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When a service returns an empty XML wrapper element (e.g., <Blobs />), unwrapped list properties deserialize to null because the list is only initialized inside the foreach loop upon matching a child element. For required, non-nullable properties this should yield an empty list.

Changes

• MrwSerializationTypeDefinition.cs: In GetPropertyVariableDeclarations(), added IsXmlUnwrappedRequiredCollection helper that initializes unwrapped, required, non-nullable collection properties with new List<T>() at variable declaration time instead of default
• MrwSerializationTypeDefinition.Xml.cs: In CreateXmlDeserializeListAssignment(), added isPreInitialized parameter to skip the redundant if (list == null) null-check when the list is already initialized at declaration time
• XmlDeserializationTests.cs: Added XmlDeserializationMethodHandlesOptionalUnwrappedListProperty test using FilteredMethodsTypeProvider to verify optional properties retain the null-check and are not affected
• Test data / regenerated code: Updated expected output and regenerated XmlAdvancedModel.Serialization.cs

Generated output

// Variable initialized at declaration instead of default
IList<string> colors = new List<string>();
IDictionary<string, BinaryData> additionalBinaryDataProperties = new ChangeTrackingDictionary<string, BinaryData>();

foreach (var child in element.Elements())
{
    string localName = child.Name.LocalName;
    if (localName == "colors")
    {
        // No null check needed — colors is already initialized
        colors.Add((string)child);
        continue;
    }
}
return new TestXmlModel(colors, additionalBinaryDataProperties);

Original prompt

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This section details on the original issue you should resolve

<issue_title>Always Initialize Non-Optional Unwrapped Items in XML Deserialization</issue_title>
<issue_description>Consider this tsp:

@Xml.name("EnumerationResults")
model ListBlobsResponse {
  /** The service endpoint. */
  @Xml.attribute
  @Xml.name("ServiceEndpoint")
  serviceEndpoint: string;

  /** The container name. */
  @Xml.attribute
  @Xml.name("ContainerName")
  containerName: string;

  /** The prefix of the blobs. */
  @Xml.name("Prefix") prefix?: string;

  /** The marker of the blobs. */
  @Xml.name("Marker") marker?: string;

  /** The max results of the blobs. */
  @Xml.name("MaxResults") maxResults?: int32;

  /** The blob segment. */
  @Xml.name("Blobs")
  segment: BlobFlatListSegment;

  /** The next marker of the blobs. */
  @continuationToken
  @Xml.name("NextMarker")
  nextMarker?: string;
}

/** The blob flat list segment. */
model BlobFlatListSegment {
  /** The blob items. */
  @pageItems
  @Xml.name("Blob")
  @Xml.unwrapped
  blobItems: BlobItem[];
}

The service can returns <Blobs /> and the current XML deserialization code will deserialize segment into null since there are no Blob elements in Blobs. Since blobItems is unwrapped and non-nullable, we should be initializing it to an empty list by default.

We should update the xml deserialization to initialize collection properties to an empty collection when they are non-nullable, required, and unwrapped.</issue_description>

Comments on the Issue (you are Copilot in this section)

• Fixes Always Initialize Non-Optional Unwrapped Items in XML Deserialization #9967

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