Native AOT Testing

Overview

The OPC UA .NET Standard stack supports Native AOT (Ahead-of-Time) compilation, which produces a self-contained executable that is compiled to native code at publish time rather than at run time. This eliminates the need for a JIT compiler and the .NET runtime on the target machine, resulting in faster startup and a smaller deployment footprint.

The Opc.Ua.Aot.Tests project verifies that the core OPC UA libraries work correctly when published as a Native AOT binary. The tests exercise encoding, sessions, subscriptions, monitored items, discovery, security, events, history, diagnostics, batch operations, node cache, complex types, GDS client operations, and client sample patterns — all running inside a single ahead-of-time compiled executable.

Prerequisites

.NET SDK

• .NET 10.0 SDK (or later LTS) is required.

Platform-Specific Native Toolchain

Native AOT compilation requires a C/C++ toolchain on the build machine.

Platform	Requirement
Windows	Visual Studio 2022+ with the Desktop development with C++ workload, or the equivalent Build Tools package.
Linux	clang and zlib1g-dev (Debian/Ubuntu) or the equivalent packages for your distribution.
macOS	Xcode Command Line Tools (xcode-select --install).

See the official Microsoft documentation for full details: https://learn.microsoft.com/en-us/dotnet/core/deploying/native-aot/?tabs=windows%2Cnet8plus

Project Structure

Tests/Opc.Ua.Aot.Tests/
├── Opc.Ua.Aot.Tests.csproj   # Project file (PublishAot=true, net10.0)
├── AotServerFixture.cs        # Lightweight AOT-compatible server host
├── AotTestFixture.cs          # Shared fixture: starts server + client session
├── GdsTestFixture.cs          # GDS-specific test fixture
├── EncodingAotTests.cs        # Binary & JSON encoding round-trips
├── DataTypeAotTests.cs        # OPC UA data type verification
├── SessionAotTests.cs         # Session lifecycle & reconnect
├── SubscriptionAotTests.cs    # Subscription create / modify / delete
├── MonitoredItemAotTests.cs   # Monitored item operations
├── DiscoveryAotTests.cs       # Endpoint & server discovery
├── SecurityAotTests.cs        # Security policy negotiation
├── EventsAotTests.cs          # Event subscription & filtering
├── HistoryAotTests.cs         # Historical read operations
├── DiagnosticsAotTests.cs     # Server diagnostics
├── BatchOperationsAotTests.cs # Batch read / write / call
├── NodeCacheAotTests.cs       # Client-side node cache
├── ComplexTypeAotTests.cs     # Complex type loading & serialization
├── GdsClientAotTests.cs       # Global Discovery Server client
├── ClientSamplesAotTests.cs   # End-to-end client sample patterns
├── AotClientSamples.cs        # Helper methods for client samples
├── PubSubAotTests.cs          # Part 14 PubSub publisher / subscriber round-trips
├── WebApiAotFixture.cs        # Kestrel host for REST endpoint round-trips
├── WebApiAotTests.cs          # REST binding (Opc.Ua.Bindings.Https) smoke + Basic auth
└── StubWebApiServer.cs        # IWebApiServer stub backing WebApiAotFixture

Why TUnit Instead of NUnit?

The project uses the TUnit test framework instead of NUnit. TUnit relies on source generation rather than runtime reflection for test discovery, which makes it fully compatible with Native AOT and the IL trimmer. NUnit (and most traditional .NET test frameworks) depend heavily on reflection, which is not supported in trimmed / AOT-published applications.

Test Fixture Pattern

AotTestFixture implements IAsyncInitializer and IAsyncDisposable from TUnit. It is shared across all test classes via the attribute:

[ClassDataSource<AotTestFixture>(Shared = SharedType.PerTestSession)]

On initialization the fixture:

1. Starts a ReferenceServer in-process using AotServerFixture<T>.
2. Creates a client ApplicationConfiguration programmatically.
3. Connects an ISession to the server over opc.tcp.

Every test class receives the fixture through its primary constructor and reuses the same server and session, keeping the test suite fast.

AotServerFixture<T> is a cut-down, AOT-compatible version of ServerFixture<T> from the NUnit test infrastructure. It avoids transitive references to BenchmarkDotNet, Moq, and other packages that are not AOT-friendly.

How to Build and Run

1. Publish the Native AOT Binary

dotnet publish Tests/Opc.Ua.Aot.Tests/Opc.Ua.Aot.Tests.csproj --configuration Release

The publish step compiles the entire application (tests, server, and all referenced OPC UA libraries) into a single native executable. This can take several minutes depending on the machine.

2. Run the Tests

Windows (x64):

./Tests/Opc.Ua.Aot.Tests/bin/Release/net10.0/win-x64/publish/Opc.Ua.Aot.Tests.exe

Linux (x64):

./Tests/Opc.Ua.Aot.Tests/bin/Release/net10.0/linux-x64/publish/Opc.Ua.Aot.Tests

The executable discovers and runs all tests, producing TUnit console output and writing results to a TestResults directory.

Build + Run in a Single Step (Development)

For iterative development you can combine both commands:

# Windows
dotnet publish Tests/Opc.Ua.Aot.Tests/Opc.Ua.Aot.Tests.csproj -c Release && ^
  Tests\Opc.Ua.Aot.Tests\bin\Release\net10.0\win-x64\publish\Opc.Ua.Aot.Tests.exe

# Linux / macOS
dotnet publish Tests/Opc.Ua.Aot.Tests/Opc.Ua.Aot.Tests.csproj -c Release && \
  ./Tests/Opc.Ua.Aot.Tests/bin/Release/net10.0/linux-x64/publish/Opc.Ua.Aot.Tests

Note: dotnet test and dotnet run do not perform AOT compilation. You must use dotnet publish followed by direct execution of the resulting binary.

CI Integration

The GitHub Actions workflow .github/workflows/buildandtest.yml defines an aot-test job that runs on both ubuntu-latest and windows-latest. The steps are:

1. Checkout the repository.
2. Setup .NET 10.0 SDK.
3. Publish the project with dotnet publish in Release configuration.
4. Execute the platform-specific binary directly.
5. Upload any TestResults artifacts.

The job runs in a separate matrix from the main dotnet test build so that AOT failures are isolated and clearly visible.

Writing New AOT Tests

1. Choose or Create a Test Class

Place AOT tests in Tests/Opc.Ua.Aot.Tests/. Each file should focus on a single area (encoding, sessions, etc.). Apply the shared fixture:

[ClassDataSource<AotTestFixture>(Shared = SharedType.PerTestSession)]
public class MyFeatureAotTests(AotTestFixture fixture)
{
    [Test]
    public async Task MyTestAsync()
    {
        // Use fixture.Session, fixture.ServerUrl, etc.
        await Assert.That(fixture.Session.Connected).IsTrue();
    }
}

2. Use TUnit Attributes and Assertions

• Mark tests with [Test] (not NUnit's [Test]—they are different types).
• Use await Assert.That(…) for assertions. TUnit assertions are async.
• Do not reference NUnit, xUnit, or MSTest assemblies.

3. Keep Code AOT-Compatible

• Avoid unbounded reflection. Do not use Type.GetType(), Activator.CreateInstance(), or similar APIs unless the types are statically reachable.
• Avoid dynamic code generation. Reflection.Emit, System.Linq.Expressions.Expression.Compile(), and similar APIs are not supported.
• Prefer concrete generic instantiations. The trimmer must see every generic type combination at compile time.
• Annotate when necessary. Use [DynamicallyAccessedMembers] or [RequiresUnreferencedCode] attributes to preserve metadata the trimmer would otherwise remove.

4. Handle Trimming Warnings

The project suppresses IL2104 (see the .csproj), which comes from third-party packages that are not yet trim-annotated. For warnings in your own code, fix the root cause rather than suppressing.

Troubleshooting

Publish Fails with Linker Errors

Ensure the platform-specific C/C++ toolchain is installed (see Prerequisites). On Windows, verify the Desktop development with C++ workload is present in the Visual Studio Installer.

TypeInitializationException or MissingMetadataException at Runtime

The IL trimmer removed type metadata that is needed at runtime. Common fixes:

• Add [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] to parameters or fields that hold types accessed via reflection.
• Add an explicit rd.xml file or [DynamicDependency] attribute to preserve specific types.
• Ensure the type is statically referenced somewhere in the code path.

Tests Pass Under dotnet test but Fail Under AOT

dotnet test runs with JIT and full reflection. Some APIs silently work under JIT but are unsupported in AOT. Compare the stack trace from the AOT binary to identify which API is problematic, then refactor to a trim-safe alternative.

Slow Publish Times

Native AOT compilation is inherently slower than JIT builds because it performs whole-program optimization. On CI this is expected. For local development, consider running the NUnit tests with dotnet test for fast feedback, and reserve AOT publish for final validation.

IL2104 or Other Trimming Warnings

Warnings prefixed with IL come from the IL trimmer/linker. The project suppresses IL2104 for third-party packages. If you see new warnings from OPC UA code, investigate and fix the root cause. Suppress only as a last resort and document the reason in a code comment.