Marc Schier
26ef554283
# Description Makes the server data-change **and** event push path fully asynchronous, so a monitored node whose value is read through an asynchronous read handler (`OnReadValueAsync` / `OnSimpleReadValueAsync`) is honored without a thread ever blocking, and the per-node notification channel applies back-pressure by awaiting rather than blocking a caller. This is the deeper follow-up to the review discussion on #3922, where the synchronous change-notification path was silently bypassing asynchronous value read handlers. The change is **additive and non-breaking**: every existing synchronous API and sink keeps working unchanged. **Asynchronous `NodeState` change-notification API** (`Stack/Opc.Ua.Types`) - New `ClearChangeMasksAsync`, `OnStateChangedAsync` / `StateChangedAsync`, and `ReportEventAsync` / `OnReportEventAsync` (plus the `BaseInstanceState` parent-recursion override). - The synchronous `ClearChangeMasks` / `ReportEvent` now also drive the asynchronous sinks: they complete inline for a synchronously-readable node with channel capacity, and block the calling thread only when a genuinely asynchronous read is in flight or the channel is full (so only synchronous-API callers ever pay that cost). **Asynchronous `MonitoredNode2` producers** (`Libraries/Opc.Ua.Server`) - `OnMonitoredNodeChangedAsync` / `OnReportEventAsync` read each monitored attribute at enqueue time via `ReadAttributeAsync` — honoring an asynchronous read handler and materializing the value **before** the change is enqueued (a strict enqueue-time snapshot) — and enqueue by awaiting the bounded per-node channel. Synchronous wrappers are retained for compatibility. - FIFO/LIFO discard and the `Overflow` status bit remain a per-monitored-item queue concern; the shared per-node channel stays a lossless conduit. **Asynchronous event source-report path** - New `IServerInternal.ReportEventAsync` / `ServerInternalData.ReportEventAsync`, and an asynchronous root-notifier `OnReportEventAsync` in `AsyncCustomNodeManager` (event propagation to the Server object is now awaited). The rare model-change `Raise*ChangeEvent` helpers remain synchronous — they drive the async producer via the sync wrapper, and events do not perform an asynchronous read, so a full async cascade through the model-change/audit subsystems would be near-zero value. **Internal flush migration** - The hot value-write flush (`AsyncCustomNodeManager.WriteAsync`) and all other asynchronous-context `ClearChangeMasks` call sites now await `ClearChangeMasksAsync`. Genuinely synchronous contexts (builders, diagnostics timers, the synchronous `CustomNodeManager`, supervision) remain on the synchronous wrapper (correct — not hot paths). **Performance:** synchronous reads are unchanged (they complete inline); asynchronous reads no longer block a thread (proven by a unit test asserting the producer's `ValueTask` is not completed while a slow asynchronous read is in flight); enqueue replaces a blocking wait / per-event `AsTask` with an awaited write. **Docs:** a new "Fully asynchronous change notifications" section in `Docs/AsyncServerSupport.md`. ## Related Issues Follow-up to the review feedback on #3922 (asynchronous value read handlers were bypassed on the synchronous change-notification push path). - Fixes # ## Checklist _Put an `x` in the boxes that apply. You can complete these step by step after opening the PR._ - [ ] I have signed the [CLA](https://opcfoundation.org/license/cla/ContributorLicenseAgreementv1.0.pdf) and read the [CONTRIBUTING](https://github.com/OPCFoundation/UA-.NETStandard/blob/master/CONTRIBUTING.md) doc. - [x] I have added tests that prove my fix is effective or that my feature works and increased code coverage. - [x] I have added all necessary documentation. - [x] I have verified that my changes do not introduce (new) build or analyzer warnings. - [ ] I ran **all** tests locally using the **UA.slnx** solution against at least .net **framework** and .net **10**, and all passed. - [ ] I fixed **all** failing and flaky tests in the CI pipelines and **all** CodeQL warnings. - [ ] I have addressed **all** PR feedback received.
OPC UA .NET Standard Stack
🆕 This is version 2.0 of the OPC UA .NET Standard Stack (current
master).Looking for the supported 1.x release? It lives on the
master378branch (last release:1.5.378). All new feature work happens here onmaster;master378continues to receive security and critical-bug fixes for the 1.x line.
The official OPC Foundation reference implementation of OPC UA for .NET — a certified, cross-platform stack with client, server, PubSub, GDS, complex types, and source-generated tooling. Used in production across industrial control, manufacturing, energy, and IoT systems.
📦 What it is
- A full-stack OPC UA implementation — Core / Client / Server / PubSub / GDS / LDS / Complex Types / Device Integration libraries built on .NET, with UA-TCP and HTTPS transports.
- Cross-platform — runs on .NET 10, .NET 9, .NET 8 (LTS), .NET Framework 4.8, and .NET Standard 2.1; ships Native-AOT-friendly assemblies.
- Certified for compliance — the reference server has been certified through an OPC Foundation Certification Test Lab and is continuously verified against the latest Compliance Test Tool (CTT).
- Companion-spec coverage — Part 9 (Alarms & Conditions), Part 11 (Historical Access), Part 13 (Aggregates), Part 16 (State Machines), Part 17 (Alias Names), Part 18 (Role Management), Part 20 (File Transfer), Part 100 (Device Integration), OPC 10100-1 (WoT Connectivity).
- Modern developer surface — first-class
Microsoft.Extensions.DependencyInjectionhosting (services.AddOpcUa()), fluent server + client builders, source-generated NodeManagers and DataTypes, and an MCP server so LLMs / Copilot can drive an OPC UA client.
For the full feature breakdown see OPC UA Profiles and Facets and the What's New in 2.0 tour.
🚀 Getting started
You need the .NET 10 SDK to build the repo. From the repository root:
dotnet restore UA.slnx
dotnet build UA.slnx
For the supported target frameworks and platform notes see
Platform Support. The official NuGet packages
are published to nuget.org under the OPCFoundation.NetStandard
prefix — the meta package
OPCFoundation.NetStandard.Opc.Ua
pulls in everything, or pick the split
OPCFoundation.NetStandard.Opc.Ua.Core / .Client / .Server /
.Bindings.Https packages directly. Preview builds for every successful
master build are available in the
Azure DevOps opcua-preview feed.
Sample applications
Each sample has its own README.md with build and run instructions.
Reference applications
- Console Reference Server — the certified reference server (with Quickstarts, CTT, and Mono configs). Also ships as a Docker container.
- Console Reference Client — cross-platform reference client demonstrating sessions, subscriptions, browsing, and method calls.
- Console LDS Server — a standalone
Local Discovery Server built on
Opc.Ua.Lds.Server. - MCP Server — Model Context Protocol server that exposes OPC UA client operations as MCP tools, so an LLM / Copilot can browse, read, write, subscribe, and call methods on any OPC UA server.
PubSub samples
- Console Reference PubSub Client —
one executable with
publisher,subscriber, andexternal(external-server adapter) modes across the supported transport profiles.
Minimal / Device-Integration samples
- Minimal Calc Server — minimal server built on the source-generated NodeManager pipeline (Calc model).
- Minimal Boiler Server — minimal Boiler-model server with the fluent state-machine builder; Native-AOT publishable.
- Pump Device Integration Server —
minimal Device Integration (Part 100) server using
Opc.Ua.Di.Server's fluent builder.
More sample projects are maintained in the companion OPC UA .NET Samples repository.
🔧 Migrating from 1.5.378 to 2.0
The 2.0 release introduces breaking API changes, and comes with a full prescriptive migration guide that links to per-area documentation covering telemetry, packages, source generation, types, encoders, node states, identity, certificates, configuration, sessions / subscriptions, alarms / model change, and TimeProvider.
Most of the mechanical migration work is automated:
OPCFoundation.NetStandard.Opc.Ua.MigrationAnalyzerNuGet — install it in your project to get analyzer warnings (UA0001–UA0022)- one-click code fixes for the patterns in the guide. Setup steps are in the package's NugetREADME.md.
- Migration agent skill — the
opcua-v20-migrationskill walks Copilot / Claude / any coding agent through installing the NuGet, runningdotnet format analyzersto apply auto-fixes, and handling the small residual manual patterns. The skill knows which sub-doc to load for each symptom so it stays context-efficient.
If you are still on 1.x and not ready to upgrade, stay on the
master378
branch — it continues to receive security and critical-bug fixes.
🤝 Contributing and license
Community contributions are welcome. Fork the repository, make your changes on a branch, and open a pull request; see CONTRIBUTING.md for the contribution workflow.
Contributors must sign the OPC Foundation Contributor License Agreement (CLA). The CLA "I AGREE" gate is presented automatically on your first PR.
The project is licensed under the OPC Foundation MIT License. Report security vulnerabilities via the process documented in SECURITY.md.
📚 Further reading
- Documentation index — every per-feature doc with a one-line description.
- What's New in 2.0 — narrative tour of the 1.5.378 → 2.0 changes grouped by theme and layer.
- OPC UA Profiles and Facets — facets / transports / security policies the stack implements.
- Migration Guide — prescriptive
per-version migration reference (links to
Docs/migrate/2.0.x/). - OPC UA Online Reference — the official OPC 10000 series specification index.
- OPC UA .NET Samples — companion repository with more sample applications.
- Preview NuGet feed — prerelease builds from Azure DevOps.