Centralize C# type attribute back-compat merging in TypeProvider

[Copilot]

Attribute back-compat merging (generated + last-contract + custom-code) was owned locally by CanonicalTypeProvider.BuildAttributes(). This moves that logic into a shared TypeProvider API so it follows the existing centralized back-compat pattern (Build*ForBackCompatibility).

Changes

• TypeProvider: Added BuildAttributesForBackCompatibility() that:
  • restores compatible attributes from the last contract
  • merges generated (Attributes) and custom-code (CustomCodeView.Attributes) attributes,
  • filters CodeGen-specific attributes (CodeGenSuppress/CodeGenMember/CodeGenType/CodeGenSerialization) that should not be preserved,
  • deduplicates by rendered display string.
• CanonicalTypeProvider: BuildAttributes() now delegates to the shared API instead of owning merge/dedup logic.
• Tests: Added coverage for the merge/dedup behavior.

Because the method reads Attributes, CustomCodeView, LastContractView, and DeclarationModifiers from the provider, invoking it on the generated provider reproduces the previous merge exactly:

protected override IReadOnlyList<MethodBodyStatement> BuildAttributes()
    => _generatedTypeProvider.BuildAttributesForBackCompatibility();

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commit: 68a9a9b

[Copilot]

Pull request overview

Centralizes attribute back-compat handling in the C# generator’s TypeProvider so attribute restoration/merging from the last contract happens through the same back-compat pipeline as other members, and adds targeted ClientModel filtering for generator-owned attributes that must be recomputed.

Changes:

• Added TypeProvider.BuildAttributesForBackCompatibility(...) and wired attribute restoration into ProcessTypeForBackCompatibility().
• Added ClientModel-specific filtering to drop recomputed generator-owned attributes (e.g., proxy/buildable attributes) after restoration.
• Added/expanded unit tests + golden test data for restore/skip/dedup scenarios.

Reviewed changes

Copilot reviewed 36 out of 36 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/TestHelpers/TestTypeProvider.cs	Extends test provider to supply attributes for back-compat tests.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TypeProviderTests.cs	Adds unit tests covering attribute back-compat restore/skip/dedup behavior.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsExperimentalAttribute/BackCompatAttributeType.cs	Last-contract input for Experimental attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsExperimentalAttribute.cs	Expected generated output for Experimental attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsEditorBrowsableAttribute/BackCompatAttributeType.cs	Last-contract input for EditorBrowsable attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsEditorBrowsableAttribute.cs	Expected generated output for EditorBrowsable attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsDefaultMemberAttribute/BackCompatAttributeType.cs	Last-contract input for DefaultMember attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsDefaultMemberAttribute.cs	Expected generated output for DefaultMember attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsCodeGenAttributes/BackCompatAttributeType.cs	Last-contract input for CodeGen attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsCodeGenAttributes.cs	Expected generated output for CodeGen attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsAttributesAlreadyInCustomCode/BackCompatAttributeType.cs	Last-contract input where an attribute exists in custom code already.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsAttributesAlreadyInCustomCode(Custom)/BackCompatAttributeType.cs	Custom-code input for “already in custom code” scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsAttributesAlreadyInCustomCode.cs	Expected generated output for “already in custom code” scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsAllCodeGenPrefixedAttributes/BackCompatAttributeType.cs	Last-contract input for CodeGen-prefixed attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilitySkipsAllCodeGenPrefixedAttributes.cs	Expected generated output for CodeGen-prefixed attribute skip scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilityReturnsGeneratedAttributesWhenNoLastContract.cs	Expected output when no last contract is present.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilityDoesNotDuplicateExistingAttribute/BackCompatAttributeType.cs	Last-contract input for duplicate-prevention scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilityDoesNotDuplicateExistingAttribute.cs	Expected output for duplicate-prevention scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilityAddsAttributeFromLastContract/BackCompatAttributeType.cs	Last-contract input for add-new-attribute scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/TestData/TypeProviderTests/BuildAttributesForBackCompatibilityAddsAttributeFromLastContract.cs	Expected output for add-new-attribute scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/SourceInput/CodeGenAttributes.cs	Adds shared CodeGen prefix constant for filtering.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/TypeProvider.cs	Implements attribute back-compat restore logic and integrates it into processing/update.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/TestHelpers/MockHelpers.cs	Extends generator mock setup to allow injecting serialization providers in tests.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/TestData/ScmModelProviderTests/BuildAttributesForBackCompatibilityDoesNotRestoreProxyAttribute/Pet.cs	Last-contract input for proxy-attribute filtering in ScmModelProvider.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/TestData/ScmModelProviderTests/BuildAttributesForBackCompatibilityDoesNotRestoreProxyAttribute.cs	Expected output verifying proxy attribute is not restored.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ScmModelProvider/ScmModelProviderTests.cs	Adds test ensuring proxy attributes aren’t restored for models.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/BackCompatibilityTests/BuildAttributesForBackCompatibilityDoesNotRestoreProxyAttribute/Pet.cs	Last-contract input for proxy-attribute filtering in MRW serialization provider.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/BackCompatibilityTests/BuildAttributesForBackCompatibilityDoesNotRestoreProxyAttribute.cs	Expected output verifying proxy attribute is not restored in MRW serialization provider.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/BackCompatibilityTests.cs	Adds MRW serialization-provider back-compat attribute filtering test.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/Definitions/TestData/ModelReaderWriterContextDefinitionTests/BuildAttributesForBackCompatibilityDoesNotRestoreBuildableAttribute/SampleContext.cs	Last-contract input for buildable-attribute filtering in MRW context definition.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/Definitions/TestData/ModelReaderWriterContextDefinitionTests/BuildAttributesForBackCompatibilityDoesNotRestoreBuildableAttribute.cs	Expected output verifying buildable attribute is not restored.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/Definitions/ModelReaderWriterContextDefinitionTests.cs	Adds test ensuring buildable attributes aren’t restored for MRW context definition.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Utilities/ScmBackCompatibilityHelpers.cs	Adds helper to remove newly-restored generator-owned attributes.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/ScmModelProvider.cs	Overrides attribute back-compat to drop restored proxy attribute for models.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs	Overrides attribute back-compat to drop restored proxy attribute for serialization providers.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/ModelReaderWriterContextDefinition.cs	Overrides attribute back-compat to drop restored buildable attribute for contexts.

[jorgerangel-msft]

@copilot there was a crash with our changes:

It looks like there's also a critical bug in our code when regenerating one of the azure libraries:

Sync and generate from a TypeSpec project

  

  Options:

    -d, --debug            Enable debug logging                          [boolean]

    -o, --output-dir       Specify an alternate output directory for the generated

                           files.                          [string] [default: "."]

    -y, --no-prompt        Skip any interactive prompts.                 [boolean]

        --help             Show help                                     [boolean]

        --repo             Repository where the project is defined        [string]

        --commit           Commit hash to be used                         [string]

        --tsp-config       Path to tspconfig.yaml                         [string]

        --local-spec-repo  Path to local spec repo                        [string]

        --emitter-options  The options to pass to the emitter             [string]

        --save-inputs      Don't clean up the temp directory after generation

                                                                         [boolean]

        --skip-install     Skip installing dependencies                  [boolean]

        --trace            Enable tracing during compile                   [array]

    -v, --version          Show version number                           [boolean]

  

  ExternalError: Emitter "@azure-typespec/http-client-csharp" crashed! This is a bug.

  Please file an issue at [https://github.com/azure-sdk/azure-sdk-for-net/issues](https://github.com/azure-sdk/azure-sdk-for-net/issues)

  

Error : error : Failed to generate the library. Exit code: 1. [/mnt/vss/_work/_temp/azure-sdk-for-net-20260717-165539/sdk/appconfiguration/Azure.Data.AppConfiguration/src/Azure.Data.AppConfiguration.csproj]

  StackTrace: 

  The method or operation is not implemented.

     at Microsoft.TypeSpec.Generator.Expressions.LiteralExpression.Write(CodeWriter writer)

     at Microsoft.TypeSpec.Generator.Statements.AttributeStatement.Write(CodeWriter writer)

     at Microsoft.TypeSpec.Generator.Statements.MethodBodyStatement.GetDebuggerDisplay()

     at Microsoft.TypeSpec.Generator.Statements.MethodBodyStatement.ToDisplayString()

     at Microsoft.TypeSpec.Generator.Providers.TypeProvider.BuildAttributesForBackCompatibility(IEnumerable`1 originalAttributes)

     at Microsoft.TypeSpec.Generator.Providers.TypeProvider.ProcessTypeForBackCompatibility()

     at Microsoft.TypeSpec.Generator.CSharpGen.ExecuteAsync()

     at Microsoft.TypeSpec.Generator.GeneratorRunner.RunAsync(CommandLineOptions options)

     at Microsoft.TypeSpec.Generator.Program.Run(CommandLineOptions options, GeneratorRunner runner)

  

      at generate (file:///mnt/vss/_work/_temp/azure-sdk-for-net-20260717-165539/sdk/appconfiguration/Azure.Data.AppConfiguration/TempTypeSpecFiles/node_modules/@typespec/http-client-csharp/dist/emitter/src/emit-generate.js:69:19)

      at process.processTicksAndRejections (node:internal/process/task_queues:103:5)

      at async emitCodeModel (file:///mnt/vss/_work/_temp/azure-sdk-for-net-20260717-165539/sdk/appconfiguration/Azure.Data.AppConfiguration/TempTypeSpecFiles/node_modules/@typespec/http-client-csharp/dist/emitter/src/emitter.js:70:17)

      at async emitAzureCodeModel (file:///mnt/vss/_work/_temp/azure-sdk-for-net-20260717-165539/sdk/appconfiguration/Azure.Data.AppConfiguration/TempTypeSpecFiles/node_modules/@azure-typespec/http-client-csharp/dist/emitter/src/emitter.js:48:12)

      at async Object.$onEmit [as emitFunction] (file:///mnt/vss/_work/_temp/azure-sdk-for-net-20260717-165539/sdk/appconfiguration/Azure.Data.AppConfiguration/TempTypeSpecFiles/node_modules/@azure-typespec/http-client-csharp/dist/emitter/src/emitter.js:8:29)

      at async runEmitter (file:///mnt/vss/_work/_temp/azure-sdk-for-net-20260717-165539/sdk/appconfiguration/Azure.Data.AppConfiguration/TempTypeSpecFiles/node_modules/@typespec/compiler/dist/src/core/program.js:684:9)

      at async file:///mnt/vss/_work/_temp/azure-sdk-for-net-20260717-165539/sdk/appconfiguration/Azure.Data.AppConfiguration/TempTypeSpecFiles/node_modules/@typespec/compiler/dist/src/core/program.js:655:37

      at async emit (file:///mnt/vss/_work/_temp/azure-sdk-for-net-20260717-165539/sdk/appconfiguration/Azure.Data.AppConfiguration/TempTypeSpecFiles/node_modules/@typespec/compiler/dist/src/core/program.js:653:12)

[Copilot]

@copilot there was a crash with our changes:

...

Fixed. Root cause: back-compat restoration renders each last-contract attribute via ToDisplayString() to build the dedup key, and LiteralExpression.Write threw NotImplementedException for integral literal kinds it didn't handle. Azure.Data.AppConfiguration has a last-contract attribute with an integral (e.g. byte/uint) argument, which hit that gap.

Two changes:

• LiteralExpression.Write now handles byte/sbyte/short/ushort/uint/ulong, so such attributes render correctly and are restored.
• BuildAttributesForBackCompatibility is now resilient: if an attribute still fails to render (e.g. an unresolved type or unsupported argument kind), it's skipped rather than crashing the whole emitter.

Added unit tests: AttributeStatementWithIntegralArguments and BuildAttributesForBackCompatibilityRestoresAttributeWithIntegralLiteralArguments.

[jorgerangel-msft]

@copilot I'd like to see some unit tests where a type has attributes in back compat, custom code, and current generated source. We should validate there are no duplicates and things work as expected

[Copilot]

@copilot I'd like to see some unit tests where a type has attributes in back compat, custom code, and current generated source. We should validate there are no duplicates and things work as expected

Added BuildAttributesForBackCompatibilityDeduplicatesAcrossGeneratedCustomAndLastContract. The type has [Obsolete("This is obsolete")] in generated source, [CLSCompliant(true)] in custom code, and the last contract declares all three of [Obsolete("This is obsolete")], [CLSCompliant(true)], and [Restorable]. The test asserts the merged result restores only [Restorable] (no duplicate Obsolete/CLSCompliant), validated via TestData.

[Copilot]

Pull request overview

Copilot reviewed 46 out of 46 changed files in this pull request and generated 4 comments.

[jorgerangel-msft]

@copilot update the description to be accurate

[Copilot]

The accurate wording is: attribute restoration only applies to externally visible types (public / protected), while generated and custom-code attributes are still merged and generator-owned / CodeGen attributes are skipped. I can’t edit the PR body directly from this environment, but that is the text that should replace the old “non-public types only” wording.

[Copilot]

@copilot analyze the regen pr Azure/azure-sdk-for-net#6108 for additions in attributes on types. Give me a summary of any attributes that were added that we didn't explicitly f...

I checked the link, but it currently redirects to Azure/azure-sdk-for-net issue #6108 (Review Authentication Support Against Guidelines) rather than a PR/files diff, so I can’t reliably enumerate attribute additions from that URL. Based on our current filters, a type-level attribute would still be restored unless it either contains CodeGen, is in TypeProvider’s non-restorable set (EditorBrowsable, Experimental, Obsolete, Serializable, DefaultMember, and the filtered System.Text.Json.Serialization attributes), or is one of the SCM-owned attributes filtered by the ClientModel providers. If you send the correct regen PR link or the relevant diff, I can cross-check it and list the exact unfiltered additions.

[jorgerangel-msft]

@copilot address this comment #11237 (comment)

[Copilot]

@copilot address this comment #11237 (comment)

The referenced thread is about the PR description, not the implementation. The correct wording is: attribute restoration only applies to externally visible types (public / protected), while generated and custom-code attributes are still merged and generator-owned / CodeGen attributes are skipped. I can’t edit the PR body from this environment, so that text still needs to be applied in the description itself.