[http-client-csharp] Optional nullable properties cannot emit explicit JSON null

[JoshLove-msft]

Description

The C# emitter collapses an optional nullable property's omitted and explicitly null states. Setting the generated property to null omits it from the request instead of writing JSON null.

This breaks services where omission selects a default but explicit null disables a feature. For example, Foundry voice-agent turn detection defaults to enabled and requires "turn_detection": null to disable it.

Input and reproduction

The pinned TypeSpec declarations correctly model both optionality and nullability:

noise_reduction?: VoiceAgentNoiseReduction | null;
turn_detection?: VoiceAgentTurnDetectionConfig | null;
transcription?: VoiceAgentInputTranscription | null;

Using the generated model in Azure/azure-sdk-for-net#62884:

using System;
using System.ClientModel.Primitives;
using Azure.AI.Projects.Agents;

#pragma warning disable AAIP001
var input = new VoiceAgentAudioInputConfig
{
    TurnDetection = null
};

Console.WriteLine(ModelReaderWriter.Write(
    input,
    new ModelReaderWriterOptions("W"),
    AzureAIProjectsAgentsContext.Default).ToString());

Actual: {}

Expected: {"turn_detection":null}

Leaving TurnDetection unset should still produce {}. The generated model needs to preserve the distinction rather than serializing all unset nullable properties as null.

The same omission occurs for NoiseReduction and Transcription. Serialization through the containing voice-agent definition produces "audio":{"input":{}}, so this also affects actual agent request bodies. The omission was reproduced against the compiled SDK and its real dependencies without service calls.

Upstream implementation

In the pinned generator, MrwSerializationTypeDefinition.CreateConditionalSerializationStatement uses Optional.IsDefined(property) and generates an explicit-null branch only for isRequired && isNullable. Optional nullable properties fall through to an if (IsDefined(...)) guard, and these generated reference-type properties have no explicit-presence tracking.

The SDK's active custom visitors only add experimental attributes or adjust virtual/override modifiers; they do not change these property types or serialization conditions.

Environment

• Emitter: @typespec/http-client-csharp 1.0.0-alpha.20260910.2
• Generator: Microsoft.TypeSpec.Generator.ClientModel 1.0.0-alpha.20260910.2
• Generator source revision, verified from the NuGet package metadata: 9b8c51de8c15b6dc748ea9cf576d87f91ef058c4
• SDK reproduction: Generate code from the latest typespec Azure/azure-sdk-for-net#62884 at efa46fdc6e3fca32a195fc575b873fd696a7b572
• Original SDK review finding

Expected coverage

Please cover omitted, explicitly null, and non-null values for optional nullable model properties, including nested wire serialization and preservation of explicit null when reading and then writing a model.

[github-actions]

🎯 Issue Triage

The C# client emitter cannot preserve the three wire states of an optional nullable model property: omitted, explicitly null, and non-null. Assigning null currently serializes as omission, preventing callers from disabling service features whose defaults apply only when the property is absent.
Category: Bug
Area: emitter:client:csharp

🔗 [Playground Reproduction]((typespec.io/redacted)

📋 Reproduction code

noise_reduction?: VoiceAgentNoiseReduction | null;
turn_detection?: VoiceAgentTurnDetectionConfig | null;
transcription?: VoiceAgentInputTranscription | null;

🔍 Analysis

The affected component is Microsoft.TypeSpec.Generator.ClientModel serialization generated by @typespec/http-client-csharp. MrwSerializationTypeDefinition.CreateConditionalSerializationStatement guards optional properties with Optional.IsDefined(property), but generated nullable reference properties do not retain independent presence state. Assigning null is therefore indistinguishable from never assigning the property, including in nested request bodies and read-then-write scenarios.

Targeted searches found related nullable-reference and serialization issues (#5811, #5988, and #3005), but none describe this same optional-property presence-tracking defect.

📝 Suggested next steps

• Add generator coverage for omitted, explicitly null, and non-null optional nullable properties.
• Cover nested wire serialization and read-then-write preservation of explicit null.
• Introduce presence tracking or equivalent serialization logic without changing omission behavior for unset properties.

Generated by Agentic Triage