C# emitter: discriminated base type with no public derived models emits a dangling "instantiation are: " with trailing whitespace

[JoshLove-msft]

Describe the bug

ModelProvider.BuildDescription appends the sentence "...The derived classes available for instantiation are: " for any discriminated base type, and then loops over the public derived models to fill in the list:

https://github.com/microsoft/typespec/blob/main/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs

var publicDerivedModels = _derivedModels.Where(m => m.DeclarationModifiers.HasFlag(TypeSignatureModifiers.Public)).ToList();
var derivedClassesDescription = DeclarationModifiers.HasFlag(TypeSignatureModifiers.Abstract)
    ? "Please note this is the abstract base class. The derived classes available for instantiation are: "
    : "Please note this is the base class. The derived classes available for instantiation are: ";

for (int i = 0; i < publicDerivedModels.Count; i++) { ... }

description = $"{description}\n{derivedClassesDescription}";

When every derived model is internal, publicDerivedModels is empty, the loop body never executes, and the sentence is emitted with nothing after it. Two problems result:

1. Semantic — the doc promises a list of instantiable derived classes and then provides none. There are no public derived classes, so the sentence should not be emitted at all.
2. Trailing whitespace — the description ends with the dangling "are: ", so the generated /// line has a trailing space. This trips git diff --check and any whitespace linting on generated output.

Reproduction

A discriminated base type whose only derived type is the generated Unknown* fallback (which is internal). Generated output:

/// <summary>
/// LimitJson abstract class.
/// Please note this is the abstract base class. The derived classes available for instantiation are: 
/// </summary>

Note the trailing space after are:. Compare with a type that does have a public derived model, which renders correctly:

/// Please note this is the abstract base class. The derived classes available for instantiation are: <see cref="Models.TypeAPolyDevice"/>.

Real-world impact

This is present in committed output today. In Azure/azure-sdk-for-net, on main:

• eng/packages/http-client-csharp-mgmt/generator/TestProjects/Local/Mgmt-TypeSpec/src/Generated/Models/LimitJsonObject.cs:16
• eng/packages/http-client-csharp-mgmt/generator/TestProjects/Local/Mgmt-TypeSpec/src/Generated/Models/LimitJsonObject.Serialization.cs:17

and in shipped SDKs, e.g. sdk/machinelearningservices/Azure.Provisioning.MachineLearning/src/Generated/Models/MachineLearningDatastoreSecrets.cs:14 and sdk/servicefabric/Azure.Provisioning.ServiceFabric/src/Generated/Models/ServicePlacementPolicyDescription.cs:14.

Suggested fix

Skip the appended sentence entirely when publicDerivedModels.Count == 0, which resolves both the dangling prose and the trailing whitespace.

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that reports the same bug

[github-actions]

🎯 Issue Triage

In ModelProvider.BuildDescription, when all derived models are internal, the "derived classes available for instantiation" sentence is emitted unconditionally with nothing after it, producing dangling prose and trailing whitespace in generated XML doc comments.

Category: Bug

Area: emitter:client:csharp

⚠️ No reproduction provided. (This is a C# code generation bug — no TypeSpec repro applies; the fix is in ModelProvider.cs.)

🔍 Analysis

• Affected file: packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs, BuildDescription method
• Root cause: The description suffix is always appended after the loop, regardless of whether publicDerivedModels.Count > 0. When only internal derived models exist (e.g., the auto-generated Unknown* fallback), the list is empty, leaving the sentence incomplete.
• Impact: Two issues — misleading XML doc comments, and trailing whitespace that breaks git diff --check and whitespace linters. The author confirmed this appears in shipped Azure SDK packages.
• Fix: Guard the appended sentence with if (publicDerivedModels.Count > 0).

📝 Suggested next steps

• In ModelProvider.BuildDescription, wrap the derivedClassesDescription append in a publicDerivedModels.Count > 0 guard
• Update/add a unit test covering the case where all derived models are internal
• Regenerate affected test snapshots

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