Handle custom external model bases

[live1206]

Summary

• Remove the synthetic custom-base SystemObjectModelProvider path and keep custom C# bases represented through existing type metadata.
• Update MRW serialization so models with custom C# bases derive serialization behavior from the base type hierarchy:
  • emit JsonModelWriteCore as an override when the custom base already defines it;
  • keep create-core return types rooted at the meaningful base type, such as ResourceData;
  • only override create-core methods when the base actually participates in that method chain.
• Tighten XML serialization override detection so XmlModelWriteCore only overrides when the base exposes a compatible, accessible, non-sealed XML core method:
  • use XML-specific detection instead of the JSON MRW override predicate;
  • account for generated base custom replacements that filter out generated methods;
  • reject wrong-signature, non-virtual, protected/public, sealed, reflected external, and shadowed ancestor XML core methods.
• Preserve existing BaseModelProvider behavior for real input-model inheritance and external @alternateType model mappings.
• Add coverage for canonical duplicate wire-path filtering from custom/system bases while preserving non-canonical same-wire SDK properties.

Validation

• dotnet test ./generator/Microsoft.TypeSpec.Generator.ClientModel/test/Microsoft.TypeSpec.Generator.ClientModel.Tests.csproj --no-restore from packages/http-client-csharp
• dotnet test ./generator/Microsoft.TypeSpec.Generator/test/Microsoft.TypeSpec.Generator.Tests.csproj --no-restore from packages/http-client-csharp
• Validated from azure-sdk-for-net with temporary local project references: full Azure SDK mgmt generator tests passed.

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/http-client-csharp@11183

commit: c4a19ce

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No changes needing a change description found.

[live1206]

No diff in regen: Azure/azure-sdk-for-net#60807

[Copilot]

Pull request overview

This PR refines how the C# generator models and serializes types that derive from custom/external base classes, removing reliance on a synthetic base-provider path and instead using existing type metadata and hierarchy-aware detection (especially for MRW XML serialization). It also adds test coverage around external base chains and canonical wire-path deduplication.

Changes:

• Extend model/provider plumbing to better represent external/custom base type chains (base type + constructor-parameter propagation behavior).
• Improve MRW serialization override decisions (JSON root return type discovery; XML override detection with signature/accessibility/overridability checks).
• Add tests + test data for external base behaviors and canonical duplicate wire-path filtering.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/SystemObjectModelProviderTests.cs	Adds coverage for representing an external base chain via type metadata without duplicating inherited properties.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/TestData/ModelCustomizationTests/CanFilterCanonicalDuplicateWirePathFromSystemObjectModelProviderBase/MockInputModel.cs	Adds test customization source defining a framework-like base (ResourceData) and a derived partial model.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelProviders/ModelCustomizationTests.cs	Adds test ensuring canonical duplicate wire-path properties are filtered when a system/custom base already maps that wire path.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Writers/CodeWriter.cs	Adds sealed emission support for method/property signatures.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/TypeProvider.cs	Enhances customized-member filtering to consider canonical duplicate wire paths (not just names).
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/SystemObjectModelProvider.cs	Adds base-type propagation and derived-constructor parameter behavior control for wrapped system types.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/NamedTypeSymbolProvider.cs	Adds interface list materialization and propagates sealed into method signature modifiers.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs	Gates base-constructor parameter propagation based on the base provider’s preference.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Primitives/MethodSignatureModifiers.cs	Adds Sealed to the modifier bitflags.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/XmlSerializationCustomizationTests.cs	Adds extensive XML override-detection tests across multiple base scenarios.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/XmlSerializationCustomizationTests/XmlModelWriteCoreOverridesGeneratedBaseWithCustomReplacement/BaseModel.cs	Adds test data for a generated base with a compatible virtual XML core method.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/XmlSerializationCustomizationTests/XmlModelWriteCoreDoesNotOverrideProtectedCustomExternalBase/MockInputModel.cs	Adds test data for a protected XML base method that must not be overridden by an internal method.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/XmlSerializationCustomizationTests/XmlModelWriteCoreDoesNotOverrideJsonOnlyCustomBase/MockInputModel.cs	Adds test data for a JSON-only base to ensure XML methods are not overridden incorrectly.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/XmlSerializationCustomizationTests/XmlModelWriteCoreDoesNotOverrideGeneratedBaseWithSealedCustomMethod/BaseModel.cs	Adds test data ensuring a sealed override in a customized base prevents derived overrides.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/XmlSerializationCustomizationTests/XmlModelWriteCoreDoesNotOverrideGeneratedBaseWithProtectedCustomMethod/BaseModel.cs	Adds test data ensuring protected custom methods don’t trigger internal overrides.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/XmlSerializationCustomizationTests/XmlModelWriteCoreDoesNotOverrideGeneratedBaseWithNonVirtualCustomMethod/BaseModel.cs	Adds test data ensuring non-virtual methods don’t trigger overrides.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/SystemObjectModelSerializationTests.cs	Adjusts system-base MRW tests to explicitly model interface implementation via a specialized provider.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.Xml.cs	Reworks XML override detection to be XML-specific and hierarchy-aware.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs	Improves MRW override decisions and root-type discovery for models with custom/external bases.

[jorgerangel-msft]

I'm wondering if we can avoid all of these changes by just changing the accessibility of BaseTypeProvider:

public TypeProvider? BaseTypeProvider => BaseTypeProviderTypeProvider;
private protected virtual TypeProvider? BaseTypeProviderTypeProvider  => null;

Then in this file we can add a helper:

private bool BaseDeclaresOverridable(string methodName)
{
    for (var provider = _model.BaseTypeProvider; provider is not null; provider = provider.BaseTypeProvider)
    {
        // Prefer the custom view so hand-authored replacements/hooks win over generated members.
        var source = provider.CustomCodeView ?? provider;
        var method = source.Methods.FirstOrDefault(m => m.Signature.Name == methodName);
        if (method is null)
        {
            continue; // not declared at this level; keep walking up
        }

        var mods = method.Signature.Modifiers;
        return !mods.HasFlag(MethodSignatureModifiers.Sealed)
            && (mods.HasFlag(MethodSignatureModifiers.Virtual)
                || mods.HasFlag(MethodSignatureModifiers.Override)
                || mods.HasFlag(MethodSignatureModifiers.Abstract));
    }

    return false;
}

Will this help simplify the proposed changes or am I missing something ?

[jorgerangel-msft]

👍

[jorgerangel-msft]

why is this needed?