.NET Native AOT Compatibility

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

dotnet build <project.csproj> -f <net8.0-or-later-tfm> --no-incremental 2>&1 | grep 'IL[0-9]\{4\}'

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:

using System.Diagnostics.CodeAnalysis;

// Before (warns IL2070):
void Process(Type t) {
    var method = t.GetMethod("Foo");  // trimmer can't verify
}

// After (clean):
void Process([DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] Type t) {
    var method = t.GetMethod("Foo");  // trimmer preserves public methods
}

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:

// BROKEN: Type boxed into object[], annotation lost
void Process(object[] args) {
    Type t = (Type)args[0];  // IL2072: annotation lost through boxing
    Evaluate(t, ...);
}

// FIXED: Pass Type as a separate, annotated parameter
void Process(
    object[] args,
    [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] Type calleeType,
    ...) {
    Evaluate(calleeType, ...);  // annotation flows cleanly
}

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