Abstract VSTest discovery sink in PlatformServices (Phase 3 of platform-agnostic effort)

[Amaury Levé (Evangelink)]

Why

Part of the multi-PR initiative to make src/Adapter/MSTestAdapter.PlatformServices platform-agnostic by removing its dependency on the VSTest object model (Microsoft.TestPlatform.ObjectModel). VSTest coupling moves up into the adapter boundary; the platform-services engine becomes neutral.

This is Phase 3 (discovery output / sink). It follows the exact pattern established by:

• Phase 1 — IAdapterMessageLogger (interface) + AdapterMessageLoggerExtensions (bridge) — merged to main (Abstract VSTest message logging in PlatformServices (Phase 1 of platform-agnostic effort) #9548)
• Phase 5 — ITestResultRecorder (interface) + TestResultRecorderExtensions (bridge) — merged to main (Abstract VSTest result reporting in PlatformServices (Phase 5 of platform-agnostic effort) #9550)
• Phase 2 — ITestElementFilter (interface) + TestMethodFilter.GetTestElementFilter (bridge) — open standalone PR Abstract VSTest test filtering in PlatformServices (Phase 2 of platform-agnostic effort) #9567

What

The discovery engine (UnitTestDiscoverer) now emits the neutral UnitTestElement model to a new platform-agnostic sink instead of building VSTest TestCases and pushing them to ITestCaseDiscoverySink.

• NewInterfaces/IUnitTestElementSink.cs — neutral sink (namespace …PlatformServices.Interface) with void SendTestElement(UnitTestElement testElement). The name deliberately carries no VSTest/TestCase/ObjectModel terms.
• NewServices/UnitTestElementSinkExtensions.cs — the single VSTest translation point: ToUnitTestElementSink(this ITestCaseDiscoverySink) wraps the host sink and materializes element.ToTestCase() at the boundary.
• Discovery/UnitTestDiscoverer.cs — DiscoverTests/DiscoverTestsInSource/SendTestCases now take IUnitTestElementSink; the engine no longer references ITestCaseDiscoverySink or calls ToTestCase().
• VSTestAdapter/MSTestDiscoverer.cs — wraps the host's real ITestCaseDiscoverySink via .ToUnitTestElementSink() at the adapter boundary.
• Execution/TestCaseDiscoverySink.cs — the internal execution-side collector now implements IUnitTestElementSink but still materializes TestCase into its ICollection<TestCase> Tests, preserving the execution path.
• Tests updated: Mock<ITestCaseDiscoverySink> kept and wrapped at call sites (so existing Verify assertions stay meaningful); TestCaseDiscoverySinkTests rewritten for the new API; integration helper CLITestBase.discovery.cs wraps its sink.

No behavior change

Pure refactor. element.ToTestCase() is still invoked exactly once per emitted element in both the discovery and execution paths, the filter application point and the Phase 2 filterHasError bail-out semantics are unchanged, and discovered TestCase fields/Id/properties/traits/ordering are byte-for-byte identical. Execution still consumes TestCase internally (that decoupling is Phase 4, intentionally out of scope here).

Remaining move-to-adapter work (future phases)

• The Services/UnitTestElementSinkExtensions bridge still lives in PlatformServices for now (same as the Phase 1/2/5 bridges); it is expected to move fully into the adapter layer once discovery no longer flows VSTest sinks through platform services.
• The execution-side TestCaseDiscoverySink still materializes TestCase because execution consumes TestCase end-to-end — that is Phase 4.

Verification

• .\build.cmd -c Debug green across all TFMs (net462, net8.0, net9.0, UWP, WinUI) — 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net8.0): 893/893 passed.
• MSTestAdapter.UnitTests (net8.0): 21/21 passed.
• expert-reviewer run on the diff: no blocking/major findings; two doc-consistency nits addressed.

⚠️ Stacked PR

Phase 1 (#9548) and Phase 5 (#9550) are already on main. This PR is stacked on Phase 2 (#9567) and should be reviewed/merged after#9567 lands on main.

Current base is the integration branch dev/amauryleve/vstest-decoupling-base (= main + Phase 2), which keeps the diff clean today. Once #9567 merges, this PR can be retargeted to base=main directly — that is the intended final base. Please do not rebase/reset the base branch in the meantime.

[Amaury Levé (Evangelink)]

Why

Part of the multi-PR initiative to make src/Adapter/MSTestAdapter.PlatformServices platform-agnostic by removing its dependency on the VSTest object model (Microsoft.TestPlatform.ObjectModel). VSTest coupling moves up into the adapter boundary; the platform-services engine becomes neutral.

This is Phase 3 (discovery output / sink). It follows the exact pattern established by:

• Phase 1 — IAdapterMessageLogger (interface) + AdapterMessageLoggerExtensions (bridge) — merged to main (Abstract VSTest message logging in PlatformServices (Phase 1 of platform-agnostic effort) #9548)
• Phase 5 — ITestResultRecorder (interface) + TestResultRecorderExtensions (bridge) — merged to main (Abstract VSTest result reporting in PlatformServices (Phase 5 of platform-agnostic effort) #9550)
• Phase 2 — ITestElementFilter (interface) + TestMethodFilter.GetTestElementFilter (bridge) — open standalone PR Abstract VSTest test filtering in PlatformServices (Phase 2 of platform-agnostic effort) #9567

What

The discovery engine (UnitTestDiscoverer) now emits the neutral UnitTestElement model to a new platform-agnostic sink instead of building VSTest TestCases and pushing them to ITestCaseDiscoverySink.

• NewInterfaces/IUnitTestElementSink.cs — neutral sink (namespace …PlatformServices.Interface) with void SendTestElement(UnitTestElement testElement). The name deliberately carries no VSTest/TestCase/ObjectModel terms.
• NewServices/UnitTestElementSinkExtensions.cs — the single VSTest translation point: ToUnitTestElementSink(this ITestCaseDiscoverySink) wraps the host sink and materializes element.ToTestCase() at the boundary.
• Discovery/UnitTestDiscoverer.cs — DiscoverTests/DiscoverTestsInSource/SendTestCases now take IUnitTestElementSink; the engine no longer references ITestCaseDiscoverySink or calls ToTestCase().
• VSTestAdapter/MSTestDiscoverer.cs — wraps the host's real ITestCaseDiscoverySink via .ToUnitTestElementSink() at the adapter boundary.
• Execution/TestCaseDiscoverySink.cs — the internal execution-side collector now implements IUnitTestElementSink but still materializes TestCase into its ICollection<TestCase> Tests, preserving the execution path.
• Tests updated: Mock<ITestCaseDiscoverySink> kept and wrapped at call sites (so existing Verify assertions stay meaningful); TestCaseDiscoverySinkTests rewritten for the new API; integration helper CLITestBase.discovery.cs wraps its sink.

No behavior change

Pure refactor. element.ToTestCase() is still invoked exactly once per emitted element in both the discovery and execution paths, the filter application point and the Phase 2 filterHasError bail-out semantics are unchanged, and discovered TestCase fields/Id/properties/traits/ordering are byte-for-byte identical. Execution still consumes TestCase internally (that decoupling is Phase 4, intentionally out of scope here).

Remaining move-to-adapter work (future phases)

• The Services/UnitTestElementSinkExtensions bridge still lives in PlatformServices for now (same as the Phase 1/2/5 bridges); it is expected to move fully into the adapter layer once discovery no longer flows VSTest sinks through platform services.
• The execution-side TestCaseDiscoverySink still materializes TestCase because execution consumes TestCase end-to-end — that is Phase 4.

Verification

• .\build.cmd -c Debug green across all TFMs (net462, net8.0, net9.0, UWP, WinUI) — 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net8.0): 893/893 passed.
• MSTestAdapter.UnitTests (net8.0): 21/21 passed.
• expert-reviewer run on the diff: no blocking/major findings; two doc-consistency nits addressed.

⚠️ Stacked PR

Phase 1 (#9548) and Phase 5 (#9550) are already on main. This PR is stacked on Phase 2 (#9567) and should be reviewed/merged after#9567 lands on main.

Current base is the integration branch dev/amauryleve/vstest-decoupling-base (= main + Phase 2), which keeps the diff clean today. Once #9567 merges, this PR can be retargeted to base=main directly — that is the intended final base. Please do not rebase/reset the base branch in the meantime.

[Amaury Levé (Evangelink)]

Why

Part of the multi-PR initiative to make src/Adapter/MSTestAdapter.PlatformServices platform-agnostic by removing its dependency on the VSTest object model (Microsoft.TestPlatform.ObjectModel). VSTest coupling moves up into the adapter boundary; the platform-services engine becomes neutral.

This is Phase 3 (discovery output / sink). It follows the exact pattern established by:

• Phase 1 — IAdapterMessageLogger (interface) + AdapterMessageLoggerExtensions (bridge) — merged to main (Abstract VSTest message logging in PlatformServices (Phase 1 of platform-agnostic effort) #9548)
• Phase 5 — ITestResultRecorder (interface) + TestResultRecorderExtensions (bridge) — merged to main (Abstract VSTest result reporting in PlatformServices (Phase 5 of platform-agnostic effort) #9550)
• Phase 2 — ITestElementFilter (interface) + TestMethodFilter.GetTestElementFilter (bridge) — open standalone PR Abstract VSTest test filtering in PlatformServices (Phase 2 of platform-agnostic effort) #9567

What

The discovery engine (UnitTestDiscoverer) now emits the neutral UnitTestElement model to a new platform-agnostic sink instead of building VSTest TestCases and pushing them to ITestCaseDiscoverySink.

• NewInterfaces/IUnitTestElementSink.cs — neutral sink (namespace …PlatformServices.Interface) with void SendTestElement(UnitTestElement testElement). The name deliberately carries no VSTest/TestCase/ObjectModel terms.
• NewServices/UnitTestElementSinkExtensions.cs — the single VSTest translation point: ToUnitTestElementSink(this ITestCaseDiscoverySink) wraps the host sink and materializes element.ToTestCase() at the boundary.
• Discovery/UnitTestDiscoverer.cs — DiscoverTests/DiscoverTestsInSource/SendTestCases now take IUnitTestElementSink; the engine no longer references ITestCaseDiscoverySink or calls ToTestCase().
• VSTestAdapter/MSTestDiscoverer.cs — wraps the host's real ITestCaseDiscoverySink via .ToUnitTestElementSink() at the adapter boundary.
• Execution/TestCaseDiscoverySink.cs — the internal execution-side collector now implements IUnitTestElementSink but still materializes TestCase into its ICollection<TestCase> Tests, preserving the execution path.
• Tests updated: Mock<ITestCaseDiscoverySink> kept and wrapped at call sites (so existing Verify assertions stay meaningful); TestCaseDiscoverySinkTests rewritten for the new API; integration helper CLITestBase.discovery.cs wraps its sink.

No behavior change

Pure refactor. element.ToTestCase() is still invoked exactly once per emitted element in both the discovery and execution paths, the filter application point and the Phase 2 filterHasError bail-out semantics are unchanged, and discovered TestCase fields/Id/properties/traits/ordering are byte-for-byte identical. Execution still consumes TestCase internally (that decoupling is Phase 4, intentionally out of scope here).

Remaining move-to-adapter work (future phases)

• The Services/UnitTestElementSinkExtensions bridge still lives in PlatformServices for now (same as the Phase 1/2/5 bridges); it is expected to move fully into the adapter layer once discovery no longer flows VSTest sinks through platform services.
• The execution-side TestCaseDiscoverySink still materializes TestCase because execution consumes TestCase end-to-end — that is Phase 4.

Verification

• .\build.cmd -c Debug green across all TFMs (net462, net8.0, net9.0, UWP, WinUI) — 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net8.0): 893/893 passed.
• MSTestAdapter.UnitTests (net8.0): 21/21 passed.
• expert-reviewer run on the diff: no blocking/major findings; two doc-consistency nits addressed.

⚠️ Stacked PR

Phase 1 (#9548) and Phase 5 (#9550) are already on main. This PR is stacked on Phase 2 (#9567) and should be reviewed/merged after#9567 lands on main.

Current base is the integration branch dev/amauryleve/vstest-decoupling-base (= main + Phase 2), which keeps the diff clean today. Once #9567 merges, this PR can be retargeted to base=main directly — that is the intended final base. Please do not rebase/reset the base branch in the meantime.

[Amaury Levé (Evangelink)]

Why

Part of the multi-PR initiative to make src/Adapter/MSTestAdapter.PlatformServices platform-agnostic by removing its dependency on the VSTest object model (Microsoft.TestPlatform.ObjectModel). VSTest coupling moves up into the adapter boundary; the platform-services engine becomes neutral.

This is Phase 3 (discovery output / sink). It follows the exact pattern established by:

• Phase 1 — IAdapterMessageLogger (interface) + AdapterMessageLoggerExtensions (bridge) — merged to main (Abstract VSTest message logging in PlatformServices (Phase 1 of platform-agnostic effort) #9548)
• Phase 5 — ITestResultRecorder (interface) + TestResultRecorderExtensions (bridge) — merged to main (Abstract VSTest result reporting in PlatformServices (Phase 5 of platform-agnostic effort) #9550)
• Phase 2 — ITestElementFilter (interface) + TestMethodFilter.GetTestElementFilter (bridge) — open standalone PR Abstract VSTest test filtering in PlatformServices (Phase 2 of platform-agnostic effort) #9567

What

The discovery engine (UnitTestDiscoverer) now emits the neutral UnitTestElement model to a new platform-agnostic sink instead of building VSTest TestCases and pushing them to ITestCaseDiscoverySink.

• NewInterfaces/IUnitTestElementSink.cs — neutral sink (namespace …PlatformServices.Interface) with void SendTestElement(UnitTestElement testElement). The name deliberately carries no VSTest/TestCase/ObjectModel terms.
• NewServices/UnitTestElementSinkExtensions.cs — the single VSTest translation point: ToUnitTestElementSink(this ITestCaseDiscoverySink) wraps the host sink and materializes element.ToTestCase() at the boundary.
• Discovery/UnitTestDiscoverer.cs — DiscoverTests/DiscoverTestsInSource/SendTestCases now take IUnitTestElementSink; the engine no longer references ITestCaseDiscoverySink or calls ToTestCase().
• VSTestAdapter/MSTestDiscoverer.cs — wraps the host's real ITestCaseDiscoverySink via .ToUnitTestElementSink() at the adapter boundary.
• Execution/TestCaseDiscoverySink.cs — the internal execution-side collector now implements IUnitTestElementSink but still materializes TestCase into its ICollection<TestCase> Tests, preserving the execution path.
• Tests updated: Mock<ITestCaseDiscoverySink> kept and wrapped at call sites (so existing Verify assertions stay meaningful); TestCaseDiscoverySinkTests rewritten for the new API; integration helper CLITestBase.discovery.cs wraps its sink.

No behavior change

Pure refactor. element.ToTestCase() is still invoked exactly once per emitted element in both the discovery and execution paths, the filter application point and the Phase 2 filterHasError bail-out semantics are unchanged, and discovered TestCase fields/Id/properties/traits/ordering are byte-for-byte identical. Execution still consumes TestCase internally (that decoupling is Phase 4, intentionally out of scope here).

Remaining move-to-adapter work (future phases)

• The Services/UnitTestElementSinkExtensions bridge still lives in PlatformServices for now (same as the Phase 1/2/5 bridges); it is expected to move fully into the adapter layer once discovery no longer flows VSTest sinks through platform services.
• The execution-side TestCaseDiscoverySink still materializes TestCase because execution consumes TestCase end-to-end — that is Phase 4.

Verification

• .\build.cmd -c Debug green across all TFMs (net462, net8.0, net9.0, UWP, WinUI) — 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net8.0): 893/893 passed.
• MSTestAdapter.UnitTests (net8.0): 21/21 passed.
• expert-reviewer run on the diff: no blocking/major findings; two doc-consistency nits addressed.

⚠️ Stacked PR

Phase 1 (#9548) and Phase 5 (#9550) are already on main. This PR is stacked on Phase 2 (#9567) and should be reviewed/merged after#9567 lands on main.

Current base is the integration branch dev/amauryleve/vstest-decoupling-base (= main + Phase 2), which keeps the diff clean today. Once #9567 merges, this PR can be retargeted to base=main directly — that is the intended final base. Please do not rebase/reset the base branch in the meantime.

[Amaury Levé (Evangelink)]

Why

Part of the multi-PR initiative to make src/Adapter/MSTestAdapter.PlatformServices platform-agnostic by removing its dependency on the VSTest object model (Microsoft.TestPlatform.ObjectModel). VSTest coupling moves up into the adapter boundary; the platform-services engine becomes neutral.

This is Phase 3 (discovery output / sink). It follows the exact pattern established by:

• Phase 1 — IAdapterMessageLogger (interface) + AdapterMessageLoggerExtensions (bridge) — merged to main (Abstract VSTest message logging in PlatformServices (Phase 1 of platform-agnostic effort) #9548)
• Phase 5 — ITestResultRecorder (interface) + TestResultRecorderExtensions (bridge) — merged to main (Abstract VSTest result reporting in PlatformServices (Phase 5 of platform-agnostic effort) #9550)
• Phase 2 — ITestElementFilter (interface) + TestMethodFilter.GetTestElementFilter (bridge) — open standalone PR Abstract VSTest test filtering in PlatformServices (Phase 2 of platform-agnostic effort) #9567

What

The discovery engine (UnitTestDiscoverer) now emits the neutral UnitTestElement model to a new platform-agnostic sink instead of building VSTest TestCases and pushing them to ITestCaseDiscoverySink.

• NewInterfaces/IUnitTestElementSink.cs — neutral sink (namespace …PlatformServices.Interface) with void SendTestElement(UnitTestElement testElement). The name deliberately carries no VSTest/TestCase/ObjectModel terms.
• NewServices/UnitTestElementSinkExtensions.cs — the single VSTest translation point: ToUnitTestElementSink(this ITestCaseDiscoverySink) wraps the host sink and materializes element.ToTestCase() at the boundary.
• Discovery/UnitTestDiscoverer.cs — DiscoverTests/DiscoverTestsInSource/SendTestCases now take IUnitTestElementSink; the engine no longer references ITestCaseDiscoverySink or calls ToTestCase().
• VSTestAdapter/MSTestDiscoverer.cs — wraps the host's real ITestCaseDiscoverySink via .ToUnitTestElementSink() at the adapter boundary.
• Execution/TestCaseDiscoverySink.cs — the internal execution-side collector now implements IUnitTestElementSink but still materializes TestCase into its ICollection<TestCase> Tests, preserving the execution path.
• Tests updated: Mock<ITestCaseDiscoverySink> kept and wrapped at call sites (so existing Verify assertions stay meaningful); TestCaseDiscoverySinkTests rewritten for the new API; integration helper CLITestBase.discovery.cs wraps its sink.

No behavior change

Pure refactor. element.ToTestCase() is still invoked exactly once per emitted element in both the discovery and execution paths, the filter application point and the Phase 2 filterHasError bail-out semantics are unchanged, and discovered TestCase fields/Id/properties/traits/ordering are byte-for-byte identical. Execution still consumes TestCase internally (that decoupling is Phase 4, intentionally out of scope here).

Remaining move-to-adapter work (future phases)

• The Services/UnitTestElementSinkExtensions bridge still lives in PlatformServices for now (same as the Phase 1/2/5 bridges); it is expected to move fully into the adapter layer once discovery no longer flows VSTest sinks through platform services.
• The execution-side TestCaseDiscoverySink still materializes TestCase because execution consumes TestCase end-to-end — that is Phase 4.

Verification

• .\build.cmd -c Debug green across all TFMs (net462, net8.0, net9.0, UWP, WinUI) — 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net8.0): 893/893 passed.
• MSTestAdapter.UnitTests (net8.0): 21/21 passed.
• expert-reviewer run on the diff: no blocking/major findings; two doc-consistency nits addressed.

⚠️ Stacked PR

Phase 1 (#9548) and Phase 5 (#9550) are already on main. This PR is stacked on Phase 2 (#9567) and should be reviewed/merged after#9567 lands on main.

Current base is the integration branch dev/amauryleve/vstest-decoupling-base (= main + Phase 2), which keeps the diff clean today. Once #9567 merges, this PR can be retargeted to base=main directly — that is the intended final base. Please do not rebase/reset the base branch in the meantime.

[Amaury Levé (Evangelink)]

Why

Part of the multi-PR initiative to make src/Adapter/MSTestAdapter.PlatformServices platform-agnostic by removing its dependency on the VSTest object model (Microsoft.TestPlatform.ObjectModel). VSTest coupling moves up into the adapter boundary; the platform-services engine becomes neutral.

This is Phase 3 (discovery output / sink). It follows the exact pattern established by:

• Phase 1 — IAdapterMessageLogger (interface) + AdapterMessageLoggerExtensions (bridge) — merged to main (Abstract VSTest message logging in PlatformServices (Phase 1 of platform-agnostic effort) #9548)
• Phase 5 — ITestResultRecorder (interface) + TestResultRecorderExtensions (bridge) — merged to main (Abstract VSTest result reporting in PlatformServices (Phase 5 of platform-agnostic effort) #9550)
• Phase 2 — ITestElementFilter (interface) + TestMethodFilter.GetTestElementFilter (bridge) — open standalone PR Abstract VSTest test filtering in PlatformServices (Phase 2 of platform-agnostic effort) #9567

What

The discovery engine (UnitTestDiscoverer) now emits the neutral UnitTestElement model to a new platform-agnostic sink instead of building VSTest TestCases and pushing them to ITestCaseDiscoverySink.

• NewInterfaces/IUnitTestElementSink.cs — neutral sink (namespace …PlatformServices.Interface) with void SendTestElement(UnitTestElement testElement). The name deliberately carries no VSTest/TestCase/ObjectModel terms.
• NewServices/UnitTestElementSinkExtensions.cs — the single VSTest translation point: ToUnitTestElementSink(this ITestCaseDiscoverySink) wraps the host sink and materializes element.ToTestCase() at the boundary.
• Discovery/UnitTestDiscoverer.cs — DiscoverTests/DiscoverTestsInSource/SendTestCases now take IUnitTestElementSink; the engine no longer references ITestCaseDiscoverySink or calls ToTestCase().
• VSTestAdapter/MSTestDiscoverer.cs — wraps the host's real ITestCaseDiscoverySink via .ToUnitTestElementSink() at the adapter boundary.
• Execution/TestCaseDiscoverySink.cs — the internal execution-side collector now implements IUnitTestElementSink but still materializes TestCase into its ICollection<TestCase> Tests, preserving the execution path.
• Tests updated: Mock<ITestCaseDiscoverySink> kept and wrapped at call sites (so existing Verify assertions stay meaningful); TestCaseDiscoverySinkTests rewritten for the new API; integration helper CLITestBase.discovery.cs wraps its sink.

No behavior change

Pure refactor. element.ToTestCase() is still invoked exactly once per emitted element in both the discovery and execution paths, the filter application point and the Phase 2 filterHasError bail-out semantics are unchanged, and discovered TestCase fields/Id/properties/traits/ordering are byte-for-byte identical. Execution still consumes TestCase internally (that decoupling is Phase 4, intentionally out of scope here).

Remaining move-to-adapter work (future phases)

• The Services/UnitTestElementSinkExtensions bridge still lives in PlatformServices for now (same as the Phase 1/2/5 bridges); it is expected to move fully into the adapter layer once discovery no longer flows VSTest sinks through platform services.
• The execution-side TestCaseDiscoverySink still materializes TestCase because execution consumes TestCase end-to-end — that is Phase 4.

Verification

• .\build.cmd -c Debug green across all TFMs (net462, net8.0, net9.0, UWP, WinUI) — 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net8.0): 893/893 passed.
• MSTestAdapter.UnitTests (net8.0): 21/21 passed.
• expert-reviewer run on the diff: no blocking/major findings; two doc-consistency nits addressed.

⚠️ Stacked PR

Phase 1 (#9548) and Phase 5 (#9550) are already on main. This PR is stacked on Phase 2 (#9567) and should be reviewed/merged after#9567 lands on main.

Current base is the integration branch dev/amauryleve/vstest-decoupling-base (= main + Phase 2), which keeps the diff clean today. Once #9567 merges, this PR can be retargeted to base=main directly — that is the intended final base. Please do not rebase/reset the base branch in the meantime.

[Amaury Levé (Evangelink)]

Why

Part of the multi-PR initiative to make src/Adapter/MSTestAdapter.PlatformServices platform-agnostic by removing its dependency on the VSTest object model (Microsoft.TestPlatform.ObjectModel). VSTest coupling moves up into the adapter boundary; the platform-services engine becomes neutral.

This is Phase 3 (discovery output / sink). It follows the exact pattern established by:

• Phase 1 — IAdapterMessageLogger (interface) + AdapterMessageLoggerExtensions (bridge) — merged to main (Abstract VSTest message logging in PlatformServices (Phase 1 of platform-agnostic effort) #9548)
• Phase 5 — ITestResultRecorder (interface) + TestResultRecorderExtensions (bridge) — merged to main (Abstract VSTest result reporting in PlatformServices (Phase 5 of platform-agnostic effort) #9550)
• Phase 2 — ITestElementFilter (interface) + TestMethodFilter.GetTestElementFilter (bridge) — open standalone PR Abstract VSTest test filtering in PlatformServices (Phase 2 of platform-agnostic effort) #9567

What

The discovery engine (UnitTestDiscoverer) now emits the neutral UnitTestElement model to a new platform-agnostic sink instead of building VSTest TestCases and pushing them to ITestCaseDiscoverySink.

• NewInterfaces/IUnitTestElementSink.cs — neutral sink (namespace …PlatformServices.Interface) with void SendTestElement(UnitTestElement testElement). The name deliberately carries no VSTest/TestCase/ObjectModel terms.
• NewServices/UnitTestElementSinkExtensions.cs — the single VSTest translation point: ToUnitTestElementSink(this ITestCaseDiscoverySink) wraps the host sink and materializes element.ToTestCase() at the boundary.
• Discovery/UnitTestDiscoverer.cs — DiscoverTests/DiscoverTestsInSource/SendTestCases now take IUnitTestElementSink; the engine no longer references ITestCaseDiscoverySink or calls ToTestCase().
• VSTestAdapter/MSTestDiscoverer.cs — wraps the host's real ITestCaseDiscoverySink via .ToUnitTestElementSink() at the adapter boundary.
• Execution/TestCaseDiscoverySink.cs — the internal execution-side collector now implements IUnitTestElementSink but still materializes TestCase into its ICollection<TestCase> Tests, preserving the execution path.
• Tests updated: Mock<ITestCaseDiscoverySink> kept and wrapped at call sites (so existing Verify assertions stay meaningful); TestCaseDiscoverySinkTests rewritten for the new API; integration helper CLITestBase.discovery.cs wraps its sink.

No behavior change

Pure refactor. element.ToTestCase() is still invoked exactly once per emitted element in both the discovery and execution paths, the filter application point and the Phase 2 filterHasError bail-out semantics are unchanged, and discovered TestCase fields/Id/properties/traits/ordering are byte-for-byte identical. Execution still consumes TestCase internally (that decoupling is Phase 4, intentionally out of scope here).

Remaining move-to-adapter work (future phases)

• The Services/UnitTestElementSinkExtensions bridge still lives in PlatformServices for now (same as the Phase 1/2/5 bridges); it is expected to move fully into the adapter layer once discovery no longer flows VSTest sinks through platform services.
• The execution-side TestCaseDiscoverySink still materializes TestCase because execution consumes TestCase end-to-end — that is Phase 4.

Verification

• .\build.cmd -c Debug green across all TFMs (net462, net8.0, net9.0, UWP, WinUI) — 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net8.0): 893/893 passed.
• MSTestAdapter.UnitTests (net8.0): 21/21 passed.
• expert-reviewer run on the diff: no blocking/major findings; two doc-consistency nits addressed.

⚠️ Stacked PR

Phase 1 (#9548) and Phase 5 (#9550) are already on main. This PR is stacked on Phase 2 (#9567) and should be reviewed/merged after#9567 lands on main.

Current base is the integration branch dev/amauryleve/vstest-decoupling-base (= main + Phase 2), which keeps the diff clean today. Once #9567 merges, this PR can be retargeted to base=main directly — that is the intended final base. Please do not rebase/reset the base branch in the meantime.

[Amaury Levé (Evangelink)]

Why

Part of the multi-PR initiative to make src/Adapter/MSTestAdapter.PlatformServices platform-agnostic by removing its dependency on the VSTest object model (Microsoft.TestPlatform.ObjectModel). VSTest coupling moves up into the adapter boundary; the platform-services engine becomes neutral.

This is Phase 3 (discovery output / sink). It follows the exact pattern established by:

• Phase 1 — IAdapterMessageLogger (interface) + AdapterMessageLoggerExtensions (bridge) — merged to main (Abstract VSTest message logging in PlatformServices (Phase 1 of platform-agnostic effort) #9548)
• Phase 5 — ITestResultRecorder (interface) + TestResultRecorderExtensions (bridge) — merged to main (Abstract VSTest result reporting in PlatformServices (Phase 5 of platform-agnostic effort) #9550)
• Phase 2 — ITestElementFilter (interface) + TestMethodFilter.GetTestElementFilter (bridge) — open standalone PR Abstract VSTest test filtering in PlatformServices (Phase 2 of platform-agnostic effort) #9567

What

The discovery engine (UnitTestDiscoverer) now emits the neutral UnitTestElement model to a new platform-agnostic sink instead of building VSTest TestCases and pushing them to ITestCaseDiscoverySink.

• NewInterfaces/IUnitTestElementSink.cs — neutral sink (namespace …PlatformServices.Interface) with void SendTestElement(UnitTestElement testElement). The name deliberately carries no VSTest/TestCase/ObjectModel terms.
• NewServices/UnitTestElementSinkExtensions.cs — the single VSTest translation point: ToUnitTestElementSink(this ITestCaseDiscoverySink) wraps the host sink and materializes element.ToTestCase() at the boundary.
• Discovery/UnitTestDiscoverer.cs — DiscoverTests/DiscoverTestsInSource/SendTestCases now take IUnitTestElementSink; the engine no longer references ITestCaseDiscoverySink or calls ToTestCase().
• VSTestAdapter/MSTestDiscoverer.cs — wraps the host's real ITestCaseDiscoverySink via .ToUnitTestElementSink() at the adapter boundary.
• Execution/TestCaseDiscoverySink.cs — the internal execution-side collector now implements IUnitTestElementSink but still materializes TestCase into its ICollection<TestCase> Tests, preserving the execution path.
• Tests updated: Mock<ITestCaseDiscoverySink> kept and wrapped at call sites (so existing Verify assertions stay meaningful); TestCaseDiscoverySinkTests rewritten for the new API; integration helper CLITestBase.discovery.cs wraps its sink.

No behavior change

Pure refactor. element.ToTestCase() is still invoked exactly once per emitted element in both the discovery and execution paths, the filter application point and the Phase 2 filterHasError bail-out semantics are unchanged, and discovered TestCase fields/Id/properties/traits/ordering are byte-for-byte identical. Execution still consumes TestCase internally (that decoupling is Phase 4, intentionally out of scope here).

Remaining move-to-adapter work (future phases)

• The Services/UnitTestElementSinkExtensions bridge still lives in PlatformServices for now (same as the Phase 1/2/5 bridges); it is expected to move fully into the adapter layer once discovery no longer flows VSTest sinks through platform services.
• The execution-side TestCaseDiscoverySink still materializes TestCase because execution consumes TestCase end-to-end — that is Phase 4.

Verification

• .\build.cmd -c Debug green across all TFMs (net462, net8.0, net9.0, UWP, WinUI) — 0 warnings, 0 errors.
• MSTestAdapter.PlatformServices.UnitTests (net8.0): 893/893 passed.
• MSTestAdapter.UnitTests (net8.0): 21/21 passed.
• expert-reviewer run on the diff: no blocking/major findings; two doc-consistency nits addressed.

⚠️ Stacked PR

Phase 1 (#9548) and Phase 5 (#9550) are already on main. This PR is stacked on Phase 2 (#9567) and should be reviewed/merged after#9567 lands on main.

Current base is the integration branch dev/amauryleve/vstest-decoupling-base (= main + Phase 2), which keeps the diff clean today. Once #9567 merges, this PR can be retargeted to base=main directly — that is the intended final base. Please do not rebase/reset the base branch in the meantime.