UA-.NETStandard/docs/ModelDependencies.md

# Cross-Assembly Model Dependencies

## Contents

- [Attribute shape](#attribute-shape)
- [Payload wire format (`ModelDependencyV1`)](#payload-wire-format-modeldependencyv1)
- [Diagnostics](#diagnostics)
- [Implementation](#implementation)

The OPC UA source generator emits `[assembly: Opc.Ua.ModelDependencyAttribute(...)]`
metadata on every assembly that has nodesets or design files in `<AdditionalFiles>`.
A single attribute carries both the lightweight dependency-closure information
**and**, on the assembly's self-declaration entry, a compact binary type-table
payload that downstream source generators can decode without re-walking
`AdditionalFiles`. Each instance of the attribute records, for one model the
assembly emits or transitively consumes:

- the model URI,
- the C# namespace prefix the generator used,
- (when known) the version and publication date,
- the C# identifier of the assembly's `Namespaces` class entry for the model,
- and — on self-declaration entries only — a base64-encoded Deflate-compressed
  `ModelDependencyV1` type-table payload.

Downstream consumers that reference such an assembly do not need to re-add
those upstream nodesets to their own `<AdditionalFiles>`. The generator scans the
attributes on referenced assemblies and uses them to:

1. **Suppress the missing-dependency error** in nodeset dependency collection
   when the missing namespace is known to be supplied by a referenced assembly.
2. **Apply override resolution** when a local `<AdditionalFiles>` entry refers
   to a model URI that is also in a referenced assembly:
   - **Same C# prefix** → local generation is silently skipped (the referenced
     assembly already supplies the types; duplicate emission would cause
     `CS0101`).
   - **Different C# prefix** → local generation proceeds; types live in a
     different C# namespace so no symbol conflict occurs.
3. **Cross-namespace prefix mapping** — when a local generator loads a
   NodeSet2 that depends on another model, it auto-generates dependency
   prefixes (e.g. `Opc.Ua.DI`) that may not match the C# namespace the
   referenced assembly actually uses (`Opc.Ua.Di`). The override step rewrites
   the dependency namespace's `Prefix` (and `Name`) to match the values
   published by the referenced assembly's `[ModelDependencyAttribute]`, so
   cross-namespace type references emitted into the local code compile
   correctly. The target namespace itself is never rewritten.
4. **Tie-break** when more than one referenced assembly provides the same model
   URI by selecting the entry with the highest `(Version, PublicationDate)`
   lexicographic tuple. The losing entries are reported as `MODELGEN012` info
   diagnostics.
5. **Import the type-table payload** on self-declaration entries so the
   validator's node table is pre-populated with the upstream's types,
   children, method arguments, DataType fields, and effective variable
   metadata. Cross-namespace
   `BaseType` / `TypeDefinition` / `DataType` references in the consumer's
   own models then resolve against the imported types without needing the
   upstream NodeSet2/ModelDesign XML in `AdditionalFiles`.

## Attribute shape

The attribute constructor accepts six arguments (the trailing four are
optional):

```csharp
[assembly: Opc.Ua.ModelDependencyAttribute(
    modelUri:        "http://opcfoundation.org/UA/DI/",
    prefix:          "Opc.Ua.Di",
    version:         "1.05.0",
    publicationDate: "2025-11-15T00:00:00Z",
    name:            "OpcUaDi",
    payload:         "qscBA…<base64 ModelDependencyV1>…AAA=")]
```

The `name` parameter records the C# identifier the assembly used inside its
`Namespaces` class — i.e. the `name` consumers must use when emitting
`global::{Prefix}.Namespaces.{Name}` cross-namespace constant references.

The `payload` parameter is non-null only on the assembly's own
self-declaration entry. Transitive-dependency entries (the one-per-referenced-
model rows the generator re-emits) carry `null`. The producing assembly is the
canonical source of its type-table description.

## Payload wire format (`ModelDependencyV1`)

Encoding lives in
`tools/Opc.Ua.SourceGeneration.Core/Dependency/ModelDependencyV1.cs`:

- Magic header: `0xAA 0xC7`
- Version byte: `0x01`
- Compression byte: `0x01` (Deflate)
- Body (compressed): `ModelUri` string + node array, each carrying symbolic
  name/namespace, class name, kind, base-type chain, numeric/string NodeId,
  abstract / enumeration flags, DataType fields, and declared instance
  children. Variable children also carry access and explicit user-access
  bitmasks, minimum sampling interval, historizing state, the corresponding
  presence flags, and serialized default-value XML. Method children carry
  their argument lists and effective method identity. The payload is
  deterministically sorted by
  `(SymbolicNamespace, SymbolicName)` so the produced base64 string is
  byte-reproducible across builds.

Readers reject unknown versions cleanly and the downstream pipeline falls
back to explicit `AdditionalFiles` resolution when a payload cannot be
decoded.

## Diagnostics

| ID            | Severity | Meaning                                                            |
| ------------- | -------- | ------------------------------------------------------------------ |
| `MODELGEN010` | Warning  | A `[NodeManager]` attribute could not be bound to a model (unmatched selector, or ambiguous when the project has multiple models). |
| `MODELGEN012` | Info     | Multiple referenced assemblies provide the same model URI.         |
| `MODELGEN013` | Info     | Local model skipped because a referenced assembly provides it.     |

## Implementation

The Roslyn-side scan lives in
`tools/Opc.Ua.SourceGeneration/ReferencedModelDependencyScanner.cs` and uses
`IAssemblySymbol.GetAttributes()` so it works for both
`PortableExecutableReference` and `CompilationReference`, hooks Roslyn's
per-symbol incremental cache, avoids file IO, and is AOT-safe inside the
generator. Each attribute is read into a `ModelDependencyReference` whose
`GetDependency()` method lazily decodes the payload through a memoising
`ConditionalWeakTable` keyed on the payload string.

The emitter lives in
`tools/Opc.Ua.SourceGeneration.Core/Generators/ModelDependencyGenerator.cs`
and produces one `{prefix}.ModelDependencies.g.cs` per generated model
containing assembly-attribute lines for the model itself (with the
`ModelDependencyV1` payload) and every model it consumes (with a `null`
payload). The template lives in `ModelDependencyTemplates.cs` and uses the
shared `Token` infrastructure for replacement.

The cross-namespace prefix override step lives in
`tools/Opc.Ua.SourceGeneration.Core/Generators.cs` as
`OverrideDependencyPrefixes`. It runs after `OpenModelDesign` and before
generation so that all downstream emitters see the harmonised prefix/name
values.

Both `GenerateCode` passes (NodeSet2 and ModelDesign) take the referenced
models as a single per-URI map of `ModelDependencyReference` and derive the
decoded payload map from it themselves via `BuildReferencedDependencyMap` —
the reference carries the raw base64 payload and memoises `GetDependency()`,
so the payload map is never threaded as a separate argument.

The payload-import surface lives directly on
`tools/Opc.Ua.SourceGeneration.Core/Schema/ModelDesignValidator.cs` (the
former `ModelDesignValidator.SnapshotImport.cs` partial was folded into the
main file). The validator's `ImportDependency(dependency, prefix, name)` API
queues a `ModelDependencyV1` for ingestion; `ApplyPendingDependencies()`
materialises the carried types into the validator's node table before the
dependency-loading pass walks `AdditionalFiles`, so consumer types can
resolve `BaseType` / `TypeDefinition` / `DataType` references against the
upstream's published types without those upstream models being present in
`AdditionalFiles`. Payloads for models that *are* present as design files in
the current compilation are skipped — the explicit file is authoritative.
After all design files are loaded, the validator links the
payload-materialised data types (base types, field data types, structure /
enumeration classification), so a local `ModelDesign` structure can subtype
a structure published only through a referenced assembly's payload.

Design files loaded as dependencies of the current target skip full
dictionary validation, so the validator links them the same way after
loading: their data types (as above) and their instances — the
`TypeDefinition` / `DataType` node references of the types' children,
variable-type data type restrictions, and method arguments including the
`InputArguments` / `OutputArguments` argument properties. A target
`ModelDesign` can therefore declare an `Object` or `Variable` whose
`TypeDefinition` is a type from another design file and have its inherited
children generate exactly as if the type were declared locally. Access level,
explicit user access, minimum sampling interval, historizing state, and
default values are retained when those inherited children come only from a
referenced assembly's payload.