[http-client-csharp] Implement support for external type references (framework types only)

[Copilot]

External Types Implementation for http-client-csharp

This PR implements support for external type references in the C# http-client-csharp emitter, enabling the use of the @alternateType decorator with ExternalType objects as defined in Azure/typespec-azure#3233.

Note: Only .NET framework types are supported as external type references. Non-framework external types will emit a warning diagnostic and be skipped during code generation to ensure buildable libraries.

Changes Made

• Add InputExternalType interface to TypeScript input-type.ts
• Update InputType union to include InputExternalType
• Implement external type detection and conversion in type-converter.ts
• Add InputExternalType C# class to the generator
• Add InputExternalTypeConverter for JSON deserialization
• Update InputTypeConverter to handle "external" kind
• Update TypeFactory.CreateCSharpTypeCore to handle InputExternalType
• Add CreateExternalType method to create CSharpType from external references
• Add comprehensive test coverage for external types
• Add diagnostic for unsupported external types (emitted from generator code)
• Return null for non-framework external types to prevent non-buildable code generation
• Add unit tests in ModelProviderTests validating external type functionality

Implementation Details

Emitter (TypeScript):

• Added InputExternalType interface with identity, package, and minVersion properties
• Modified fromSdkType to detect when an SDK type has an external property and convert it to InputExternalType
• Added fromSdkExternalType helper function that converts external type information to InputExternalType

Generator (C#):

• Created InputExternalType class in Microsoft.TypeSpec.Generator.Input
• Created InputExternalTypeConverter for JSON deserialization of InputExternalType from the TypeScript emitter output
• Updated InputTypeConverter to handle "external" kind and route to InputExternalTypeConverter
• Updated TypeFactory.CreateCSharpTypeCore switch statement to handle InputExternalType
• Implemented CreateExternalType method that:
  • Attempts to resolve external types as framework types using CreateFrameworkType
  • Emits a warning diagnostic using CodeModelGenerator.Instance.Emitter.ReportDiagnostic for types that cannot be resolved
  • Returns null for types that cannot be resolved as framework types (preventing generation of non-buildable code)

Usage Example

import "@azure-tools/typespec-client-generator-core";

using Azure.ClientGenerator.Core;

// ✅ Supported: Framework types will be resolved successfully
@alternateType({
  identity: "System.Uri",
}, "csharp")
model ExternalUri {
  value: string;
}

// ⚠️ Warning: Non-framework types will emit a diagnostic and be skipped
@alternateType({
  identity: "Azure.Core.Expressions.DataFactoryExpression",
  package: "Azure.Core.Expressions",
  minVersion: "1.0.0",
}, "csharp")
union Dfe<T> {
  T,
  DfeExpression: string
}

Framework types like System.Uri, System.BinaryData, System.Text.Json.JsonElement will be successfully resolved and referenced in the generated code. Non-framework external types will emit a warning during code generation and be skipped, ensuring that all generated libraries are buildable.

Testing

TypeScript Tests (in type-converter.test.ts):

• External types on unions
• External types on models
• Package and version information handling

C# Generator Tests (in ModelProviderTests.cs):

• ExternalTypeModelUsedAsProperty: Tests a model decorated with @alternateType that references System.Uri, used as a property in another model
• ExternalTypePropertyIsResolved: Tests a property decorated with @alternateType referencing System.Net.IPAddress
• UnsupportedExternalTypeEmitsDiagnostic: Tests that unsupported external types (non-framework) emit diagnostics and are properly skipped

Diagnostics

The warning diagnostic unsupported-external-type is emitted from the generator code (not the emitter) when:

• An external type cannot be resolved as a framework type
• The diagnostic uses a simplified message: "External type '{identity}' is not currently supported."

Fixes #8187

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[http-client-csharp] Reference external types Implementation</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #8187

---

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[JoshLove-msft]

@copilot there are build failures related to this line.

[Copilot]

Fixed in commit 74e3f90. The issue was an incorrect call to the CSharpType constructor. I updated the CreateExternalType method to properly parse the fully qualified name and use the internal constructor with all required parameters.

[JoshLove-msft]

@copilot this is not going to result in buildable libraries. We should emit a diagnostic saying that an unsupported external type was used and just return null.

[Copilot]

Fixed in commit c8a6e4b. Updated CreateExternalType to return null for external types that cannot be resolved as framework types. Also added a diagnostic warning in the TypeScript emitter for non-framework external types, preventing generation of non-buildable code.

[JoshLove-msft]

@copilot The diagnostic can be emitted directly from the generator code. We should not add this to the emitter code. We can use the CodeModelGenerator.Instance.Emitter property.

[Copilot]

Fixed in commit 3e8bf94. Moved the diagnostic emission from the TypeScript emitter to the C# generator code. The diagnostic is now emitted using CodeModelGenerator.Instance.Emitter.ReportDiagnostic in the CreateExternalType method, and the diagnostic code has been added to DiagnosticCodes.cs.

[github-actions]

No changes needing a change description found.

[JoshLove-msft]

@copilot this can be removed.

[Copilot]

Removed in commit 8e63fa5. The constant was not being used since the diagnostic code is passed as a string literal.

[JoshLove-msft]

@copilot let's add a few tests that validate the new functionality:

• simulate a model that is decorated with alternateType. The model should be used as a property in a different model.
• simulate a property that is decorated with alternateType;
  These should go in ModelProviderTests.

[Copilot]

Added in commit fc47d2e. Three tests added to ModelProviderTests.cs:

1. TestExternalType_ModelDecoratedWithAlternateType - validates model decorated with @alternateType using System.Uri as property in another model
2. TestExternalType_PropertyDecoratedWithAlternateType - validates property decorated with @alternateType using System.Net.IPAddress
3. TestExternalType_UnsupportedExternalType - validates unsupported external types emit diagnostics and are skipped

All tests pass successfully.