Add Assert.AddValueFormatter for customizing assertion failure rendering

[Amaury Levé (Evangelink)]

Fixes#9089

Adds two public Assert.AddValueFormatter overloads that let users register custom formatters used by the structured assertion failure message renderer introduced by RFC-012 (#8964):

• AddValueFormatter<T>(Func<T, string?>) — typed sugar. Return null to fall through to the next formatter / built-in renderer.
• AddValueFormatter(Func<Func<object?, string>, Func<object?, string>>) — chain-of-responsibility factory that receives the next formatter.

Both overloads return an IDisposable whose disposal removes the registration. Registrations are anchored on AsyncLocal<T>, so the scope follows the test context (per-test, per-class, per-assembly), and they stack newest-first so the most recently registered formatter wins.

Example

[TestMethod]publicvoidDateTimeFailureShowsKind(){usingvar_=Assert.AddValueFormatter<DateTime>(dt =>$""{dt:O} [{dt.Kind}]"");Assert.AreEqual(d1,d2);// failure message now annotates DateTime.Kind}

Implementation

• New AssertionValueFormatterRegistry (internal, AsyncLocal-backed) holds the chain. Nodes are append-only with a mutable IsRemoved flag so Dispose works for any position in the stack (including out-of-order disposal).
• AssertionValueRenderer.RenderValue consults the registry ahead of the built-in switch, and only when at least one formatter is registered (zero-allocation fast-path for the common no-formatter case).
• null is always rendered as ""null"" and is never exposed to user formatters, to avoid spurious NullReferenceExceptions inside user code.
• User-formatter exceptions are caught and annotated so the underlying assertion failure remains useful.
• New API is marked [Experimental(""MSTESTEXP"")] consistent with Assert.Scope().

Tests

15 new AssertTests.AddValueFormatter tests cover:

• ArgumentNullException on both overloads
• End-to-end assertion failure-message customization
• Disposal removes the registration (including out-of-order disposal)
• Idempotent Dispose
• Newest-first stacking + fall-through ordering
• Null values bypass the registry
• Non-matching types bypass the typed overload
• Collection items get per-item formatting
• The factory overload composes with the typed overload
• Throwing formatters do not derail rendering

Verified passing on net9.0, net8.0, and net48.

Out of scope (per the issue)

• No discovery via attributes/interfaces (deferred).
• No built-in defaults changed.
• No equality customization (existing IEqualityComparer<T> overloads cover that).

[Copilot]

Pull request overview

Adds an experimental public API to MSTest (Assert.AddValueFormatter) to let users customize how values are rendered in the structured assertion failure messages (via an AsyncLocal-scoped, newest-first formatter stack), and updates the central renderer to consult those formatters before the built-in formatting logic.

Changes:

• Introduces Assert.AddValueFormatter<T>(Func<T, string?>) and Assert.AddValueFormatter(Func<Func<object?,string>, Func<object?,string>>) returning IDisposable for scoped registration/removal.
• Adds an internal AssertionValueFormatterRegistry (AsyncLocal-backed) and wires it into AssertionValueRenderer.RenderValue.
• Adds unit tests covering registration, stacking, disposal semantics, null bypass, and failure-message customization.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AddValueFormatter.cs	New unit tests for both overloads, ordering, disposal, and rendering behavior.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new experimental public API signatures.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Consults the formatter registry before built-in rendering (with a no-formatters fast path).
src/TestFramework/TestFramework/Assertions/AssertionValueFormatterRegistry.cs	New internal AsyncLocal registry that composes formatter factories and supports removal via disposal.
src/TestFramework/TestFramework/Assertions/Assert.AddValueFormatter.cs	Implements the two new public overloads and documents scoping/stacking semantics.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Amaury Levé (Evangelink)]

The Windows Debug failure was an unrelated, intermittent infra flake in MSBuildTests_KnownExtensionRegistration.Microsoft_Testing_Platform_Extensions_ShouldBe_Correctly_Registered: 2 of the 4 net10.0 matrix entries threw Sequence contains no matching element because the _GenerateSelfRegisteredExtensions target was absent from the captured binlog, while the other 2 net10.0 entries (and all other TFMs) passed. This PR only touches Assert code in TestFramework and cannot affect MSBuild extension registration. Pushed an empty commit to re-run CI.

[Amaury Levé (Evangelink)]

🧪 Test quality grade — PR #9148

17 test methods graded across 2 files; all 17 earn A (90–100). Tests are well-named, single-behavior, and cover the formatter registry's lifecycle end-to-end — null guards, type specificity, LIFO stacking, fall-through chaining, dispose idempotency, fast-path restoration, fault isolation, and acceptance-level scope flow-down. No issues found across the board.

Δ	Test	Grade	Band	Notes
new	ValueFormatterFlowTests. ValueFormatterScopesFlowDownCorrectly	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ AppliesToCollectionItems	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeIsIdempotent	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeRemovesOnlyThatRegistration	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposingAllRegistrationsRestoresFastPath	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ChainsWithTypedOverload	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ReceivesNextDelegate	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Factory_ NullFactory_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ NullValuesAlwaysRenderedAsNullLiteral	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksFallThroughInOrder	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksNewestFirst	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ ThrowingFormatterDoesNotDerailRendering	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ CustomizesFailureMessage	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ DoesNotMatchUnrelatedTypes	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ FormatterRemovedAfterDispose	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ NullFormatter_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ ReturningNullFallsThroughToBuiltIn	A	90–100	No issues found.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

🤖 Automated content by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Grade Tests on PR (on open / sync) workflow. · 286.5 AIC · ⌖ 13.4 AIC · [◷]( · ◷)

[Amaury Levé (Evangelink)]

Fixes#9089

Adds two public Assert.AddValueFormatter overloads that let users register custom formatters used by the structured assertion failure message renderer introduced by RFC-012 (#8964):

• AddValueFormatter<T>(Func<T, string?>) — typed sugar. Return null to fall through to the next formatter / built-in renderer.
• AddValueFormatter(Func<Func<object?, string>, Func<object?, string>>) — chain-of-responsibility factory that receives the next formatter.

Both overloads return an IDisposable whose disposal removes the registration. Registrations are anchored on AsyncLocal<T>, so the scope follows the test context (per-test, per-class, per-assembly), and they stack newest-first so the most recently registered formatter wins.

Example

[TestMethod]publicvoidDateTimeFailureShowsKind(){usingvar_=Assert.AddValueFormatter<DateTime>(dt =>$""{dt:O} [{dt.Kind}]"");Assert.AreEqual(d1,d2);// failure message now annotates DateTime.Kind}

Implementation

• New AssertionValueFormatterRegistry (internal, AsyncLocal-backed) holds the chain. Nodes are append-only with a mutable IsRemoved flag so Dispose works for any position in the stack (including out-of-order disposal).
• AssertionValueRenderer.RenderValue consults the registry ahead of the built-in switch, and only when at least one formatter is registered (zero-allocation fast-path for the common no-formatter case).
• null is always rendered as ""null"" and is never exposed to user formatters, to avoid spurious NullReferenceExceptions inside user code.
• User-formatter exceptions are caught and annotated so the underlying assertion failure remains useful.
• New API is marked [Experimental(""MSTESTEXP"")] consistent with Assert.Scope().

Tests

15 new AssertTests.AddValueFormatter tests cover:

• ArgumentNullException on both overloads
• End-to-end assertion failure-message customization
• Disposal removes the registration (including out-of-order disposal)
• Idempotent Dispose
• Newest-first stacking + fall-through ordering
• Null values bypass the registry
• Non-matching types bypass the typed overload
• Collection items get per-item formatting
• The factory overload composes with the typed overload
• Throwing formatters do not derail rendering

Verified passing on net9.0, net8.0, and net48.

Out of scope (per the issue)

• No discovery via attributes/interfaces (deferred).
• No built-in defaults changed.
• No equality customization (existing IEqualityComparer<T> overloads cover that).

[Copilot]

Pull request overview

Adds an experimental public API to MSTest (Assert.AddValueFormatter) to let users customize how values are rendered in the structured assertion failure messages (via an AsyncLocal-scoped, newest-first formatter stack), and updates the central renderer to consult those formatters before the built-in formatting logic.

Changes:

• Introduces Assert.AddValueFormatter<T>(Func<T, string?>) and Assert.AddValueFormatter(Func<Func<object?,string>, Func<object?,string>>) returning IDisposable for scoped registration/removal.
• Adds an internal AssertionValueFormatterRegistry (AsyncLocal-backed) and wires it into AssertionValueRenderer.RenderValue.
• Adds unit tests covering registration, stacking, disposal semantics, null bypass, and failure-message customization.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AddValueFormatter.cs	New unit tests for both overloads, ordering, disposal, and rendering behavior.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new experimental public API signatures.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Consults the formatter registry before built-in rendering (with a no-formatters fast path).
src/TestFramework/TestFramework/Assertions/AssertionValueFormatterRegistry.cs	New internal AsyncLocal registry that composes formatter factories and supports removal via disposal.
src/TestFramework/TestFramework/Assertions/Assert.AddValueFormatter.cs	Implements the two new public overloads and documents scoping/stacking semantics.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Amaury Levé (Evangelink)]

The Windows Debug failure was an unrelated, intermittent infra flake in MSBuildTests_KnownExtensionRegistration.Microsoft_Testing_Platform_Extensions_ShouldBe_Correctly_Registered: 2 of the 4 net10.0 matrix entries threw Sequence contains no matching element because the _GenerateSelfRegisteredExtensions target was absent from the captured binlog, while the other 2 net10.0 entries (and all other TFMs) passed. This PR only touches Assert code in TestFramework and cannot affect MSBuild extension registration. Pushed an empty commit to re-run CI.

[Amaury Levé (Evangelink)]

🧪 Test quality grade — PR #9148

17 test methods graded across 2 files; all 17 earn A (90–100). Tests are well-named, single-behavior, and cover the formatter registry's lifecycle end-to-end — null guards, type specificity, LIFO stacking, fall-through chaining, dispose idempotency, fast-path restoration, fault isolation, and acceptance-level scope flow-down. No issues found across the board.

Δ	Test	Grade	Band	Notes
new	ValueFormatterFlowTests. ValueFormatterScopesFlowDownCorrectly	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ AppliesToCollectionItems	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeIsIdempotent	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeRemovesOnlyThatRegistration	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposingAllRegistrationsRestoresFastPath	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ChainsWithTypedOverload	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ReceivesNextDelegate	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Factory_ NullFactory_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ NullValuesAlwaysRenderedAsNullLiteral	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksFallThroughInOrder	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksNewestFirst	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ ThrowingFormatterDoesNotDerailRendering	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ CustomizesFailureMessage	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ DoesNotMatchUnrelatedTypes	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ FormatterRemovedAfterDispose	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ NullFormatter_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ ReturningNullFallsThroughToBuiltIn	A	90–100	No issues found.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

🤖 Automated content by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Grade Tests on PR (on open / sync) workflow. · 286.5 AIC · ⌖ 13.4 AIC · [◷]( · ◷)

[Amaury Levé (Evangelink)]

Fixes#9089

Adds two public Assert.AddValueFormatter overloads that let users register custom formatters used by the structured assertion failure message renderer introduced by RFC-012 (#8964):

• AddValueFormatter<T>(Func<T, string?>) — typed sugar. Return null to fall through to the next formatter / built-in renderer.
• AddValueFormatter(Func<Func<object?, string>, Func<object?, string>>) — chain-of-responsibility factory that receives the next formatter.

Both overloads return an IDisposable whose disposal removes the registration. Registrations are anchored on AsyncLocal<T>, so the scope follows the test context (per-test, per-class, per-assembly), and they stack newest-first so the most recently registered formatter wins.

Example

[TestMethod]publicvoidDateTimeFailureShowsKind(){usingvar_=Assert.AddValueFormatter<DateTime>(dt =>$""{dt:O} [{dt.Kind}]"");Assert.AreEqual(d1,d2);// failure message now annotates DateTime.Kind}

Implementation

• New AssertionValueFormatterRegistry (internal, AsyncLocal-backed) holds the chain. Nodes are append-only with a mutable IsRemoved flag so Dispose works for any position in the stack (including out-of-order disposal).
• AssertionValueRenderer.RenderValue consults the registry ahead of the built-in switch, and only when at least one formatter is registered (zero-allocation fast-path for the common no-formatter case).
• null is always rendered as ""null"" and is never exposed to user formatters, to avoid spurious NullReferenceExceptions inside user code.
• User-formatter exceptions are caught and annotated so the underlying assertion failure remains useful.
• New API is marked [Experimental(""MSTESTEXP"")] consistent with Assert.Scope().

Tests

15 new AssertTests.AddValueFormatter tests cover:

• ArgumentNullException on both overloads
• End-to-end assertion failure-message customization
• Disposal removes the registration (including out-of-order disposal)
• Idempotent Dispose
• Newest-first stacking + fall-through ordering
• Null values bypass the registry
• Non-matching types bypass the typed overload
• Collection items get per-item formatting
• The factory overload composes with the typed overload
• Throwing formatters do not derail rendering

Verified passing on net9.0, net8.0, and net48.

Out of scope (per the issue)

• No discovery via attributes/interfaces (deferred).
• No built-in defaults changed.
• No equality customization (existing IEqualityComparer<T> overloads cover that).

[Copilot]

Pull request overview

Adds an experimental public API to MSTest (Assert.AddValueFormatter) to let users customize how values are rendered in the structured assertion failure messages (via an AsyncLocal-scoped, newest-first formatter stack), and updates the central renderer to consult those formatters before the built-in formatting logic.

Changes:

• Introduces Assert.AddValueFormatter<T>(Func<T, string?>) and Assert.AddValueFormatter(Func<Func<object?,string>, Func<object?,string>>) returning IDisposable for scoped registration/removal.
• Adds an internal AssertionValueFormatterRegistry (AsyncLocal-backed) and wires it into AssertionValueRenderer.RenderValue.
• Adds unit tests covering registration, stacking, disposal semantics, null bypass, and failure-message customization.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AddValueFormatter.cs	New unit tests for both overloads, ordering, disposal, and rendering behavior.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new experimental public API signatures.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Consults the formatter registry before built-in rendering (with a no-formatters fast path).
src/TestFramework/TestFramework/Assertions/AssertionValueFormatterRegistry.cs	New internal AsyncLocal registry that composes formatter factories and supports removal via disposal.
src/TestFramework/TestFramework/Assertions/Assert.AddValueFormatter.cs	Implements the two new public overloads and documents scoping/stacking semantics.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Amaury Levé (Evangelink)]

The Windows Debug failure was an unrelated, intermittent infra flake in MSBuildTests_KnownExtensionRegistration.Microsoft_Testing_Platform_Extensions_ShouldBe_Correctly_Registered: 2 of the 4 net10.0 matrix entries threw Sequence contains no matching element because the _GenerateSelfRegisteredExtensions target was absent from the captured binlog, while the other 2 net10.0 entries (and all other TFMs) passed. This PR only touches Assert code in TestFramework and cannot affect MSBuild extension registration. Pushed an empty commit to re-run CI.

[Amaury Levé (Evangelink)]

🧪 Test quality grade — PR #9148

17 test methods graded across 2 files; all 17 earn A (90–100). Tests are well-named, single-behavior, and cover the formatter registry's lifecycle end-to-end — null guards, type specificity, LIFO stacking, fall-through chaining, dispose idempotency, fast-path restoration, fault isolation, and acceptance-level scope flow-down. No issues found across the board.

Δ	Test	Grade	Band	Notes
new	ValueFormatterFlowTests. ValueFormatterScopesFlowDownCorrectly	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ AppliesToCollectionItems	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeIsIdempotent	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeRemovesOnlyThatRegistration	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposingAllRegistrationsRestoresFastPath	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ChainsWithTypedOverload	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ReceivesNextDelegate	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Factory_ NullFactory_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ NullValuesAlwaysRenderedAsNullLiteral	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksFallThroughInOrder	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksNewestFirst	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ ThrowingFormatterDoesNotDerailRendering	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ CustomizesFailureMessage	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ DoesNotMatchUnrelatedTypes	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ FormatterRemovedAfterDispose	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ NullFormatter_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ ReturningNullFallsThroughToBuiltIn	A	90–100	No issues found.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

🤖 Automated content by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Grade Tests on PR (on open / sync) workflow. · 286.5 AIC · ⌖ 13.4 AIC · [◷]( · ◷)

[Amaury Levé (Evangelink)]

Fixes#9089

Adds two public Assert.AddValueFormatter overloads that let users register custom formatters used by the structured assertion failure message renderer introduced by RFC-012 (#8964):

• AddValueFormatter<T>(Func<T, string?>) — typed sugar. Return null to fall through to the next formatter / built-in renderer.
• AddValueFormatter(Func<Func<object?, string>, Func<object?, string>>) — chain-of-responsibility factory that receives the next formatter.

Both overloads return an IDisposable whose disposal removes the registration. Registrations are anchored on AsyncLocal<T>, so the scope follows the test context (per-test, per-class, per-assembly), and they stack newest-first so the most recently registered formatter wins.

Example

[TestMethod]publicvoidDateTimeFailureShowsKind(){usingvar_=Assert.AddValueFormatter<DateTime>(dt =>$""{dt:O} [{dt.Kind}]"");Assert.AreEqual(d1,d2);// failure message now annotates DateTime.Kind}

Implementation

• New AssertionValueFormatterRegistry (internal, AsyncLocal-backed) holds the chain. Nodes are append-only with a mutable IsRemoved flag so Dispose works for any position in the stack (including out-of-order disposal).
• AssertionValueRenderer.RenderValue consults the registry ahead of the built-in switch, and only when at least one formatter is registered (zero-allocation fast-path for the common no-formatter case).
• null is always rendered as ""null"" and is never exposed to user formatters, to avoid spurious NullReferenceExceptions inside user code.
• User-formatter exceptions are caught and annotated so the underlying assertion failure remains useful.
• New API is marked [Experimental(""MSTESTEXP"")] consistent with Assert.Scope().

Tests

15 new AssertTests.AddValueFormatter tests cover:

• ArgumentNullException on both overloads
• End-to-end assertion failure-message customization
• Disposal removes the registration (including out-of-order disposal)
• Idempotent Dispose
• Newest-first stacking + fall-through ordering
• Null values bypass the registry
• Non-matching types bypass the typed overload
• Collection items get per-item formatting
• The factory overload composes with the typed overload
• Throwing formatters do not derail rendering

Verified passing on net9.0, net8.0, and net48.

Out of scope (per the issue)

• No discovery via attributes/interfaces (deferred).
• No built-in defaults changed.
• No equality customization (existing IEqualityComparer<T> overloads cover that).

[Copilot]

Pull request overview

Adds an experimental public API to MSTest (Assert.AddValueFormatter) to let users customize how values are rendered in the structured assertion failure messages (via an AsyncLocal-scoped, newest-first formatter stack), and updates the central renderer to consult those formatters before the built-in formatting logic.

Changes:

• Introduces Assert.AddValueFormatter<T>(Func<T, string?>) and Assert.AddValueFormatter(Func<Func<object?,string>, Func<object?,string>>) returning IDisposable for scoped registration/removal.
• Adds an internal AssertionValueFormatterRegistry (AsyncLocal-backed) and wires it into AssertionValueRenderer.RenderValue.
• Adds unit tests covering registration, stacking, disposal semantics, null bypass, and failure-message customization.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AddValueFormatter.cs	New unit tests for both overloads, ordering, disposal, and rendering behavior.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new experimental public API signatures.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Consults the formatter registry before built-in rendering (with a no-formatters fast path).
src/TestFramework/TestFramework/Assertions/AssertionValueFormatterRegistry.cs	New internal AsyncLocal registry that composes formatter factories and supports removal via disposal.
src/TestFramework/TestFramework/Assertions/Assert.AddValueFormatter.cs	Implements the two new public overloads and documents scoping/stacking semantics.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Amaury Levé (Evangelink)]

The Windows Debug failure was an unrelated, intermittent infra flake in MSBuildTests_KnownExtensionRegistration.Microsoft_Testing_Platform_Extensions_ShouldBe_Correctly_Registered: 2 of the 4 net10.0 matrix entries threw Sequence contains no matching element because the _GenerateSelfRegisteredExtensions target was absent from the captured binlog, while the other 2 net10.0 entries (and all other TFMs) passed. This PR only touches Assert code in TestFramework and cannot affect MSBuild extension registration. Pushed an empty commit to re-run CI.

[Amaury Levé (Evangelink)]

🧪 Test quality grade — PR #9148

17 test methods graded across 2 files; all 17 earn A (90–100). Tests are well-named, single-behavior, and cover the formatter registry's lifecycle end-to-end — null guards, type specificity, LIFO stacking, fall-through chaining, dispose idempotency, fast-path restoration, fault isolation, and acceptance-level scope flow-down. No issues found across the board.

Δ	Test	Grade	Band	Notes
new	ValueFormatterFlowTests. ValueFormatterScopesFlowDownCorrectly	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ AppliesToCollectionItems	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeIsIdempotent	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeRemovesOnlyThatRegistration	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposingAllRegistrationsRestoresFastPath	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ChainsWithTypedOverload	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ReceivesNextDelegate	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Factory_ NullFactory_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ NullValuesAlwaysRenderedAsNullLiteral	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksFallThroughInOrder	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksNewestFirst	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ ThrowingFormatterDoesNotDerailRendering	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ CustomizesFailureMessage	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ DoesNotMatchUnrelatedTypes	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ FormatterRemovedAfterDispose	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ NullFormatter_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ ReturningNullFallsThroughToBuiltIn	A	90–100	No issues found.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

🤖 Automated content by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Grade Tests on PR (on open / sync) workflow. · 286.5 AIC · ⌖ 13.4 AIC · [◷]( · ◷)

[Amaury Levé (Evangelink)]

Fixes#9089

Adds two public Assert.AddValueFormatter overloads that let users register custom formatters used by the structured assertion failure message renderer introduced by RFC-012 (#8964):

• AddValueFormatter<T>(Func<T, string?>) — typed sugar. Return null to fall through to the next formatter / built-in renderer.
• AddValueFormatter(Func<Func<object?, string>, Func<object?, string>>) — chain-of-responsibility factory that receives the next formatter.

Both overloads return an IDisposable whose disposal removes the registration. Registrations are anchored on AsyncLocal<T>, so the scope follows the test context (per-test, per-class, per-assembly), and they stack newest-first so the most recently registered formatter wins.

Example

[TestMethod]publicvoidDateTimeFailureShowsKind(){usingvar_=Assert.AddValueFormatter<DateTime>(dt =>$""{dt:O} [{dt.Kind}]"");Assert.AreEqual(d1,d2);// failure message now annotates DateTime.Kind}

Implementation

• New AssertionValueFormatterRegistry (internal, AsyncLocal-backed) holds the chain. Nodes are append-only with a mutable IsRemoved flag so Dispose works for any position in the stack (including out-of-order disposal).
• AssertionValueRenderer.RenderValue consults the registry ahead of the built-in switch, and only when at least one formatter is registered (zero-allocation fast-path for the common no-formatter case).
• null is always rendered as ""null"" and is never exposed to user formatters, to avoid spurious NullReferenceExceptions inside user code.
• User-formatter exceptions are caught and annotated so the underlying assertion failure remains useful.
• New API is marked [Experimental(""MSTESTEXP"")] consistent with Assert.Scope().

Tests

15 new AssertTests.AddValueFormatter tests cover:

• ArgumentNullException on both overloads
• End-to-end assertion failure-message customization
• Disposal removes the registration (including out-of-order disposal)
• Idempotent Dispose
• Newest-first stacking + fall-through ordering
• Null values bypass the registry
• Non-matching types bypass the typed overload
• Collection items get per-item formatting
• The factory overload composes with the typed overload
• Throwing formatters do not derail rendering

Verified passing on net9.0, net8.0, and net48.

Out of scope (per the issue)

• No discovery via attributes/interfaces (deferred).
• No built-in defaults changed.
• No equality customization (existing IEqualityComparer<T> overloads cover that).

[Copilot]

Pull request overview

Adds an experimental public API to MSTest (Assert.AddValueFormatter) to let users customize how values are rendered in the structured assertion failure messages (via an AsyncLocal-scoped, newest-first formatter stack), and updates the central renderer to consult those formatters before the built-in formatting logic.

Changes:

• Introduces Assert.AddValueFormatter<T>(Func<T, string?>) and Assert.AddValueFormatter(Func<Func<object?,string>, Func<object?,string>>) returning IDisposable for scoped registration/removal.
• Adds an internal AssertionValueFormatterRegistry (AsyncLocal-backed) and wires it into AssertionValueRenderer.RenderValue.
• Adds unit tests covering registration, stacking, disposal semantics, null bypass, and failure-message customization.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AddValueFormatter.cs	New unit tests for both overloads, ordering, disposal, and rendering behavior.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new experimental public API signatures.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Consults the formatter registry before built-in rendering (with a no-formatters fast path).
src/TestFramework/TestFramework/Assertions/AssertionValueFormatterRegistry.cs	New internal AsyncLocal registry that composes formatter factories and supports removal via disposal.
src/TestFramework/TestFramework/Assertions/Assert.AddValueFormatter.cs	Implements the two new public overloads and documents scoping/stacking semantics.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Amaury Levé (Evangelink)]

The Windows Debug failure was an unrelated, intermittent infra flake in MSBuildTests_KnownExtensionRegistration.Microsoft_Testing_Platform_Extensions_ShouldBe_Correctly_Registered: 2 of the 4 net10.0 matrix entries threw Sequence contains no matching element because the _GenerateSelfRegisteredExtensions target was absent from the captured binlog, while the other 2 net10.0 entries (and all other TFMs) passed. This PR only touches Assert code in TestFramework and cannot affect MSBuild extension registration. Pushed an empty commit to re-run CI.

[Amaury Levé (Evangelink)]

🧪 Test quality grade — PR #9148

17 test methods graded across 2 files; all 17 earn A (90–100). Tests are well-named, single-behavior, and cover the formatter registry's lifecycle end-to-end — null guards, type specificity, LIFO stacking, fall-through chaining, dispose idempotency, fast-path restoration, fault isolation, and acceptance-level scope flow-down. No issues found across the board.

Δ	Test	Grade	Band	Notes
new	ValueFormatterFlowTests. ValueFormatterScopesFlowDownCorrectly	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ AppliesToCollectionItems	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeIsIdempotent	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeRemovesOnlyThatRegistration	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposingAllRegistrationsRestoresFastPath	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ChainsWithTypedOverload	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ReceivesNextDelegate	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Factory_ NullFactory_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ NullValuesAlwaysRenderedAsNullLiteral	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksFallThroughInOrder	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksNewestFirst	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ ThrowingFormatterDoesNotDerailRendering	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ CustomizesFailureMessage	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ DoesNotMatchUnrelatedTypes	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ FormatterRemovedAfterDispose	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ NullFormatter_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ ReturningNullFallsThroughToBuiltIn	A	90–100	No issues found.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

🤖 Automated content by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Grade Tests on PR (on open / sync) workflow. · 286.5 AIC · ⌖ 13.4 AIC · [◷]( · ◷)

[Amaury Levé (Evangelink)]

Fixes#9089

Adds two public Assert.AddValueFormatter overloads that let users register custom formatters used by the structured assertion failure message renderer introduced by RFC-012 (#8964):

• AddValueFormatter<T>(Func<T, string?>) — typed sugar. Return null to fall through to the next formatter / built-in renderer.
• AddValueFormatter(Func<Func<object?, string>, Func<object?, string>>) — chain-of-responsibility factory that receives the next formatter.

Both overloads return an IDisposable whose disposal removes the registration. Registrations are anchored on AsyncLocal<T>, so the scope follows the test context (per-test, per-class, per-assembly), and they stack newest-first so the most recently registered formatter wins.

Example

[TestMethod]publicvoidDateTimeFailureShowsKind(){usingvar_=Assert.AddValueFormatter<DateTime>(dt =>$""{dt:O} [{dt.Kind}]"");Assert.AreEqual(d1,d2);// failure message now annotates DateTime.Kind}

Implementation

• New AssertionValueFormatterRegistry (internal, AsyncLocal-backed) holds the chain. Nodes are append-only with a mutable IsRemoved flag so Dispose works for any position in the stack (including out-of-order disposal).
• AssertionValueRenderer.RenderValue consults the registry ahead of the built-in switch, and only when at least one formatter is registered (zero-allocation fast-path for the common no-formatter case).
• null is always rendered as ""null"" and is never exposed to user formatters, to avoid spurious NullReferenceExceptions inside user code.
• User-formatter exceptions are caught and annotated so the underlying assertion failure remains useful.
• New API is marked [Experimental(""MSTESTEXP"")] consistent with Assert.Scope().

Tests

15 new AssertTests.AddValueFormatter tests cover:

• ArgumentNullException on both overloads
• End-to-end assertion failure-message customization
• Disposal removes the registration (including out-of-order disposal)
• Idempotent Dispose
• Newest-first stacking + fall-through ordering
• Null values bypass the registry
• Non-matching types bypass the typed overload
• Collection items get per-item formatting
• The factory overload composes with the typed overload
• Throwing formatters do not derail rendering

Verified passing on net9.0, net8.0, and net48.

Out of scope (per the issue)

• No discovery via attributes/interfaces (deferred).
• No built-in defaults changed.
• No equality customization (existing IEqualityComparer<T> overloads cover that).

[Copilot]

Pull request overview

Adds an experimental public API to MSTest (Assert.AddValueFormatter) to let users customize how values are rendered in the structured assertion failure messages (via an AsyncLocal-scoped, newest-first formatter stack), and updates the central renderer to consult those formatters before the built-in formatting logic.

Changes:

• Introduces Assert.AddValueFormatter<T>(Func<T, string?>) and Assert.AddValueFormatter(Func<Func<object?,string>, Func<object?,string>>) returning IDisposable for scoped registration/removal.
• Adds an internal AssertionValueFormatterRegistry (AsyncLocal-backed) and wires it into AssertionValueRenderer.RenderValue.
• Adds unit tests covering registration, stacking, disposal semantics, null bypass, and failure-message customization.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AddValueFormatter.cs	New unit tests for both overloads, ordering, disposal, and rendering behavior.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new experimental public API signatures.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Consults the formatter registry before built-in rendering (with a no-formatters fast path).
src/TestFramework/TestFramework/Assertions/AssertionValueFormatterRegistry.cs	New internal AsyncLocal registry that composes formatter factories and supports removal via disposal.
src/TestFramework/TestFramework/Assertions/Assert.AddValueFormatter.cs	Implements the two new public overloads and documents scoping/stacking semantics.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Amaury Levé (Evangelink)]

The Windows Debug failure was an unrelated, intermittent infra flake in MSBuildTests_KnownExtensionRegistration.Microsoft_Testing_Platform_Extensions_ShouldBe_Correctly_Registered: 2 of the 4 net10.0 matrix entries threw Sequence contains no matching element because the _GenerateSelfRegisteredExtensions target was absent from the captured binlog, while the other 2 net10.0 entries (and all other TFMs) passed. This PR only touches Assert code in TestFramework and cannot affect MSBuild extension registration. Pushed an empty commit to re-run CI.

[Amaury Levé (Evangelink)]

🧪 Test quality grade — PR #9148

17 test methods graded across 2 files; all 17 earn A (90–100). Tests are well-named, single-behavior, and cover the formatter registry's lifecycle end-to-end — null guards, type specificity, LIFO stacking, fall-through chaining, dispose idempotency, fast-path restoration, fault isolation, and acceptance-level scope flow-down. No issues found across the board.

Δ	Test	Grade	Band	Notes
new	ValueFormatterFlowTests. ValueFormatterScopesFlowDownCorrectly	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ AppliesToCollectionItems	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeIsIdempotent	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeRemovesOnlyThatRegistration	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposingAllRegistrationsRestoresFastPath	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ChainsWithTypedOverload	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ReceivesNextDelegate	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Factory_ NullFactory_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ NullValuesAlwaysRenderedAsNullLiteral	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksFallThroughInOrder	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksNewestFirst	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ ThrowingFormatterDoesNotDerailRendering	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ CustomizesFailureMessage	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ DoesNotMatchUnrelatedTypes	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ FormatterRemovedAfterDispose	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ NullFormatter_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ ReturningNullFallsThroughToBuiltIn	A	90–100	No issues found.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

🤖 Automated content by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Grade Tests on PR (on open / sync) workflow. · 286.5 AIC · ⌖ 13.4 AIC · [◷]( · ◷)

[Amaury Levé (Evangelink)]

Fixes#9089

Adds two public Assert.AddValueFormatter overloads that let users register custom formatters used by the structured assertion failure message renderer introduced by RFC-012 (#8964):

• AddValueFormatter<T>(Func<T, string?>) — typed sugar. Return null to fall through to the next formatter / built-in renderer.
• AddValueFormatter(Func<Func<object?, string>, Func<object?, string>>) — chain-of-responsibility factory that receives the next formatter.

Both overloads return an IDisposable whose disposal removes the registration. Registrations are anchored on AsyncLocal<T>, so the scope follows the test context (per-test, per-class, per-assembly), and they stack newest-first so the most recently registered formatter wins.

Example

[TestMethod]publicvoidDateTimeFailureShowsKind(){usingvar_=Assert.AddValueFormatter<DateTime>(dt =>$""{dt:O} [{dt.Kind}]"");Assert.AreEqual(d1,d2);// failure message now annotates DateTime.Kind}

Implementation

• New AssertionValueFormatterRegistry (internal, AsyncLocal-backed) holds the chain. Nodes are append-only with a mutable IsRemoved flag so Dispose works for any position in the stack (including out-of-order disposal).
• AssertionValueRenderer.RenderValue consults the registry ahead of the built-in switch, and only when at least one formatter is registered (zero-allocation fast-path for the common no-formatter case).
• null is always rendered as ""null"" and is never exposed to user formatters, to avoid spurious NullReferenceExceptions inside user code.
• User-formatter exceptions are caught and annotated so the underlying assertion failure remains useful.
• New API is marked [Experimental(""MSTESTEXP"")] consistent with Assert.Scope().

Tests

15 new AssertTests.AddValueFormatter tests cover:

• ArgumentNullException on both overloads
• End-to-end assertion failure-message customization
• Disposal removes the registration (including out-of-order disposal)
• Idempotent Dispose
• Newest-first stacking + fall-through ordering
• Null values bypass the registry
• Non-matching types bypass the typed overload
• Collection items get per-item formatting
• The factory overload composes with the typed overload
• Throwing formatters do not derail rendering

Verified passing on net9.0, net8.0, and net48.

Out of scope (per the issue)

• No discovery via attributes/interfaces (deferred).
• No built-in defaults changed.
• No equality customization (existing IEqualityComparer<T> overloads cover that).

[Copilot]

Pull request overview

Adds an experimental public API to MSTest (Assert.AddValueFormatter) to let users customize how values are rendered in the structured assertion failure messages (via an AsyncLocal-scoped, newest-first formatter stack), and updates the central renderer to consult those formatters before the built-in formatting logic.

Changes:

• Introduces Assert.AddValueFormatter<T>(Func<T, string?>) and Assert.AddValueFormatter(Func<Func<object?,string>, Func<object?,string>>) returning IDisposable for scoped registration/removal.
• Adds an internal AssertionValueFormatterRegistry (AsyncLocal-backed) and wires it into AssertionValueRenderer.RenderValue.
• Adds unit tests covering registration, stacking, disposal semantics, null bypass, and failure-message customization.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AddValueFormatter.cs	New unit tests for both overloads, ordering, disposal, and rendering behavior.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new experimental public API signatures.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Consults the formatter registry before built-in rendering (with a no-formatters fast path).
src/TestFramework/TestFramework/Assertions/AssertionValueFormatterRegistry.cs	New internal AsyncLocal registry that composes formatter factories and supports removal via disposal.
src/TestFramework/TestFramework/Assertions/Assert.AddValueFormatter.cs	Implements the two new public overloads and documents scoping/stacking semantics.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Amaury Levé (Evangelink)]

The Windows Debug failure was an unrelated, intermittent infra flake in MSBuildTests_KnownExtensionRegistration.Microsoft_Testing_Platform_Extensions_ShouldBe_Correctly_Registered: 2 of the 4 net10.0 matrix entries threw Sequence contains no matching element because the _GenerateSelfRegisteredExtensions target was absent from the captured binlog, while the other 2 net10.0 entries (and all other TFMs) passed. This PR only touches Assert code in TestFramework and cannot affect MSBuild extension registration. Pushed an empty commit to re-run CI.

[Amaury Levé (Evangelink)]

🧪 Test quality grade — PR #9148

17 test methods graded across 2 files; all 17 earn A (90–100). Tests are well-named, single-behavior, and cover the formatter registry's lifecycle end-to-end — null guards, type specificity, LIFO stacking, fall-through chaining, dispose idempotency, fast-path restoration, fault isolation, and acceptance-level scope flow-down. No issues found across the board.

Δ	Test	Grade	Band	Notes
new	ValueFormatterFlowTests. ValueFormatterScopesFlowDownCorrectly	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ AppliesToCollectionItems	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeIsIdempotent	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeRemovesOnlyThatRegistration	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposingAllRegistrationsRestoresFastPath	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ChainsWithTypedOverload	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ReceivesNextDelegate	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Factory_ NullFactory_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ NullValuesAlwaysRenderedAsNullLiteral	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksFallThroughInOrder	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksNewestFirst	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ ThrowingFormatterDoesNotDerailRendering	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ CustomizesFailureMessage	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ DoesNotMatchUnrelatedTypes	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ FormatterRemovedAfterDispose	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ NullFormatter_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ ReturningNullFallsThroughToBuiltIn	A	90–100	No issues found.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

🤖 Automated content by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Grade Tests on PR (on open / sync) workflow. · 286.5 AIC · ⌖ 13.4 AIC · [◷]( · ◷)

[Amaury Levé (Evangelink)]

Fixes#9089

Adds two public Assert.AddValueFormatter overloads that let users register custom formatters used by the structured assertion failure message renderer introduced by RFC-012 (#8964):

• AddValueFormatter<T>(Func<T, string?>) — typed sugar. Return null to fall through to the next formatter / built-in renderer.
• AddValueFormatter(Func<Func<object?, string>, Func<object?, string>>) — chain-of-responsibility factory that receives the next formatter.

Both overloads return an IDisposable whose disposal removes the registration. Registrations are anchored on AsyncLocal<T>, so the scope follows the test context (per-test, per-class, per-assembly), and they stack newest-first so the most recently registered formatter wins.

Example

[TestMethod]publicvoidDateTimeFailureShowsKind(){usingvar_=Assert.AddValueFormatter<DateTime>(dt =>$""{dt:O} [{dt.Kind}]"");Assert.AreEqual(d1,d2);// failure message now annotates DateTime.Kind}

Implementation

• New AssertionValueFormatterRegistry (internal, AsyncLocal-backed) holds the chain. Nodes are append-only with a mutable IsRemoved flag so Dispose works for any position in the stack (including out-of-order disposal).
• AssertionValueRenderer.RenderValue consults the registry ahead of the built-in switch, and only when at least one formatter is registered (zero-allocation fast-path for the common no-formatter case).
• null is always rendered as ""null"" and is never exposed to user formatters, to avoid spurious NullReferenceExceptions inside user code.
• User-formatter exceptions are caught and annotated so the underlying assertion failure remains useful.
• New API is marked [Experimental(""MSTESTEXP"")] consistent with Assert.Scope().

Tests

15 new AssertTests.AddValueFormatter tests cover:

• ArgumentNullException on both overloads
• End-to-end assertion failure-message customization
• Disposal removes the registration (including out-of-order disposal)
• Idempotent Dispose
• Newest-first stacking + fall-through ordering
• Null values bypass the registry
• Non-matching types bypass the typed overload
• Collection items get per-item formatting
• The factory overload composes with the typed overload
• Throwing formatters do not derail rendering

Verified passing on net9.0, net8.0, and net48.

Out of scope (per the issue)

• No discovery via attributes/interfaces (deferred).
• No built-in defaults changed.
• No equality customization (existing IEqualityComparer<T> overloads cover that).

[Copilot]

Pull request overview

Adds an experimental public API to MSTest (Assert.AddValueFormatter) to let users customize how values are rendered in the structured assertion failure messages (via an AsyncLocal-scoped, newest-first formatter stack), and updates the central renderer to consult those formatters before the built-in formatting logic.

Changes:

• Introduces Assert.AddValueFormatter<T>(Func<T, string?>) and Assert.AddValueFormatter(Func<Func<object?,string>, Func<object?,string>>) returning IDisposable for scoped registration/removal.
• Adds an internal AssertionValueFormatterRegistry (AsyncLocal-backed) and wires it into AssertionValueRenderer.RenderValue.
• Adds unit tests covering registration, stacking, disposal semantics, null bypass, and failure-message customization.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.AddValueFormatter.cs	New unit tests for both overloads, ordering, disposal, and rendering behavior.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new experimental public API signatures.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Consults the formatter registry before built-in rendering (with a no-formatters fast path).
src/TestFramework/TestFramework/Assertions/AssertionValueFormatterRegistry.cs	New internal AsyncLocal registry that composes formatter factories and supports removal via disposal.
src/TestFramework/TestFramework/Assertions/Assert.AddValueFormatter.cs	Implements the two new public overloads and documents scoping/stacking semantics.

Copilot's findings

• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

Copilot's findings

• Files reviewed: 6/6 changed files
• Comments generated: 2

[Amaury Levé (Evangelink)]

The Windows Debug failure was an unrelated, intermittent infra flake in MSBuildTests_KnownExtensionRegistration.Microsoft_Testing_Platform_Extensions_ShouldBe_Correctly_Registered: 2 of the 4 net10.0 matrix entries threw Sequence contains no matching element because the _GenerateSelfRegisteredExtensions target was absent from the captured binlog, while the other 2 net10.0 entries (and all other TFMs) passed. This PR only touches Assert code in TestFramework and cannot affect MSBuild extension registration. Pushed an empty commit to re-run CI.

[Amaury Levé (Evangelink)]

🧪 Test quality grade — PR #9148

17 test methods graded across 2 files; all 17 earn A (90–100). Tests are well-named, single-behavior, and cover the formatter registry's lifecycle end-to-end — null guards, type specificity, LIFO stacking, fall-through chaining, dispose idempotency, fast-path restoration, fault isolation, and acceptance-level scope flow-down. No issues found across the board.

Δ	Test	Grade	Band	Notes
new	ValueFormatterFlowTests. ValueFormatterScopesFlowDownCorrectly	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ AppliesToCollectionItems	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeIsIdempotent	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposeRemovesOnlyThatRegistration	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ DisposingAllRegistrationsRestoresFastPath	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ChainsWithTypedOverload	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ FactoryOverload_ ReceivesNextDelegate	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Factory_ NullFactory_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ NullValuesAlwaysRenderedAsNullLiteral	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksFallThroughInOrder	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ StacksNewestFirst	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ ThrowingFormatterDoesNotDerailRendering	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ CustomizesFailureMessage	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ DoesNotMatchUnrelatedTypes	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ FormatterRemovedAfterDispose	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ NullFormatter_ ThrowsArgumentNullException	A	90–100	No issues found.
new	AssertTests. AddValueFormatter_ Typed_ ReturningNullFallsThroughToBuiltIn	A	90–100	No issues found.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

🤖 Automated content by GitHub Copilot. Posted via a maintainer's GitHub token, so it appears under their account — the account owner did not write or approve this content personally. Generated by the Grade Tests on PR (on open / sync) workflow. · 286.5 AIC · ⌖ 13.4 AIC · [◷]( · ◷)