[Bug]: Avoid duplicate output for required nullable properties during patch serialization

[joseharriaga]

Describe the bug

For properties defined as required but nullable (foo: string | null), the generated C# serialization code can emit the property twice during serialization when using JsonPatch. The generated pattern writes either the value or null for the property before Patch.WriteTo(writer) runs, so when the patch also contains that same property path, the patched value is emitted again, resulting in duplicate JSON properties.

The property serialization fallback should only execute when the patch does not already contain the property path. In practice, the null/value branch for a required nullable property should be gated on !Patch.Contains("$.param") so patched values are the single source of output for that property.

Reproduction

Relevant generated code pattern:

if (Optional.IsDefined(Foo) && !Patch.Contains("$.foo"u8))
{
    writer.WritePropertyName("foo"u8);
    writer.WriteStringValue(Foo);
}
else
{
    writer.WriteNull("foo"u8);
}

This else branch still writes "foo": null even when Patch.Contains("$.foo") is true. Later, Patch.WriteTo(writer) emits the patched foo value, producing duplicate foo properties.

Expected behavior:

• When Patch.Contains("$.foo") is true, neither the value branch nor the null fallback should write foo directly.
• The generated serialization should instead let Patch.WriteTo(writer) emit the property exactly once.

Likely generator location:

• packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs

Related generated/test files showing merge-patch patterns:

• packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/DynamicModel.Serialization.cs
• packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/WriteMultiplePrimitiveProperties.cs

A minimal repro is a model with a required nullable property such as foo: string | null, where the property is included in Patch and serialization is generated using the current C# client model generator.

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[github-actions]

🎯 Issue Triage

The C# client emitter generates incorrect serialization code for required nullable properties (foo: string | null) in JsonPatch models: the else null-fallback branch writes the property unconditionally, even when Patch.Contains("$.foo") is true, causing Patch.WriteTo(writer) to emit the same property a second time.

Category: Bug

Area: emitter:client:csharp

⚠️ No TypeSpec playground reproduction provided — the repro is expressed as a generated C# code pattern rather than a TypeSpec snippet.

🔍 Analysis

Root cause: In MrwSerializationTypeDefinition.cs, the generated pattern for a required nullable property wraps the value-write in if (Optional.IsDefined(Foo) && !Patch.Contains("$.foo"u8)) but leaves the else branch (writer.WriteNull("foo"u8)) unguarded. When the patch contains the property, the else branch still fires and writes "foo": null, then Patch.WriteTo(writer) emits the patched value a second time — producing invalid JSON with duplicate keys.

Expected fix: The else branch should also be wrapped with !Patch.Contains("$.foo"u8), so that neither the value branch nor the null fallback writes the property when the patch already owns it.

Affected component: packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/src/Providers/MrwSerializationTypeDefinition.cs

Related test files:

• packages/http-client-csharp/generator/TestProjects/Local/Sample-TypeSpec/src/Generated/Models/DynamicModel.Serialization.cs
• packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/WriteMultiplePrimitiveProperties.cs

📝 Suggested next steps

• Update MrwSerializationTypeDefinition.cs to gate the null-fallback else branch on !Patch.Contains(...) (same condition as the value-write branch).
• Add or update a test in WriteMultiplePrimitiveProperties.cs (or a new test) that covers a required nullable property in a patch model and asserts the property is emitted exactly once.
• Regenerate affected snapshots/golden files.

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