ILLinker Trimming Configuration Missing Constraint for Generic Type Reflection Metadata

Short answer

Use a custom trimming descriptor XML file (conventionally ILLink.Descriptors.xml) instead of scattering preservation attributes. A descriptor centralizes policy, works on third‑party assemblies you cannot annotate, and travels with the project in source control. Attributes are appropriate only for code you own where the preservation requirement is intrinsic to the type definition.

Why the descriptor wins

• Centralized policy – one file declares what the trimmer must keep; reviewers see the full surface area at a glance.
• Third‑party coverage – you cannot add attributes to NuGet packages; a descriptor can root their types.
• Versioned with the build – the XML lives in the repo, so CI always applies the same rules.

Reference it in the project file:

<ItemGroup>
  <TrimmerRootDescriptor Include="ILLink.Descriptors.xml" />
</ItemGroup>

Modern attribute alternative

The legacy [Preserve] attribute is not the standard .NET API. Current SDKs expect [DynamicallyAccessedMembers] on generic type parameters or [DynamicDependency] on members. These feed the static analyzers so the trimmer and IDE agree. Use them only on your types where reflection is a documented contract (e.g., a serializer you ship).

MetadataLoadContext is a special case

System.Reflection.MetadataLoadContext loads assemblies for inspection‑only reflection. If it appears only in build‑time or design‑time tooling, exclude that assembly from the published output (ExcludeAssets="runtime" or a separate tool project) rather than rooting it. Rooting it in a trimmed app usually indicates a design mismatch.

Trade‑off: root only what you actually reflect over

Preserve all on the specific open‑generic definitions your code activates via reflection (serializers, DI containers, configuration binders). Example descriptor snippet:

<linker>
  <assembly fullname="MyApp.Core">
    <type fullname="MyApp.Core.Services.IGenericService`1" preserve="all" />
  </assembly>
</linker>

Rooting entire assemblies or using preserve="all" broadly negates most size savings.

Verification checklist

1. Enable granular warnings: set <TrimmerSingleWarn>false</TrimmerSingleWarn> to surface every IL2xxx warning per assembly.
2. Treat warnings as errors in CI: <TreatWarningsAsErrors>true</TreatWarningsAsErrors> for the trimmed configuration.
3. Smoke‑test the trimmed output: run the published binary exercising every reflection path (serialization round‑trips, DI activation of open generics, configuration binding). Compare behavior with an untrimmed build.
4. Measure size impact: diff the published directory size before and after adding the descriptor to confirm the cost is acceptable.

One diagnostic detail needed

Are the MissingMethodException / InvalidProgramException failures reproducible in an untrimmed build? If they only appear when PublishTrimmed=true, trimming is the cause; if they also appear untrimmed, the fix is not a descriptor.