Dotnet Trimming

dotnet-trimming

Trim-safe development for .NET 8+ applications and libraries: trimming annotations ([RequiresUnreferencedCode], [DynamicallyAccessedMembers], [DynamicDependency]), ILLink descriptor XML for type preservation, TrimmerSingleWarn for granular diagnostics, testing trimmed output, fixing IL2xxx/IL3xxx warnings, and library authoring with IsTrimmable.

Version assumptions: .NET 8.0+ baseline. Trimming shipped in .NET 6, but .NET 8 provides the most complete annotation surface and analyzer coverage. .NET 9 improved warning accuracy and library compat.

Scope

• MSBuild properties for trimming (apps vs libraries)
• Trimming annotations (RequiresUnreferencedCode, DynamicallyAccessedMembers, DynamicDependency)
• ILLink descriptor XML for type preservation
• TrimmerSingleWarn for granular diagnostics
• IL2xxx/IL3xxx warning reference and fixes
• Testing trimmed output and CI gates
• Library authoring with IsTrimmable and IsAotCompatible

Out of scope

• Native AOT publish pipeline and MSBuild configuration -- see [skill:dotnet-native-aot]
• AOT-first design patterns -- see [skill:dotnet-aot-architecture]
• WASM AOT compilation -- see [skill:dotnet-aot-wasm]
• MAUI-specific AOT and trimming -- see [skill:dotnet-maui-aot]
• Source generator authoring -- see [skill:dotnet-csharp-source-generators]
• Serialization depth -- see [skill:dotnet-serialization]
• Container deployment -- see [skill:dotnet-containers]
• Performance patterns (Span, pooling) -- see [skill:dotnet-performance-patterns]

Cross-references: [skill:dotnet-native-aot] for AOT compilation pipeline, [skill:dotnet-aot-architecture] for AOT-safe design patterns, [skill:dotnet-serialization] for AOT-safe serialization, [skill:dotnet-csharp-source-generators] for source gen as trimming enabler.

MSBuild Properties: Apps vs Libraries

Apps and libraries use different MSBuild properties for trimming. This distinction is critical -- using the wrong property causes subtle issues.

For Applications

<PropertyGroup>
  <!-- Enable trimming on publish -->
  <PublishTrimmed>true</PublishTrimmed>

  <!-- Enable trim analyzer during development -->
  <EnableTrimAnalyzer>true</EnableTrimAnalyzer>

  <!-- Optional: also enable AOT analyzer if targeting AOT -->
  <EnableAotAnalyzer>true</EnableAotAnalyzer>
</PropertyGroup>

PublishTrimmed tells the linker to remove unreachable code when publishing. EnableTrimAnalyzer enables Roslyn analyzers that warn about trim-unsafe patterns during development.

For Libraries

<PropertyGroup>
  <!-- Declare the library is trim-safe (auto-enables trim analyzer) -->
  <IsTrimmable>true</IsTrimmable>

  <!-- Declare AOT compatibility (auto-enables AOT analyzer) -->
  <IsAotCompatible>true</IsAotCompatible>
</PropertyGroup>

Key difference: Libraries do not set PublishTrimmed -- they are not published as standalone applications. IsTrimmable tells consumers that the library's public API is annotated for trimming safety. Setting IsTrimmable automatically enables the trim analyzer for the library project.

Trimming Annotations

.NET provides attributes to annotate code that interacts with reflection, helping the trimmer understand what to preserve.

[RequiresUnreferencedCode]

Marks a method as unsafe for trimming. The trimmer and analyzer produce IL2026 warnings when this method is called from trim-safe code.

[RequiresUnreferencedCode("Uses reflection to discover plugins")]
public IPlugin LoadPlugin(string typeName)
{
    var type = Type.GetType(typeName)
        ?? throw new InvalidOperationException($"Type {typeName} not found");
    return (IPlugin)Activator.CreateInstance(type)!;
}

[DynamicallyAccessedMembers]

Tells the trimmer which members of a type are accessed via reflection, so they are preserved:

public T CreateInstance<[DynamicallyAccessedMembers(
    DynamicallyAccessedMemberTypes.PublicConstructors)] T>()
    where T : class
    => (T)Activator.CreateInstance(typeof(T))!;

// The trimmer preserves public constructors of T
// because the constraint tells it what's needed

[DynamicDependency]

Explicitly preserves a specific member from trimming:

// Preserve a method that is only called via reflection
[DynamicDependency(nameof(OnConfigChanged), typeof(ConfigWatcher))]
public void StartWatching() { /* reflects on OnConfigChanged */ }

// Preserve all public properties (e.g., for serialization)
[DynamicDependency(DynamicallyAccessedMemberTypes.PublicProperties,
    typeof(LegacyDto))]
public void SerializeLegacy(LegacyDto dto) { /* ... */ }

[UnconditionalSuppressMessage]

Suppresses a specific trim warning when you have verified the code is safe despite the analyzer's concern:

[UnconditionalSuppressMessage("Trimming",
    "IL2026:RequiresUnreferencedCode",
    Justification = "Type is preserved via ILLink descriptor")]
public void CallLegacyCode() { /* ... */ }

Use sparingly -- only when you have verified safety through ILLink descriptors or other means.

ILLink Descriptors

ILLink descriptor XML files tell the trimmer to preserve types, methods, or entire assemblies. Do not use legacy RD.xml -- it is a .NET Native/UWP format that is silently ignored by modern .NET trimming.

Descriptor Format

<!-- ILLink.Descriptors.xml -->
<linker>
  <!-- Preserve specific types -->
  <assembly fullname="MyApp">
    <type fullname="MyApp.Models.PluginConfig" preserve="all" />
    <type fullname="MyApp.Services.LegacyAdapter">
      <method name="Initialize" />
      <method name="ProcessRequest" />
    </type>
  </assembly>

  <!-- Preserve an entire third-party assembly -->
  <assembly fullname="LegacyLibrary" preserve="all" />
</linker>

Registration

<!-- In .csproj -->
<ItemGroup>
  <TrimmerRootDescriptor Include="ILLink.Descriptors.xml" />
</ItemGroup>

Alternative: TrimmerRootAssembly

For entire assemblies that must not be trimmed:

<ItemGroup>
  <!-- Preserve entire assembly (no trimming at all) -->
  <TrimmerRootAssembly Include="LegacyLibrary" />
</ItemGroup>

TrimmerSingleWarn

By default, the trimmer groups warnings per assembly, showing one summary line. TrimmerSingleWarn=false shows every individual warning, which is essential for fixing trim issues.

# Default: one warning per assembly (hard to debug)
dotnet publish -c Release /p:PublishTrimmed=true
# warning IL2104: Assembly 'MyApp' produced trim warnings

# Detailed: per-occurrence warnings (easier to fix)
dotnet publish -c Release /p:PublishTrimmed=true /p:TrimmerSingleWarn=false
# warning IL2026: MyApp.PluginLoader.LoadPlugin(...) requires unreferenced code
# warning IL2057: Unrecognized value passed to Type.GetType(...)

# Analysis without publishing
dotnet build /p:EnableTrimAnalyzer=true /p:TrimmerSingleWarn=false

IL2xxx/IL3xxx Warning Reference

Trim Warnings (IL2xxx)

AOT Warnings (IL3xxx)

Testing Trimmed Output

Publish and Test

# Publish with trimming
dotnet publish -c Release -r linux-x64 /p:PublishTrimmed=true

# Run the trimmed binary
./bin/Release/net8.0/linux-x64/publish/MyApp

# Verify functionality:
# 1. All endpoints respond correctly
# 2. JSON deserialization produces populated objects
# 3. DI-resolved services function
# 4. No MissingMethodException or MissingMetadataException

Trim Test in CI

# CI script: publish trimmed and run integration tests
dotnet publish src/MyApp -c Release -r linux-x64 /p:PublishTrimmed=true -o ./publish

# Run smoke tests against trimmed binary
./publish/MyApp &
APP_PID=$!
sleep 3

curl -f http://localhost:8080/health/live || (kill $APP_PID; exit 1)
curl -f http://localhost:8080/api/products || (kill $APP_PID; exit 1)

kill $APP_PID

Trim Warning CI Gate

# Fail CI if any trim warnings exist
dotnet build /p:EnableTrimAnalyzer=true /p:TrimmerSingleWarn=false \
  /warnaserror:IL2026,IL2057,IL2060,IL2067,IL2070,IL3050

Library Authoring for Trimming

Making a Library Trim-Safe

1. Set <IsTrimmable>true</IsTrimmable> in the library .csproj
2. Annotate all reflection-using APIs with [RequiresUnreferencedCode]
3. Add [DynamicallyAccessedMembers] to parameters that receive types used reflectively
4. Replace reflection with source generators where possible
5. Test by consuming the library from a trimmed application

<!-- Library .csproj -->
<PropertyGroup>
  <!-- Auto-enables trim analyzer -->
  <IsTrimmable>true</IsTrimmable>
  <!-- Auto-enables AOT analyzer; implies IsTrimmable in .NET 8+ -->
  <IsAotCompatible>true</IsAotCompatible>
</PropertyGroup>

Annotating Public APIs

// Method that uses reflection internally -- annotate honestly
[RequiresUnreferencedCode(
    "Uses reflection to discover plugin types. " +
    "Use RegisterPlugin<T>() for trim-safe plugin registration.")]
public IPlugin LoadPlugin(string typeName) { /* ... */ }

// Trim-safe alternative
public void RegisterPlugin<[DynamicallyAccessedMembers(
    DynamicallyAccessedMemberTypes.PublicConstructors)] T>()
    where T : class, IPlugin
{
    _plugins[typeof(T).Name] = () => (IPlugin)Activator.CreateInstance<T>();
}

Conditional APIs

Provide both reflection-based and trim-safe APIs when possible:

public class ServiceRegistry
{
    // Trim-safe: explicit type
    public void Register<[DynamicallyAccessedMembers(
        DynamicallyAccessedMemberTypes.PublicConstructors)] TService,
        TImplementation>()
        where TImplementation : class, TService
    { /* ... */ }

    // Not trim-safe: assembly scanning
    [RequiresUnreferencedCode("Scans assembly for service types")]
    public void RegisterFromAssembly(Assembly assembly)
    { /* ... */ }
}

Agent Gotchas

1. Do not use PublishTrimmed in library projects. Libraries use IsTrimmable to declare they are trim-safe. PublishTrimmed is for applications.
2. Do not use RD.xml for type preservation. RD.xml is a .NET Native/UWP format that is silently ignored by modern .NET trimming. Use ILLink descriptor XML files instead.
3. Do not suppress trim warnings without verifying safety. [UnconditionalSuppressMessage] hides warnings but does not fix the underlying issue. Only suppress when you have verified the code is safe (e.g., via ILLink descriptors).
4. Do not forget TrimmerSingleWarn=false when debugging trim issues. Without it, you get one summary warning per assembly, making it impossible to find the specific problematic call site.
5. Do not confuse IsTrimmable with PublishTrimmed. IsTrimmable declares a library is trim-safe and enables the analyzer. PublishTrimmed enables the linker in applications. They serve different purposes.
6. Do not add [RequiresUnreferencedCode] to methods that do not use reflection. The annotation propagates virally -- callers must also be annotated or suppress the warning. Only annotate methods that actually use trim-unsafe reflection.

References

• Trim self-contained applications
• Prepare libraries for trimming
• Introduction to trim warnings
• Trimming annotation attributes
• ILLink descriptor format