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Cross-Assembly Model Dependencies

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, and DataType fields. 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):

[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 (with method-argument lists). 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 Info Local model skipped because a referenced assembly provides it.
MODELGEN012 Info Multiple referenced assemblies provide the same model URI.

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.

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.