] is applied

Original prompt

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This section details on the original issue you should resolve

<issue_title>Article does not address the issue</issue_title>
<issue_description>### Type of issue

Typo

Description

The tests run in parallel NOT in series. I need the to run in an ORDER (ie in a series) and this article does not address this at all. At minimum the title is misleading at worse it is missing the point all together.

Page URL

https://learn.microsoft.com/en-us/dotnet/core/testing/order-unit-tests

Content source URL

https://github.com/dotnet/docs/blob/main/docs/core/testing/order-unit-tests.md

Document Version Independent Id

b82fafa9-47c1-1c5a-93a3-669782e915eb

Article author

@IEvangelist

Metadata

• ID: a17b1fbc-2e79-c453-ef83-2ffff1683d5c
• PlatformId: ce22304e-278e-cb39-2b5f-149262472864
• Service: dotnet-fundamentals

Related Issues

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Associated WorkItem - 550425</issue_description>

Comments on the Issue (you are @copilot in this section)

@gewarren cc @meaghanlewis

• Fixes Article does not address the issue #44969

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