AspNetCore.Docs/aspnetcore/fundamentals/native-aot.md

title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
ASP.NET Core support for native AOT	mitchdenny	Learn about ASP.NET Core support for native AOT	>= aspnetcore-8.0	midenn	mvc, engagement-fy23	06/19/2023	fundamentals/native-aot

ASP.NET Core support for native AOT

ASP.NET Core 8.0 introduces support for .NET native ahead-of-time (AOT).

[!WARNING] In .NET 8, not all ASP.NET Core features are compatible with native AOT.

Why use native AOT with ASP.NET Core

Publishing and deploying a native AOT app provides the following benefits:

• Minimized disk footprint: When publishing using native AOT, a single executable is produced containing just the code from external dependencies that is needed to support the program. Reduced executable size can lead to:
  • Smaller container images, for example in containerized deployment scenarios.
  • Reduced deployment time from smaller images.
• Reduced startup time: Native AOT applications can show reduced start-up times, which means
  • The app is ready to service requests quicker.
  • Improved deployment where container orchestrators need to manage transition from one version of the app to another.
• Reduced memory demand: Native AOT apps can have reduced memory demands, depending on the work done by the app. Reduced memory consumption can lead to greater deployment density and improved scalability.

The template app was run in our benchmarking lab to compare performance of an AOT published app, a trimmed runtime app, and an untrimmed runtime app. The following chart shows the results of the benchmarking:

The preceding chart shows that native AOT has lower app size, memory usage, and startup time.

ASP.NET Core and native AOT compatibility

Not all features in ASP.NET Core are currently compatible with native AOT. The following table summarizes ASP.NET Core feature compatibility with native AOT:

Feature	Fully Supported	Partially Supported	Not Supported
gRPC	✔️Fully supported
Minimal APIs		✔️Partially supported
MVC			❌Not supported
Blazor Server			❌Not supported
SignalR			❌Not supported
Authentication			❌Not supported (JWT soon)
CORS	✔️Fully supported
HealthChecks	✔️Fully supported
HttpLogging	✔️Fully supported
Localization	✔️Fully supported
OutputCaching	✔️Fully supported
RateLimiting	✔️Fully supported
RequestDecompression	✔️Fully supported
ResponseCaching	✔️Fully supported
ResponseCompression	✔️Fully supported
Rewrite	✔️Fully supported
Session			❌Not supported
Spa			❌Not supported
StaticFiles	✔️Fully supported
WebSockets	✔️Fully supported

It's important to test an app thoroughly when moving to a native AOT deployment model. The AOT deployed app should be tested to verify functionality hasn't changed from the untrimmed and JIT-compiled app. When building the app, review and correct AOT warnings. An app that issues AOT warnings during publishing is not guaranteed to work correctly. If no AOT warnings are issued at publish time, the published AOT app should work the same as when it's run in development.

For more information, see Introduction to AOT warnings.

Native AOT publishing

Native AOT is enabled with the PublishAot MSBuild property. The following example shows how to enable native AOT in a project file:

<PropertyGroup>
  <PublishAot>true</PublishAot>
</PropertyGroup>

This setting enables native AOT compilation during publish and enables dynamic code usage analysis during build and editing. A project that uses native AOT publishing uses JIT compilation when running locally. An AOT app has the following differences from a JIT-compiled app:

• Features that aren't compatible with native AOT are disabled and throw exceptions at run time.
• A source analyzer is enabled to highlight code that isn't compatible with native AOT. At publish time, the entire app, including NuGet packages, are analyzed for compatibility again.

Native AOT analysis includes all of the app's code and the libraries the app depends on. Review native AOT warnings and take corrective steps. It's a good idea to publish apps frequently to discover issues early in the development lifecycle.

In .NET 8, native AOT is supported by the following ASP.NET Core app types:

• minimal APIs - For more information, see the API template section later in this article.
• gRPC - For more information, see gRPC and native AOT.
• Worker services - For more information, see AOT in Worker Service templates.

The API template

The ASP.NET Core API template has an option to enable AOT:

• Visual Studio 2022 has an Enable native AOT publish option.

• The CLI uses the dotnet new api command and the --aot option, as in the following example:

  dotnet new api --aot -o MyFirstAotWebApi
  

This template is intended to produce a project more directly focused on cloud-native, API-first scenarios. The template differs from the Web API project template in the following ways:

• Uses minimal APIs only, as MVC isn't yet compatible with native AOT.
• Uses the xref:Microsoft.AspNetCore.Builder.WebApplication.CreateSlimBuilder API to ensure only the essential features are enabled by default, minimizing the app's deployed size.
• Is configured to listen on HTTP only, as HTTPS traffic is commonly handled by an ingress service in cloud-native deployments.
• Doesn't include a launch profile for running under IIS or IIS Express.
• Creates an .http file configured with sample HTTP requests that can be sent to the app's endpoints.
• Includes a sample Todo API instead of the weather forecast sample.

In addition to these differences, the ASP.NET Core API template has the following differences when the Enable native AOT publish option is selected:

• Adds PublishAot to the project file, as shown earlier in this article.
• Enables the JSON serializer source generators. The source generator is used to generate serialization code at build time, which is required for native AOT compilation.

Changes to support source generation

The following example shows the code added to the Program.cs file to support JSON serialization source generation:

|using MyFirstAotWebApi;
+using System.Text.Json.Serialization;

var builder = WebApplication.CreateSlimBuilder(args);

+builder.Services.ConfigureHttpJsonOptions(options =>
+{
+  options.SerializerOptions.TypeInfoResolverChain.Insert(0, AppJsonSerializerContext.Default);
+});

var app = builder.Build();

var sampleTodos = TodoGenerator.GenerateTodos().ToArray();

var todosApi = app.MapGroup("/todos");
todosApi.MapGet("/", () => sampleTodos);
todosApi.MapGet("/{id}", (int id) =>
    sampleTodos.FirstOrDefault(a => a.Id == id) is { } todo
        ? Results.Ok(todo)
        : Results.NotFound());

app.Run();

+[JsonSerializable(typeof(Todo[]))]
+internal partial class AppJsonSerializerContext : JsonSerializerContext
+{
+
+}

Without this added code, System.Text.Json uses reflection to serialize and deserialize JSON. Reflection isn't supported in native AOT.

For more information, see:

• JSON serializer source generators
• JsonSerializerOptions.TypeInfoResolverChain
• xref:System.Text.Json.JsonSerializerOptions.TypeInfoResolverChain

Changes to launchSettings.json

The API template launchSettings.json file has the iisSettings section and IIS Express profile removed:

{
  "$schema": "http://json.schemastore.org/launchsettings.json",
-  "iisSettings": {
-     "windowsAuthentication": false,
-     "anonymousAuthentication": true,
-     "iisExpress": {
-       "applicationUrl": "http://localhost:11152",
-       "sslPort": 0
-     }
-   },
  "profiles": {
    "http": {
      "commandName": "Project",
      "dotnetRunMessages": true,
      "launchBrowser": true,
      "launchUrl": "todos",
      "applicationUrl": "http://localhost:5102",
        "environmentVariables": {
          "ASPNETCORE_ENVIRONMENT": "Development"
        }
      },
-     "IIS Express": {
-       "commandName": "IISExpress",
-       "launchBrowser": true,
-       "launchUrl": "todos",
-      "environmentVariables": {
-       "ASPNETCORE_ENVIRONMENT": "Development"
-      }
-    }
  }
}

The CreateSlimBuilder method

The template uses the xref:Microsoft.AspNetCore.Builder.WebApplication.CreateSlimBuilder method instead of the xref:Microsoft.AspNetCore.Builder.WebApplication.CreateBuilder method.

:::code language="csharp" source="~/fundamentals/aot/samples/Program.cs" highlight="4":::

The CreateSlimBuilder method initializes the xref:Microsoft.AspNetCore.Builder.WebApplicationBuilder with the minimum ASP.NET Core features necessary to run an app. It's used by the template whether or not the AOT option is used.

As noted earlier, the CreateSlimBuilder method doesn't include support for HTTPS or HTTP/3. These protocols typically aren't required for apps that run behind a TLS termination proxy. For example, see TLS termination and end to end TLS with Application Gateway. HTTPS can be enabled by calling builder.WebHost.UseKestrelHttpsConfiguration HTTP/3 can be enabled by calling builder.WebHost.UseQuic.

The CreateSlimBuilder method does include the following features needed for an efficient development experience:

• JSON file configuration for appsettings.json and appsettings.{EnvironmentName}.json.
• User secrets configuration.
• Console logging.
• Logging configuration.

Including minimal features has benefits for trimming as well as AOT. For more information, see Trim self-contained deployments and executables.

Source generators

Because unused code is trimmed during publishing for native AOT, the app can't use unbounded reflection at runtime. Source generators are used to produce code that avoids the need for reflection. In some cases, source generators produce code optimized for AOT even when a generator isn't required.

To view the source code that is generated, add the EmitCompilerGeneratedFiles property to an app's .csproj file, as shown in the following example:

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <!-- Other properties omitted for brevity -->
    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
  </PropertyGroup>

</Project>

Run the dotnet build command to see the generated code. The output includes an obj/Debug/net8.0/generated/ directory that contains all the generated files for the project.

The dotnet publish command also compiles the source files and generates files that are compiled. In addition, dotnet publish passes the generated assemblies to a native IL compiler. The IL compiler produces the native executable. The native executable contains the native machine code.

See also

• xref:fundamentals/native-aot-tutorial
• Native AOT deployment

Known issues

See this GitHub issue to report or review issues with native AOT support in ASP.NET Core.