feat: Add CodeGenNamespace assembly attribute for enum namespace customization

[radhgupta]

Problem

Fixes #9086

Users need to change the namespace of fixed C# enum types, but since enums can't be partial, they must redefine ALL enum members in custom code via [CodeGenType]. This prevents them from automatically catching spec updates — any new enum values added to the spec require manual custom code updates.

Solution

Add a new [assembly: CodeGenNamespace] attribute that changes the namespace of a generated type without requiring a replacement type definition:

[assembly: CodeGenNamespace("OriginalEnum", "NewNamespace.Models")]

The generator still emits the full enum with all members — just in the new namespace. No custom enum definition needed, no member duplication.

Note on @clientNamespace

As Josh noted on the issue, the @clientNamespace TCGC decorator can also be used at the TypeSpec level to change namespaces. The InputEnumType.Namespace property already flows from TCGC → emitter → generator, so @clientNamespace should work for this scenario.

The [CodeGenNamespace] attribute complements @clientNamespace for scenarios where:

• Users cannot modify the TypeSpec spec (consuming shared/external specs)
• Users need C#-specific namespace overrides that don't apply to other languages
• Users want to stay consistent with existing C# customization patterns ([CodeGenType], [CodeGenMember])

Changes

Core implementation (5 files modified/added):

• CodeGenNamespaceAttributeDefinition.cs — New TypeProvider that generates the CodeGenNamespaceAttribute class (AttributeTargets.Assembly, AllowMultiple = true)
• SourceInputModel.cs — Scans assembly-level CodeGenNamespace attributes from custom code, builds a typeName → namespace override map
• TypeProvider.cs — Added GetResolvedNamespace() with priority chain: CustomCodeView → CodeGenNamespace override → default namespace
• CodeModelGenerator.cs — Registered new attribute; added safe GetNamespaceOverride() static accessor for initialization-time safety
• CodeGenAttributes.cs — Added constant for the new attribute name

Tests (12 test cases across 3 files):

• Fixed enum namespace change
• Extensible enum namespace change
• Member preservation after namespace change
• Model namespace change (works for all types)
• [CodeGenType] takes precedence when both attributes present
• Non-matching type name has no effect (negative case)
• Attribute instantiation test
• Stronger assertions: type identity, field counts, enum value counts

Generated:

• CodeGenNamespaceAttribute.cs in Generated/Internal/ (auto-generated via Generate.ps1)

Test Results

All 2,660 tests pass across 5 test projects (0 failures).

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@10124

commit: f0a53ce

[github-actions]

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[JoshLove-msft]

I'm not sure it is worth adding this just for enums when we can already customize in client.tsp.

[radhgupta]

I'm not sure it is worth adding this just for enums when we can already customize in client.tsp.

I see, should we mark the issue done then??

[JoshLove-msft]

I'm not sure it is worth adding this just for enums when we can already customize in client.tsp.

I see, should we mark the issue done then??

I think we can close it for now as not fixing.