dotnet-aot-wasm

WebAssembly AOT compilation for Blazor WASM and Uno WASM applications: compilation pipeline, download size vs runtime speed tradeoffs, trimming interplay, lazy loading assemblies, and Brotli pre-compression for download optimization.

Version assumptions: .NET 8.0+ baseline. Blazor WASM AOT shipped in .NET 6 and has been refined through .NET 8-10. Uno WASM uses a similar compilation pipeline with Uno-specific tooling.

Important tradeoff: Trimming and AOT have opposite effects on WASM artifact size. Trimming reduces download size by removing unused code. AOT increases artifact size (native WASM code is larger than IL) but improves runtime execution speed. Use both together for the best balance.

Out of scope: Native AOT for server-side .NET -- see [skill:dotnet-native-aot]. AOT-first design patterns -- see [skill:dotnet-aot-architecture]. Trim-safe library authoring -- see [skill:dotnet-trimming]. MAUI-specific AOT -- see [skill:dotnet-maui-aot]. Blazor component patterns and architecture -- see [skill:dotnet-blazor-patterns] (soft). Uno Platform architecture -- see [skill:dotnet-uno-platform] (soft).

Cross-references: [skill:dotnet-native-aot] for general AOT pipeline, [skill:dotnet-trimming] for trimming annotations, [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 AOT enabler, [skill:dotnet-blazor-patterns] for Blazor architecture (soft), [skill:dotnet-uno-platform] for Uno Platform patterns (soft).

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Download Size vs Runtime Speed

Understanding the size/speed tradeoff is critical for WASM AOT decisions:

Compilation Mode	Download Size	Runtime Speed	Startup Time
IL interpreter (no AOT)	Smallest	Slowest	Fastest startup
AOT (all assemblies)	Largest	Fastest	Slower startup
AOT (selective) + trimming	Balanced	Good	Moderate
Trimmed only (no AOT)	Small	Moderate (JIT interpretation)	Fast

Key insight: Trimming reduces size by removing unused IL. AOT increases total artifact size because compiled native WASM code is larger than the equivalent IL bytecode. However, AOT-compiled code executes significantly faster because it skips IL interpretation at runtime.

When to Use WASM AOT

• CPU-intensive workloads: Image processing, complex calculations, data transformation
• Predictable performance: Consistent execution speed without JIT pauses
• Hot paths: AOT-compile only performance-critical assemblies (selective AOT)

When to Skip WASM AOT

• Bandwidth-constrained users: AOT increases download size significantly
• Simple CRUD apps: IL interpretation is fast enough for UI interactions and API calls
• Rapid iteration: AOT compilation adds significant publish time

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Blazor WASM AOT

Enabling AOT

<!-- Blazor WASM .csproj -->
<PropertyGroup>
  <RunAOTCompilation>true</RunAOTCompilation>
</PropertyGroup>

# Publish with AOT (required -- AOT only applies during publish)
dotnet publish -c Release

Note: RunAOTCompilation is the Blazor WASM property (not PublishAot which is for server-side Native AOT). AOT compilation only happens during dotnet publish, not during dotnet run or dotnet build.

Selective AOT via Lazy Loading

Blazor WASM AOT compiles all non-lazy-loaded assemblies. To control which assemblies are AOT-compiled, mark non-critical assemblies as lazy-loaded -- they will use IL interpretation instead:

<PropertyGroup>
  <RunAOTCompilation>true</RunAOTCompilation>
</PropertyGroup>

<ItemGroup>
  <!-- These assemblies are NOT AOT-compiled (loaded on demand via IL interpreter) -->
  <BlazorWebAssemblyLazyLoad Include="MyApp.Reporting.wasm" />
  <BlazorWebAssemblyLazyLoad Include="MyApp.Admin.wasm" />
  <!-- All other assemblies (MyApp.Core, MyApp.Calculations, etc.) ARE AOT-compiled -->
</ItemGroup>

Trimming + AOT Together

For the best balance, use both trimming and AOT:

<PropertyGroup>
  <!-- Trimming reduces unused code (smaller download) -->
  <PublishTrimmed>true</PublishTrimmed>

  <!-- AOT compiles remaining code to native WASM (faster execution) -->
  <RunAOTCompilation>true</RunAOTCompilation>

  <!-- Detailed warnings during development -->
  <EnableTrimAnalyzer>true</EnableTrimAnalyzer>
</PropertyGroup>

The publish pipeline runs: trim unused IL first, then AOT-compile the remaining assemblies to native WASM. This produces an artifact that is larger than trimmed-only but smaller than AOT-without-trimming, with the best runtime performance.

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Uno WASM AOT

Uno Platform 5+ with .NET 8+ uses the standard .NET WASM workload, so the AOT configuration is the same as Blazor WASM.

Enabling AOT (Uno 5+ / .NET 8+)

<!-- Uno WASM head .csproj -->
<PropertyGroup Condition="'$(TargetFramework)' == 'net8.0-browserwasm'">
  <RunAOTCompilation>true</RunAOTCompilation>
</PropertyGroup>

Older Uno versions using Uno.Wasm.Bootstrap had a separate WasmShellMonoRuntimeExecutionMode property with Interpreter, InterpreterAndAOT, and FullAOT modes. On .NET 8+, use RunAOTCompilation instead.

Trimming in Uno WASM

<PropertyGroup>
  <PublishTrimmed>true</PublishTrimmed>
  <TrimMode>link</TrimMode>
</PropertyGroup>

See [skill:dotnet-uno-platform] for Uno Platform architecture patterns.

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Lazy Loading Assemblies

Lazy loading defers downloading assemblies until they are needed, reducing initial download size. This is especially effective when combined with AOT (which increases per-assembly size).

Blazor WASM Lazy Loading

<!-- Mark assemblies for lazy loading in .csproj -->
<ItemGroup>
  <BlazorWebAssemblyLazyLoad Include="MyApp.Reporting.wasm" />
  <BlazorWebAssemblyLazyLoad Include="MyApp.Admin.wasm" />
  <BlazorWebAssemblyLazyLoad Include="ChartLibrary.wasm" />
</ItemGroup>

// Load assemblies on demand in a component or router
@inject LazyAssemblyLoader LazyLoader

@code {
    private List<Assembly> _lazyLoadedAssemblies = new();

    private async Task LoadReportingModule()
    {
        var assemblies = await LazyLoader.LoadAssembliesAsync(new[]
        {
            "MyApp.Reporting.wasm"
        });
        _lazyLoadedAssemblies.AddRange(assemblies);
    }
}

Router-Based Lazy Loading

<!-- App.razor -->
@inject LazyAssemblyLoader LazyLoader

<Router AppAssembly="typeof(App).Assembly"
        AdditionalAssemblies="@_lazyLoadedAssemblies"
        OnNavigateAsync="@OnNavigateAsync">
    <Navigating>
        <div class="loading">Loading module...</div>
    </Navigating>
</Router>

@code {
    private List<Assembly> _lazyLoadedAssemblies = new();

    private async Task OnNavigateAsync(NavigationContext context)
    {
        if (context.Path.StartsWith("admin"))
        {
            var assemblies = await LazyLoader.LoadAssembliesAsync(new[]
            {
                "MyApp.Admin.wasm"
            });
            _lazyLoadedAssemblies.AddRange(assemblies);
        }
        else if (context.Path.StartsWith("reports"))
        {
            var assemblies = await LazyLoader.LoadAssembliesAsync(new[]
            {
                "MyApp.Reporting.wasm"
            });
            _lazyLoadedAssemblies.AddRange(assemblies);
        }
    }
}

Lazy Loading Strategy

Strategy	Initial Load	Feature Load	Best For
No lazy loading	All at once	Instant	Small apps (<5 MB total)
Route-based lazy loading	Core only	On navigation	Multi-module apps
Feature-based lazy loading	Core only	On demand	Apps with optional features

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Brotli Pre-Compression

Brotli pre-compression reduces WASM download size by 60-80%. Blazor WASM automatically generates Brotli-compressed files during publish.

How It Works

During dotnet publish, Blazor WASM generates .br (Brotli) and .gz (gzip) compressed versions of all static files in _framework/. The web server serves the pre-compressed file when the browser supports it.

# After publish, check compressed sizes
ls -la bin/Release/net8.0/publish/wwwroot/_framework/

# You'll see:
# MyApp.wasm       (original)
# MyApp.wasm.br    (Brotli compressed, ~60-80% smaller)
# MyApp.wasm.gz    (gzip compressed, ~50-70% smaller)

Server Configuration

The web server must be configured to serve pre-compressed files. Most Blazor hosting setups handle this automatically.

ASP.NET Core hosting:

// In the server project hosting Blazor WASM
app.UseBlazorFrameworkFiles();
app.UseStaticFiles();
// Blazor framework files are served with compression headers automatically

Nginx:

location /_framework/ {
    # Serve Brotli-compressed files when available
    gzip_static on;
    brotli_static on;

    # Set correct MIME types
    types {
        application/wasm wasm;
    }

    # Cache aggressively (files are content-hashed)
    add_header Cache-Control "public, max-age=31536000, immutable";
}

Azure Static Web Apps / GitHub Pages:

Pre-compressed .br files are served automatically when the Accept-Encoding: br header is present.

Compression Impact

Content	Original	Brotli (.br)	Reduction
.NET WASM runtime	~2.5 MB	~0.8 MB	~68%
App assemblies (IL)	varies	~70% smaller	~70%
App assemblies (AOT)	varies	~65% smaller	~65%
JavaScript glue code	~100 KB	~25 KB	~75%

Disabling Compression (Rarely Needed)

<!-- Disable Brotli pre-compression -->
<PropertyGroup>
  <BlazorEnableCompression>false</BlazorEnableCompression>
</PropertyGroup>

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WASM Size Optimization Checklist

1. Enable trimming -- removes unused IL before AOT compilation
2. Use lazy loading -- defer non-critical assemblies
3. Enable Brotli pre-compression -- 60-80% reduction in transfer size (on by default)
4. Use selective AOT -- only AOT-compile performance-critical assemblies
5. Enable invariant globalization if culture-specific formatting is not needed:

   <PropertyGroup>
     <InvariantGlobalization>true</InvariantGlobalization>
   </PropertyGroup>
   

6. Remove unused framework features:

   <PropertyGroup>
     <!-- Disable features you don't use -->
     <EventSourceSupport>false</EventSourceSupport>
     <HttpActivityPropagationSupport>false</HttpActivityPropagationSupport>
   </PropertyGroup>
   

7. Verify compression is served -- check browser DevTools Network tab for content-encoding: br

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Agent Gotchas

1. Do not confuse RunAOTCompilation with PublishAot. Blazor WASM uses RunAOTCompilation for WASM AOT. PublishAot is for server-side Native AOT and produces a different kind of binary.
2. Do not assume AOT reduces WASM download size. AOT increases artifact size because native WASM code is larger than IL bytecode. Use trimming to reduce size and AOT to improve runtime speed.
3. Do not forget to publish when testing AOT. WASM AOT only runs during dotnet publish, not dotnet run. Debug builds always use IL interpretation.
4. Do not lazy-load assemblies that are needed at startup. Only lazy-load assemblies for features accessed after initial navigation. Loading a lazy assembly triggers a network request.
5. Do not skip Brotli compression verification. Ensure your web server serves .br files. Without compression, WASM downloads are 3-5x larger than necessary. Check browser DevTools for content-encoding: br header.
6. Do not AOT-compile all assemblies when download size matters. Use BlazorWebAssemblyLazyLoad to defer non-critical assemblies -- lazy-loaded assemblies use IL interpretation instead of AOT.

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References

• ASP.NET Core Blazor WebAssembly AOT
• Lazy load assemblies in Blazor WASM
• Host and deploy Blazor WASM
• Trim self-contained applications
• Uno Platform WASM