[trimmable-type-map] Avoid unused XML references

[simonrozsival]

Summary

• create XML marshalling metadata references lazily in generated [Export] dispatch assemblies
• avoid adding System.Xml.ReaderWriter to a generated type-map assembly when exported signatures do not use XML parser mappings
• preserve the existing XML marshalling references when XmlPullParser or XmlResourceParser mappings are used

APK size and combined MAUI validation

The PR is draft because the stock MAUI sample-content application does not currently demonstrate an APK-size or packaged-assembly reduction from this change.

The first stock .NET 11 Android CoreCLR Release comparison produced an identical signed APK before and after this change:

Build	Signed APK size
Baseline	23,300,175 bytes
Patched	23,300,175 bytes
Difference	0 bytes

Three independent MAUI trimming fixes were then applied together:

• Trim unused XAML runtime code from linked apps maui#38613 trims unused runtime XAML loader/parser code
• Controls: Remove Regex from ShadowTypeConverter maui#38615 removes ShadowTypeConverter's regex dependency
• [Android] Hosting: Trim disabled MAUI Aspire support maui#38616 trims disabled MAUI Aspire support and its generated localhost regex

The combined MAUI changes reduced the sample-content signed APK by 81,920 bytes, removed XamlLoader, XamlParser, and LocalhostRegex, and reduced the retained XML/regex implementations. However, System.Xml.ReaderWriter remains rooted directly by source-generated XAML line-info calls in the application, and System.Private.Xml remains transitively and through additional framework/control references.

To exercise this PR's actual code path, the identical combined-MAUI input was then published twice with the trimmable Android type map explicitly enabled:

dotnet publish CombinedTrimApp.csproj \
  -c Release -f net11.0-android -r android-arm64 \
  -p:AndroidKeyStore=false \
  -p:PublishReadyToRun=false \
  -p:TrimMode=partial \
  -p:_AndroidTypeMapImplementation=trimmable \
  -p:AndroidPackageFormats=apk

Output	Baseline generator	Patched generator	Difference
Signed APK	21,248,079 B	21,248,079 B	0 B
Unsigned APK	21,118,994 B	21,119,021 B	+27 B packaging noise
Generated type-map DLLs	11,133,952 B	11,133,952 B	0 B
System.Xml.ReaderWriter.dll	5,120 B	5,120 B	0 B
System.Private.Xml.dll	381,440 B	381,440 B	0 B

All 86 generated type-map DLLs were byte-identical, and none had a System.Xml.ReaderWriter assembly reference. The sample-content app and its dependencies do not contain a non-XML [Export] dispatcher that triggers the bug fixed here. Its remaining System.Xml.ReaderWriter reference comes from generated XAML InitializeComponent line-info services (System.Xml.IXmlLineInfo), outside the Android type-map generator.

Therefore this change is valid generator cleanup and is covered by focused tests, but it does not currently reduce the stock MAUI sample-content APK or remove its XML assemblies. The benefit applies only to an app/library that generates a non-XML [Export] dispatcher and has no independent XML roots.

Tests

• dotnet test tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests.csproj --no-restore --nologo -v:minimal
• targeted TypeMapAssemblyGeneratorTests.Generate_Export* coverage

[Copilot]

Copilot review overview

🔵 Needs a closer look

Add coverage for XML parameter/return paths and all supported XML assembly identities.

Review effort: Lite
Findings: None

What changed in this PR

This PR lazily creates XML marshalling metadata to avoid unnecessary XML references in generated dispatch assemblies.

Changes:

• Defers XML metadata reference creation until needed.
• Preserves references for XML parser mappings.
• Adds conditional-reference regression tests.

File	Summary
tests/​Microsoft.Android.Sdk.TrimmableTypeMap.Tests/​Generator/​TypeMapAssemblyGeneratorTests.cs	Tests conditional XML reference emission.
src/​Microsoft.Android.Sdk.TrimmableTypeMap/​Generator/​ExportMethodDispatchEmitterContext.cs	Adds lazy XML metadata reference getters.
src/​Microsoft.Android.Sdk.TrimmableTypeMap/​Generator/​ExportMethodDispatchEmitter.cs	Uses lazy getters for XML marshalling emission.