# Description
Nullable reference types are now enabled once, in `common.props`, and
every project in the repository builds warning-clean with them.
**Configuration**
- `<Nullable>enable</Nullable>` lives in `common.props`. All per-project
`<Nullable>` settings are removed: 178 project files (enable,
annotations, and the `disable` in the three Aot test projects).
- 419 per-file `#nullable enable`/`restore` directives and the 4
`#nullable disable` directives are removed.
- Kept on purpose:
- The 5 `<auto-generated>` schema files (`UANodeSet.cs`, `UA Type
Dictionary.cs`, `OPCBinarySchema.cs`, `SecuredApplication.cs`,
`PInvokeHelper.cs`) keep `#nullable enable`. The compiler ignores the
project setting in generated code, so the directive is required there.
- `tests/Opc.Ua.Interop.LegacyPeer` keeps `disable`. It builds against
the 1.5.378 packages and does not import the repository props.
- `docs/DeveloperGuide.md` and `.github/copilot-instructions.md` now
describe the central setting.
**Migration** (the 43 unset and 13 annotations-only projects, roughly
10k diagnostics)
- **Production code: annotated at the declarations.** Covers
`tools/Opc.Ua.SourceGeneration.Core`, `src/Opc.Ua.Lds.Server` and the
LDS sample.
- Members, parameters and returns are nullable where null actually
flows.
- Null-check helpers carry `[NotNullWhen]` (`IsNull`, `IsValidXmlName`,
`IsMethodTypeDesign`).
- `CoreUtils.Format`, `Utils.Format`, `ITemplateWriter.WriteLine` take
`params object?[]`. `Template.AddReplacement` takes `string?` (rendering
already skips null replacements).
- `IRegisteredServerStore.RegisterAsync` returns
`Task<RegistrationEntry?>`: it returns null when a registration is
removed.
- Dereferences backed by an invariant (for example, the validated model)
use `!`, so runtime behaviour is unchanged.
- **Behavioural changes in production code** (small, deliberate):
- `NodeSetToModelDesign` throws `InvalidOperationException` when the
file is not a NodeSet. Before, it threw a NullReferenceException on the
next line.
- `ModelDesignValidator` starts its lookup tables empty instead of null.
A redundant `if (m_nodes == null) return;` guard is gone.
- **Test code: minimal annotations.**
- `?` on values that can be null, `!` where the test asserts non-null.
- `= null!` for fixture members assigned in SetUp.
- `TryGetStructure<T>(out T? x)` where the inferred `T?` broke the
`IEncodeable` constraint.
- A few redundant `== null` checks on structs (`NodeId`, `ArrayOf<T>`)
folded into the adjacent `IsNull`/`Count` checks.
**Notes for reviewers**
- The diff is large (~1,500 files) but almost entirely mechanical: `?`,
`!` and directive removal. The non-annotation changes are listed above.
- Known follow-up, not done here:
`IModelDesign.Namespaces/TargetNamespace/NamespaceUris` stay
non-nullable even though the validator returns null before it loads a
dictionary. Making them nullable would ripple through every generator
call site.
## Related Issues
- Fixes#4439
## Checklist
- [ ] 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.
- [ ] 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.
The "no new warnings" box covers net10.0 and net48: full non-incremental
builds of `UA.slnx`, `tools/SourceGeneration.slnx`,
`tools/MigrationAnalyzer.slnx` and the Aot test projects finished with 0
warnings and 0 errors under the repository's warnings-as-errors
settings. net9.0 and net8.0 also had no nullable diagnostics, but they
were not rebuilt after the final fixes. Tests have not been run locally
yet.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
---------
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
## Summary
Adds hand-written partial state classes that enforce OPC UA Part 8
§5.3.3 write validation for the `DiscreteItemType` subtypes, plus tests.
## Details
- **`MultiStateDiscreteState`** — a written value is treated as an index
into `EnumStrings`. Returns `BadOutOfRange` when the index is outside
`[0, len)`.
- **`MultiStateValueDiscreteState`** — a written value is matched
against the sparse `EnumValues` set. Returns `BadOutOfRange` when no
entry matches. On a good write, `ValueAsText` is refreshed from the
matching entry's display name.
- **`DiscreteItemState`** — provides the shared `TryGetIntegerValue`
helper that normalizes any scalar integer built-in type to `long` (the
exact-type `Variant` accessors return `default` on a built-in-type
mismatch, so a `BuiltInType` switch is required).
- **`TwoStateDiscreteType`** intentionally has no hand-written class.
These 3 classes are `public partial class` extending the
source-generated DataAccess partials already present on `master`. They
read via the typed properties (`EnumStrings`, `EnumValues`,
`ValueAsText`) rather than `FindChild`, because
`BaseDataVariableState.FindChild` special-cases the `EnumStrings`
BrowseName and never falls through to the `AddChild`'d children.
## Testing
- `dotnet build src\Opc.Ua.Core.Types\Opc.Ua.Core.Types.csproj -c
Release -f net10.0` → 0 warnings, 0 errors.
- 13 tests across `MultiStateDiscreteStateTests` and
`MultiStateValueDiscreteStateTests` pass (13 passed, 0 failed).
---------
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
romanett