[http-client-csharp] Expose extension point for generator-specific custom-code attributes

[ArcturusZhang]

Problem

Azure management-plane C# generation needs to introduce management-specific CodeGen attributes, for example CodeGenResourceDataAttribute, that are emitted into generated SDK projects and are also available while the generator compiles custom code.

The base C# generator already has built-in CodeGenTypeAttributeDefinition, CodeGenMemberAttributeDefinition, CodeGenSuppressAttributeDefinition, and CodeGenSerializationAttributeDefinition. These are included in CodeModelGenerator.CustomCodeAttributeProviders, but that provider list is not extensible from derived generators.

Because derived generators cannot add their own custom-code attribute providers, Azure management currently has to use reflection to append a TypeProvider to CustomCodeAttributeProviders. If the provider is a normal TypeProvider, it may also need to avoid source-input view lookup before SourceInputModel is initialized.

Request

Please expose a supported extension point for derived C# generators to contribute custom-code attribute providers, such as one of:

• protected virtual IReadOnlyList BuildCustomCodeAttributeProviders()
• protected void AddCustomCodeAttributeProvider(TypeProvider provider)
• another supported API that lets derived generators add generated attribute definitions used during custom-code compilation

Expected usage

Azure management generator should be able to register a provider like CodeGenResourceDataAttributeDefinition so custom code can write:

`csharp
using Microsoft.TypeSpec.Generator.Customizations;

[CodeGenResourceData(typeof(MyCustomResourceData))]
public partial class FooResource { }
`

without the SDK depending on the generator assembly and without reflection in the derived generator.

Related downstream PR

Azure SDK for .NET PR: Azure/azure-sdk-for-net#59851

[github-actions]

🎯 Issue Triage

Feature request to expose a supported extension point in the http-client-csharp CodeModelGenerator so derived generators (e.g., Azure management) can contribute custom-code attribute providers without resorting to reflection hacks.

Category: Feature Request
Area: emitter:client:csharp

🔍 Analysis

• CodeModelGenerator.CustomCodeAttributeProviders currently ships four built-in providers (CodeGenTypeAttributeDefinition, CodeGenMemberAttributeDefinition, CodeGenSuppressAttributeDefinition, CodeGenSerializationAttributeDefinition) but the list is not overridable by subclasses.
• Azure management-plane generator needs to register a CodeGenResourceDataAttributeDefinition so generated custom code can use [CodeGenResourceData(...)] without depending on the generator assembly.
• Current workaround uses reflection to append a TypeProvider, which is fragile and may have ordering issues relative to SourceInputModel initialization.
• Proposed API options from the author:
  • protected virtual IReadOnlyList<TypeProvider> BuildCustomCodeAttributeProviders()
  • protected void AddCustomCodeAttributeProvider(TypeProvider provider)
• The virtual override pattern is generally cleaner as it doesn't require the base class to maintain a mutable list; it's consistent with other Build* virtual patterns in the generator.
• Related downstream PR: Azure/azure-sdk-for-net#59851

📝 Suggested next steps

• Decide on the preferred API shape (virtual BuildCustomCodeAttributeProviders() or AddCustomCodeAttributeProvider())
• Add the extension point in CodeModelGenerator
• Update Azure management generator to use the new API and remove the reflection workaround
• Add tests covering a derived generator that registers a custom attribute provider

Generated by Agentic Triage