Fix XmlSerializer consuming sibling elements when deserializing empty XmlElement member

[Copilot]

Description

When deserializing a class with an XmlElement member followed by other members, a self-closing/empty element (e.g., <Description/>) caused the serializer to consume the next sibling element as the XmlElement's content, leaving subsequent members null.

public class Root {
    public XmlElement Description { get; set; }
    public string Name { get; set; }
}

// Bug: obj.Description = <Name>Test</Name>, obj.Name = null
var obj = serializer.Deserialize(new StringReader(
    @"<Root><Description/><Name>Test</Name></Root>"));

// Expected: obj.Description = null, obj.Name = "Test"

Root Cause

ReadXmlNode(bool wrapped) called _r.ReadStartElement() without first checking IsEmptyElement. For an empty element like <Description/>, ReadStartElement() advances the reader past the empty element to the next sibling. The subsequent MoveToContent() + NodeType != EndElement check then incorrectly read that sibling as child content.

Fix

• XmlSerializationReader.ReadXmlNode: Add IsEmptyElement guard before ReadStartElement() — matching the pattern already used in ReadNull(). If empty, Skip() and return null.

if (_r.IsEmptyElement)
{
    _r.Skip();
    return null;
}
_r.ReadStartElement();

• Test: Added TypeWithXmlElementMemberAndSibling to SerializationTypes.cs and a [Theory] regression test (Xml_XmlElementMember_EmptyElement_SiblingNotConsumed) covering non-empty, space-before-slash, and compact empty element forms.

[Copilot]

Pull request overview

Fixes an XmlSerializer deserialization bug where a self-closing/empty element mapped to an XmlElement/XmlNode member could incorrectly consume the next sibling element as its content, causing subsequent members to deserialize as null.

Changes:

• Update XmlSerializationReader.ReadXmlNode(bool wrapped) to treat wrapped empty elements as having no content (skip and return null) rather than advancing into the next sibling.
• Add a regression test in XmlSerializerTests.RuntimeOnly.cs covering non-empty and empty element forms.
• Add a new test type TypeWithXmlElementMemberAndSibling.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/System.Runtime.Serialization.Xml/tests/SerializationTypes.cs	Adds TypeWithXmlElementMemberAndSibling test type (but see comment about compilation inclusion).
src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.RuntimeOnly.cs	Adds regression coverage for empty XmlElement member not consuming a sibling.
src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializationReader.cs	Fixes wrapped empty-element handling in ReadXmlNode to avoid consuming siblings.

[Copilot]

This test only asserts obj.Name, which checks that the sibling wasn’t consumed, but it doesn’t validate the intended XmlElement behavior: for the empty element cases obj.Description should be null, and for the non-empty case it should be non-null (and ideally have expected name/content). Adding those assertions would make the regression coverage tighter.

Suggested change

	[InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description><p>text</p></Description><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", "Test")]
	[InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description /><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", "Test")]
	[InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description/><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", "Test")]
	public static void Xml_XmlElementMember_EmptyElement_SiblingNotConsumed(string xml, string expectedName)
	{
	var serializer = new XmlSerializer(typeof(TypeWithXmlElementMemberAndSibling));
	TypeWithXmlElementMemberAndSibling obj = (TypeWithXmlElementMemberAndSibling)serializer.Deserialize(new StringReader(xml));
	Assert.Equal(expectedName, obj.Name);
	[InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description><p>text</p></Description><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", "Test", true, "Description", "<p>text</p>")]
	[InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description /><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", "Test", false, null, null)]
	[InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description/><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", "Test", false, null, null)]
	public static void Xml_XmlElementMember_EmptyElement_SiblingNotConsumed(string xml, string expectedName, bool expectDescription, string expectedDescriptionName, string expectedDescriptionInnerXml)
	{
	var serializer = new XmlSerializer(typeof(TypeWithXmlElementMemberAndSibling));
	TypeWithXmlElementMemberAndSibling obj = (TypeWithXmlElementMemberAndSibling)serializer.Deserialize(new StringReader(xml));
	Assert.Equal(expectedName, obj.Name);
	if (expectDescription)
	{
	Assert.NotNull(obj.Description);
	Assert.Equal(expectedDescriptionName, obj.Description.Name);
	Assert.Equal(expectedDescriptionInnerXml, obj.Description.InnerXml);
	}
	else
	{
	Assert.Null(obj.Description);
	}

[Copilot]

TypeWithXmlElementMemberAndSibling is defined in System.Runtime.Serialization.Xml/tests/SerializationTypes.cs, but System.Private.Xml.Tests.csproj includes that file as <None> (not <Compile>), so this test likely won’t compile in this project. Consider moving the new type into SerializationTypes.RuntimeOnly.cs (which is compiled here) or changing the csproj to compile SerializationTypes.cs.