XmlSerializer - Fire UnknownAttribute for stray attributes on built-in typed elements

[StephenMolloy]

Summary

XmlSerializer's UnknownAttribute event only fired for unknown attributes on elements mapped to user-defined struct types. Elements mapped to built-in types (primitives, arrays, and collections) read their content directly and never inspected attributes, so stray attributes on those elements were silently dropped and never surfaced through the event.

This fixes #96683 by making the primitive/array/collection paths walk the element's attributes the same way the struct path already does.

Approach

When an element is mapped to a primitive, array, or collection, the reader now walks the current element's attributes and raises UnknownNode/UnknownAttribute for any non-namespace attribute, mirroring the existing struct handling in WriteAttributes. The same change is applied consistently to all three readers:

• Reflection reader (ReflectionXmlSerializationReader)
• IL-gen reader (XmlSerializationReaderILGen, the default runtime reader)
• Code-gen reader (XmlSerializationReader/sgen)

xsi:nil elements are deliberately skipped (guarded by GetNullAttr()). The nil marker is consumed by the subsequent ReadNull() call, and surfacing it would be wrong; this matches the struct path, where a nil element returns before its attributes are inspected.

Compatibility

There is no app-compat switch and no new public API surface. When no handler is subscribed, UnknownNode is a no-op, so behavior is unchanged unless a caller has explicitly subscribed to the UnknownAttribute event, in which case they now also get notified about attributes on built-in typed elements. Anyone subscribed to that event almost certainly wants this to work correctly.

Tests

Added two regression tests to XmlSerializerTests:

• XmlUnknownAttributeOnBuiltInTypedMembersTest verifies that stray attributes on string/int/nullable-int/list/array members and their items are all reported, and that they do not disrupt deserialization of the element content.
• XmlNilAttributeOnBuiltInTypedMembersNotReportedTest verifies that xsi:nil is not surfaced as an unknown attribute and still drives the expected null results.

Verified across the IL-gen and reflection-only test legs; the full XmlSerializerTests class passes (324 tests, 0 failures). The tests fail without the fix and pass with it.

Fixes #96683

Note

This pull request was authored with the assistance of GitHub Copilot.

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Pull request overview

This PR fixes a behavioral gap in XmlSerializer deserialization events: unknown (non-namespace) attributes on elements mapped to built-in types (primitives, arrays, collections) are now surfaced via UnknownNode / UnknownAttribute, aligning these paths with existing struct-mapped behavior.

Changes:

• Add attribute-walking logic for primitive / array / collection element readers so stray attributes raise UnknownNode / UnknownAttribute (skipping xsi:nil elements).
• Apply the same behavior across the three reader implementations (reflection-based, IL-gen, and source-gen).
• Add regression tests validating unknown attributes are reported on built-in typed members/items, and that xsi:nil is not reported.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs	Adds regression coverage for unknown attributes on built-in typed members/items and verifies xsi:nil is not surfaced as unknown.
src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializationReaderILGen.cs	Emits IL to walk attributes for primitive/array/collection element paths and raise unknown-attribute events consistently.
src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializationReader.cs	Updates source-gen writer to generate attribute-walking logic for primitive/array element paths.
src/libraries/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationReader.cs	Adds reflection-path attribute walking for primitive/array/collection element reads.

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Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated no new comments.

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