Fix XmlSerializer consuming sibling elements when deserializing empty XmlElement member

src/libraries/System.Private.Xml/src/System/Xml/Core/LocalAppContextSwitches.cs

@@ -89,5 +89,15 @@ public static bool IgnoreObsoleteMembers
                 return SwitchesHelpers.GetCachedSwitchValue("Switch.System.Xml.IgnoreObsoleteMembers", ref s_ignoreObsoleteMembers);
             }
         }
+
+        private static int s_useLegacyEmptyXmlElementDeserialization;
+        public static bool UseLegacyEmptyXmlElementDeserialization
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                return SwitchesHelpers.GetCachedSwitchValue("Switch.System.Xml.UseLegacyEmptyXmlElementDeserialization", ref s_useLegacyEmptyXmlElementDeserialization);
+            }
+        }
     }
 }

src/libraries/System.Private.Xml/src/System/Xml/Serialization/XmlSerializationReader.cs

@@ -905,6 +905,11 @@ protected XmlQualifiedName ReadElementQualifiedName()
             if (wrapped)
             {
                 if (ReadNull()) return null;
+                if (!LocalAppContextSwitches.UseLegacyEmptyXmlElementDeserialization && _r.IsEmptyElement)
+                {
+                    _r.Skip();
+                    return null;
+                }
                 _r.ReadStartElement();
                 _r.MoveToContent();
                 if (_r.NodeType != XmlNodeType.EndElement)

src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.RuntimeOnly.cs

@@ -3666,4 +3666,32 @@ public static void SerializePrimitiveXmlTextAttributeOnDerivedClass()
         var actual = SerializeAndDeserialize(value, "<?xml version=\"1.0\"?><PrimiveAttributeTestDerived xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">5</PrimiveAttributeTestDerived>");
         Assert.Equal(value.Number, actual.Number);
     }
+
+    [Theory]
+    [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, "p", "text")]
+    [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, false)]
+    [InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description></Description><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", "Test", false, null, null, false)]
+    [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, "p", "text", true)]
+    [InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description /><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", null, true, "Name", "Test", true)]
+    [InlineData(@"<TypeWithXmlElementMemberAndSibling xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema""><Description></Description><Name>Test</Name></TypeWithXmlElementMemberAndSibling>", "Test", false, null, null, true)]
+    public static void Xml_XmlElementMember_EmptyElement_SiblingNotConsumed(string xml, string? expectedName, bool expectDescription, string? expectedDescriptionName, string? expectedDescriptionInnerXml, bool? compatSwitch = null)
+    {
+        using (var appContextScope = compatSwitch.HasValue ? new XmlSerializerAppContextSwitchScope("Switch.System.Xml.UseLegacyEmptyXmlElementDeserialization", compatSwitch.Value) : null)
+        {
+            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);
+            }
+        }
+    }
 }

src/libraries/System.Runtime.Serialization.Xml/tests/SerializationTypes.RuntimeOnly.cs

@@ -836,6 +836,12 @@ public WithListOfXElement(bool init)
         }
     }
 
+    public class TypeWithXmlElementMemberAndSibling
+    {
+        public XmlElement Description { get; set; }
+        public string Name { get; set; }
+    }
+
     public class BaseType
     {
         public virtual string Name1 { get; set; }