dotnet-aot-compat

dotnet-aot-compat

Make .NET projects compatible with Native AOT and trimming by systematically resolving all IL trim/AOT analyzer warnings.

When to Use This Skill

• "Make this project AOT-compatible"
• "Fix trimming warnings" or "fix IL warnings"
• "Resolve IL2070 / IL2067 / IL2072 / IL2026 / IL3050 warnings"
• "Add DynamicallyAccessedMembers annotations"
• "Enable IsAotCompatible in my .csproj"
• "My project has trim analyzer warnings after upgrading to net8.0"
• "Annotate reflection code for the trimmer"

When Not to Use This Skill

Do not use this skill when the project exclusively targets .NET Framework (net4x), which does not support the trim/AOT analyzers.

Prerequisites

An existing .NET project targeting net8.0 or later (or multi-targeting with at least one net8.0+ TFM) and the corresponding .NET SDK installed.

Background: What AOT Compatibility Means

Native AOT and the IL trimmer perform static analysis to determine what code is reachable. Reflection can break this analysis because the trimmer can't see what types/members are accessed at runtime. The IsAotCompatible property enables analyzers that flag these issues as build warnings (ILXXXX codes).

Critical Rules

❌ Never suppress warnings incorrectly

• NEVER use #pragma warning disable for IL warnings. It hides warnings from the Roslyn analyzer at build time, but the IL linker and AOT compiler still see the issue. The code will fail at trim/publish time.
• NEVER use [UnconditionalSuppressMessage]. It tells both the analyzer AND the linker to ignore the warning, meaning the trimmer cannot verify safety. Raising an error at build time is always preferable to hiding the issue and having it silently break at runtime.

💡 Preferred approaches

• Prefer [DynamicallyAccessedMembers] annotations to flow type information through the call chain.
• Prefer refactoring to eliminate patterns that break annotation flow (e.g., boxing Type through object[]).
• Use [RequiresUnreferencedCode] / [RequiresDynamicCode] / [RequiresAssemblyFiles] to mark methods as fundamentally incompatible with trimming, propagating the requirement to callers. This surfaces the issue clearly rather than hiding it — callers must explicitly acknowledge the incompatibility.

Annotation flow is key

The trimmer tracks [DynamicallyAccessedMembers] annotations through assignments, parameter passing, and return values. If this flow is broken (e.g., by boxing a Type into object, storing in an untyped collection, or casting through interfaces), the trimmer loses track and warns. The fix is to preserve the flow, not suppress the warning.

Step-by-Step Procedure

Do not explore the codebase up-front. The build warnings tell you exactly which files and lines need changes. Follow a tight loop: build → pick a warning → open that file at that line → apply the fix recipe → rebuild. Reading or analyzing source files beyond what a specific warning points you to is wasted effort and leads to timeouts. Let the compiler guide you.

❌ Do NOT run find, ls, or grep to understand the project structure before building. Do NOT read README, docs, or architecture files. Your first action should be Step 1 (enable AOT analysis), then build.

Step 1: Enable AOT analysis in the .csproj

Add IsAotCompatible. If the project doesn't exclusively target net8.0+, add a TFM condition (AOT analysis requires net8.0+):

<PropertyGroup>
  <IsAotCompatible Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net8.0'))">true</IsAotCompatible>
</PropertyGroup>

This automatically sets EnableTrimAnalyzer=true and EnableAotAnalyzer=true for compatible TFMs. For multi-targeting projects (e.g., netstandard2.0;net8.0), the condition ensures no NETSDK1210 warnings on older TFMs.

Step 2: Build and collect warnings

Sort and deduplicate. Common warning codes:

• IL2070: Reflection call on a Type parameter missing [DynamicallyAccessedMembers]
• IL2067: Passing an unannotated Type to a method expecting [DynamicallyAccessedMembers]
• IL2072: Return value or extracted value missing annotation (often from unboxing)
• IL2057: Type.GetType(string) with a non-constant argument
• IL2026: Calling a method marked [RequiresUnreferencedCode]
• IL2050: P/invoke method with COM marshalling parameters
• IL2075: Return value flows into reflection without annotation
• IL2091: Generic argument missing [DynamicallyAccessedMembers] required by constraint
• IL3000: Assembly.Location returns empty string in single-file/AOT apps
• IL3050: Calling a method marked [RequiresDynamicCode]

Step 3: Triage warnings by code (do NOT read every file)

Group the warnings from Step 2 by warning code and count them. Do not open individual files yet. Identify the top 1-2 patterns by count — these drive your fix strategy:

In most real projects, IL2026/IL3050 from JsonSerializer dominate. Start with Strategy C unless the warning breakdown clearly shows otherwise. After the batch JSON fix, handle remaining warnings with Strategies A–B. Only use Strategy D as a last resort.

Step 4: Fix warnings iteratively (innermost first)

Work from the innermost reflection call outward. Each fix may cascade new warnings to callers.

Stay warning-driven. For each warning, open only the file and line the compiler reported, identify the pattern, apply the matching fix recipe below, and move on. Do not scan the codebase for similar patterns or try to understand the full architecture — fix what the compiler tells you, rebuild, and let new warnings guide the next change. Fix a small batch of warnings (5-10), then rebuild immediately to check progress.

Use sub-agents when available. If you can launch sub-agents (e.g., via a task tool), dispatch multiple sub-agents in parallel to edit different files simultaneously. Keep the main loop focused on building, parsing warnings, and dispatching — delegate actual file edits to sub-agents. For batch JSON updates, give each sub-agent 5-10 files to update in one prompt. After 2 build-fix cycles, dispatch all remaining file edits to sub-agents in parallel — do not continue fixing files sequentially. Example:

Update these files to use source-generated JSON: src/Models/Resource.Serialization.cs, src/Models/Identity.Serialization.cs, src/Models/Plan.Serialization.cs. In each file, replace JsonSerializer.Serialize(writer, value) with JsonSerializer.Serialize(writer, value, MyProjectJsonContext.Default.TypeName) and JsonSerializer.Deserialize<T>(ref reader) with JsonSerializer.Deserialize(ref reader, MyProjectJsonContext.Default.TypeName). Only edit the JsonSerializer call sites.

Strategy A: Add [DynamicallyAccessedMembers] (preferred)

When a method uses reflection on a Type parameter, annotate the parameter to tell the trimmer what members are needed:

When you annotate a parameter, all callers must now pass properly annotated types. This cascades outward — follow each caller and annotate or refactor as needed. The caller's annotation must include at least the same member types as the callee's. If the callee requires PublicConstructors | NonPublicConstructors, the caller must specify the same or a superset — using only NonPublicConstructors will produce IL2091.

Strategy B: Refactor to preserve annotation flow

When annotation flow is broken by boxing (storing Type in object, object[], or untyped collections), refactor to pass the Type directly:

Common patterns that break flow and how to fix them:

• object[] parameter bags: Extract the Type into a dedicated annotated parameter
• Dictionary/List storage: Use a typed field with annotation instead
• Interface indirection: Add annotation to the interface method's parameter
• Property with boxing getter: Annotate the property's return type

Strategy C: Source-generated JSON serialization (batch fix)

When most warnings are IL2026/IL3050 from JsonSerializer.Serialize/Deserialize, this is a single mechanical fix applied in bulk:

1. Collect affected types — grep for all JsonSerializer.Serialize and JsonSerializer.Deserialize call sites. Extract the type being serialized (the <T> in Deserialize<T>, or the runtime type of the object in Serialize).

2. Create one JsonSerializerContext with [JsonSerializable] for every type found. Skip types from external packages (e.g., ResponseError from Azure.Core) — they won't source-generate for types you don't own. Handle external types separately via Gotcha #1 below.

3. Batch-update all call sites — do not read each file individually. Apply the pattern mechanically:

   • JsonSerializer.Serialize(obj) → JsonSerializer.Serialize(obj, MyProjectJsonContext.Default.TypeName)
   • JsonSerializer.Deserialize<T>(json) → JsonSerializer.Deserialize(json, MyProjectJsonContext.Default.TypeName)

   Find and update all call sites in one pass:

   # Find all files with JsonSerializer calls
   grep -rl 'JsonSerializer\.\(Serialize\|Deserialize\)' src/ --include='*.cs'

   Then use sequential edit calls to apply the same transformation to every matching file. Do not use sed for C# code — generics like Deserialize<T>() have angle brackets and nested parentheses that sed will mangle.

4. Build once to verify. Remaining warnings will be non-serialization issues — handle those with Strategies A–B or D.

Strategy D: [RequiresUnreferencedCode] (last resort)

When a method fundamentally requires arbitrary reflection that cannot be statically described:

[RequiresUnreferencedCode("Loads plugins by name using Assembly.Load")]
public void LoadPlugin(string assemblyName) {
    var asm = Assembly.Load(assemblyName);
    // ...
}

This propagates to callers — they must also be annotated with [RequiresUnreferencedCode]. Use sparingly; it marks the entire call chain as trim-incompatible.

Step 5: Rebuild and repeat

After each small batch of fixes (5-10 warnings), rebuild with --no-incremental and check for new warnings. Do not attempt to fix all warnings before rebuilding — frequent rebuilds catch mistakes early and reveal cascading warnings. Fixes cascade — annotating an inner method may surface warnings in its callers. Repeat until 0 Warning(s).

Step 6: Validate all TFMs

Build all target frameworks to ensure:

• 0 IL warnings on net8.0+ TFMs
• No NETSDK1210 warnings (the IsAotCompatible condition handles this)
• Clean builds on older TFMs (netstandard2.0, net472, etc.)

dotnet build <project.csproj>  # builds all TFMs

Stop Signals

• Do not analyze more than 2-3 representative files per warning pattern. After identifying the fix for a pattern, apply it to all matching files without reading each one first.
• Start fixing after one build. Do not do a second analysis pass — begin implementing fixes for the most common warning pattern immediately after Step 3 triage.
• Stop after achieving 0 IL warnings for net8.0+ TFMs. Don't optimize or refactor already-clean annotations.
• If a warning requires architectural refactoring beyond annotation flow fixes (e.g., replacing an entire serialization layer), document it and stop — don't rewrite large subsystems.
• Limit to 3 build-fix iterations per warning. If annotation flow doesn't resolve it after 3 attempts, escalate to [RequiresUnreferencedCode].
• Don't chase warnings in third-party dependencies you can't modify. Note them and move on.
• If the user asked a scoped question (e.g., "fix warnings in this file"), don't expand to the entire project.

Polyfills for Older TFMs

For multi-targeting projects that include netstandard2.0 or net472, you need polyfills for DynamicallyAccessedMembersAttribute and related types. See references/polyfills.md.

Common Gotchas

1. External types without AOT-safe serialization: When a type comes from a dependency you can't modify (e.g., ResponseError from Azure.Core) and it lacks a source-generated serializer, Options.GetConverter<T>() is reflection-based and will produce IL warnings. First check if the type implements IJsonModel<T> (common in Azure SDK) — if so, bypass JsonSerializer entirely:

// Before (IL2026 — JsonSerializer uses reflection):
JsonSerializer.Serialize(writer, errorValue);
 
// After (AOT-safe — uses IJsonModel directly):
((IJsonModel<ResponseError>)errorValue).Write(writer, ModelReaderWriterOptions.Json);
 
// For deserialization:
var error = ((IJsonModel<ResponseError>)new ResponseError()).Create(ref reader, ModelReaderWriterOptions.Json);

Do not add the external type to your JsonSerializerContext — it won't source-generate for types you don't own. If the type doesn't implement IJsonModel<T>, write a custom JsonConverter<T> with manual Utf8JsonReader/Utf8JsonWriter logic and register it via [JsonSourceGenerationOptions] on your context.

2. Serialization libraries: Most reflection-based serializers (e.g., Newtonsoft.Json, XmlSerializer) are not AOT-compatible. Migrate to a source-generation-based serializer such as System.Text.Json with a JsonSerializerContext. If migration is not feasible, mark the serialization call site with [RequiresUnreferencedCode].

3. Shared projects / projitems: When source is shared between multiple projects via <Import>, annotations added to shared code affect ALL consuming projects. Verify that all consumers still build cleanly.

References

Limitations Conceptual: Understanding trimming How-to: trim compat

Checklist

• Added <IsAotCompatible> with TFM condition to .csproj
• Built with AOT analyzers enabled (net8.0+ TFM)
• Fixed all IL warnings via annotations or refactoring
• No #pragma warning disable or [UnconditionalSuppressMessage] used for any IL warning
• Polyfills present for older TFMs if needed
• All target frameworks build with 0 warnings
• Verified shared/linked source doesn't break sibling projects