[http-client-csharp] Initialize omitted required collections during deserialization

[Copilot]

Deserialization accepts JSON that omits required, non-nullable collections but leaves them null. Accessing those collections or immediately serializing the returned model can then throw NullReferenceException.

• Initialization: Extend empty-collection fallbacks to required non-nullable lists and dictionaries, including read-only collections.

  requiredCollection ?? new ChangeTrackingList<StringFixedEnum>()

• Compatibility: Use assignable fallbacks for concrete List<T> customizations. Preserve optional and required-nullable handling, populated/empty collections, and explicit-null rejection.
• Regression coverage: Add collection access/mutation, immediate J/W serialization without prior getter access, and concrete-list customization cases. Update affected generated output and baselines.

• Fixes [http-client-csharp] JSON round-trip throws NullReferenceException when a required collection is omitted #12080

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npm i https://pkg.pr.new/@typespec/http-client-csharp@12090

commit: 532e7bf

[Copilot]

Copilot review overview

🔵 Needs a closer look

The required Cop static-analysis validation remains incomplete.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes C# model deserialization so omitted required, non-nullable collections initialize empty and remain serializable.

Changes:

• Adds collection fallbacks when invoking serialization constructors.
• Regenerates affected JSON/XML model output.
• Adds regression coverage for mutation, round-tripping, and explicit null rejection.

File	Description
MrwSerializationTypeDefinition.cs	Adds required non-nullable collection fallback.
RoundTripModelTests.cs	Adds runtime regression tests.
DeserializeArrayProperties.cs	Updates array baseline.
DeserializeDictionaryProperties.cs	Updates dictionary baseline.
XmlDeserializationMethodHandlesUnwrappedListProperty.cs	Updates unwrapped XML baseline.
XmlDeserializationMethodHandlesWrappedListProperty.cs	Updates wrapped XML baseline.
DynamicModel.Serialization.cs	Regenerates collection fallbacks.
ListWithContinuationTokenHeaderResponseResult.Serialization.cs	Regenerates paging collection fallback.
ListWithContinuationTokenResult.Serialization.cs	Regenerates paging collection fallback.
ListWithNextLinkResult.Serialization.cs	Regenerates paging collection fallback.
ListWithStringNextLinkResult.Serialization.cs	Regenerates paging collection fallback.
PageThing.Serialization.cs	Regenerates page-item fallback.
RoundTripModel.Serialization.cs	Regenerates required collection fallbacks.
XmlAdvancedModel.Serialization.cs	Regenerates XML collection fallbacks.

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[Copilot]

Copilot review overview

🟡 Changes recommended

The fallback condition incorrectly converts omitted required-nullable customized collections into empty collections.

Review effort: Balanced
Findings: 1

Open (1)

• Use wire nullability for customized collection omission handling · New

[Copilot]

Copilot review overview

🟡 Changes recommended

The requested XML, multipart, and dynamic optional concrete-list regression tests remain absent.

Review effort: Balanced
Findings: 1

Open (1)

• Add coverage for concrete-list optional properties across output formats

[Copilot]

Copilot review overview

🟢 Approval recommended

The corrected behavior is consistently applied and covered across the affected serialization paths.

Review effort: Balanced
Findings: None

Resolved since last review (1)

• Add coverage for concrete-list optional properties across output formats

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation preserves nullability semantics and includes focused generated-output and runtime regression coverage.

Review effort: Balanced
Findings: None

[Jorge Rangel (jorgerangel-msft)]

Copilot this is a breaking change. Revert the modifier to public and just add a new internal overload

[Copilot]

Restored the public one-argument overload and kept the type-aware overload internal in 8d9e49a6. The targeted C# tests, generator build, and Cop checks pass.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation preserves existing nullable semantics and includes focused baseline and runtime regression coverage.

Review effort: Balanced
Findings: None

[Jorge Rangel (jorgerangel-msft)]

Copilot if this is only used in this type, lets make it private. If it's needed elsewhere in the assembly, make this internal

[Copilot]

IsConcreteCollection is also used by MrwSerializationTypeDefinition, so I changed it to internal rather than private in 532e7bf4. The generator build, 409 targeted tests, and Cop checks pass.

[Copilot]

Copilot review overview

🟢 Approval recommended

The fallback logic is type-compatible and covered across omitted, nullable, explicit-null, serialization, and customization paths.

Review effort: Balanced
Findings: None