dotnet · StephenMolloy · Jul 27, 2026 · Jul 9, 2026
diff --git a/...aries/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationReader.cs b/...aries/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationReader.cs
@@ -812,7 +812,7 @@ private void WriteMemberElementsIf(Member[] expectedMembers, Member? anyElementM
                {
                    string? ns = e!.Form == XmlSchemaForm.Qualified ? e.Namespace : string.Empty;
                    bool isList = member!.Mapping.TypeDesc!.IsArrayLike && !member.Mapping.TypeDesc.IsArray;
-                    WriteElement(e, isList && member.Mapping.TypeDesc.IsNullable, member.Mapping.ReadOnly, ns, member.FixupIndex, fixup, member);
+                    WriteElement(e, isList && member.Mapping.TypeDesc.IsNullable, member.Mapping.ReadOnly, ns, member.FixupIndex, fixup, member, elementIndex);
                }
            }
            else
@@ -828,7 +828,7 @@ private void WriteMemberElementsIf(Member[] expectedMembers, Member? anyElementM
                        if (element.Any && element.Name.Length == 0)
                        {
                            string? ns = element.Form == XmlSchemaForm.Qualified ? element.Namespace : string.Empty;
-                            WriteElement(element, false, false, ns, fixup: fixup, member: member);
+                            WriteElement(element, false, false, ns, fixup: fixup, member: member, elementIndex: i);
                            break;
                        }
                    }
@@ -841,7 +841,7 @@ private void WriteMemberElementsIf(Member[] expectedMembers, Member? anyElementM
            }
        }

-        private object? WriteElement(ElementAccessor element, bool checkForNull, bool readOnly, string? defaultNamespace, int fixupIndex = -1, Fixup? fixup = null, Member? member = null)
+        private object? WriteElement(ElementAccessor element, bool checkForNull, bool readOnly, string? defaultNamespace, int fixupIndex = -1, Fixup? fixup = null, Member? member = null, int elementIndex = -1)
        {
            object? value = null;
            if (element.Mapping is ArrayMapping arrayMapping)
@@ -1013,7 +1013,7 @@ private void WriteMemberElementsIf(Member[] expectedMembers, Member? anyElementM
                throw new InvalidOperationException(SR.XmlInternalError);
            }

-            member?.ChoiceSource?.Invoke(element.Name);
+            member?.ChoiceSource?.Invoke(elementIndex);

            if (member?.ArraySource != null)
            {
@@ -1744,19 +1744,20 @@ private static XmlSerializationCollectionFixupCallback GetCreateCollectionOfObje
                    ChoiceIdentifierAccessor? choice = mapping.ChoiceIdentifier;
                    if (choice != null && o != null)
                    {
-                        member.ChoiceSource = (elementNameObject) =>
+                        member.ChoiceSource = (elementIndex) =>
                        {
-                            string? elementName = elementNameObject as string;
-                            foreach (var name in choice.MemberIds!)
+                            // The choice enum member that corresponds to the selected element is stored
+                            // positionally in MemberIds, parallel to the member's Elements. This mirrors
+                            // how the IL-generated reader resolves the choice (choice.MemberIds[elementIndex]),
+                            // and honors [XmlEnum] aliases because the importer already resolved them.
+                            string[]? memberIds = choice.MemberIds;
+                            if (memberIds is null || (uint)elementIndex >= (uint)memberIds.Length)
                            {
-                                if (name == elementName)
-                                {
-                                    object choiceValue = Enum.Parse(choice.Mapping!.TypeDesc!.Type!, name);
-                                    SetOrAddValueToMember(o, choiceValue, choice.MemberInfo!);
-
-                                    break;
-                                }
+                                return;
                            }
+
+                            object choiceValue = Enum.Parse(choice.Mapping!.TypeDesc!.Type!, memberIds[elementIndex]);
+                            SetOrAddValueToMember(o, choiceValue, choice.MemberInfo!);
                        };
                    }

@@ -2107,7 +2108,7 @@ internal sealed class Member
            public Func<object?>? GetSource;
            public Action<object>? ArraySource;
            public Action<object?>? CheckSpecifiedSource;
-            public Action<object>? ChoiceSource;
+            public Action<int>? ChoiceSource;
            public Action<string, string>? XmlnsSource;
            public Action<object>? EnsureCollection;


diff --git a/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.RuntimeOnly.cs b/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.RuntimeOnly.cs
@@ -3152,6 +3152,22 @@ public static void Xml_TypeWithArrayPropertyHavingChoiceErrors()
        Assert.Contains("is missing enumeration value", ex.Message);
    }

+    [Fact]
+    public static void Xml_TypeWithAliasedChoiceIdentifier()
+    {
+        var value = new TypeWithAliasedChoiceIdentifier()
+        {
+            Item = 42,
+            ChoiceType = AliasedChoiceType.NumberChoice
+        };
+
+        var actual = SerializeAndDeserialize(value, WithXmlHeader("<TypeWithAliasedChoiceIdentifier xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\"><Number>42</Number></TypeWithAliasedChoiceIdentifier>"));
+
+        Assert.NotNull(actual);
+        Assert.Equal(value.Item, actual.Item);
+        Assert.Equal(value.ChoiceType, actual.ChoiceType);
+    }
+
    [Fact]
    public static void Xml_XmlIncludedTypesInTypedCollection()
    {

diff --git a/src/libraries/System.Runtime.Serialization.Xml/tests/SerializationTypes.RuntimeOnly.cs b/src/libraries/System.Runtime.Serialization.Xml/tests/SerializationTypes.RuntimeOnly.cs
@@ -2266,6 +2266,25 @@ public class TypeWithPropertyHavingChoiceError
        public MoreChoices[] ChoiceArray;
    }

+    public enum AliasedChoiceType
+    {
+        [XmlEnum("Word")]
+        WordChoice,
+        [XmlEnum("Number")]
+        NumberChoice,
+    }
+
+    public class TypeWithAliasedChoiceIdentifier
+    {
+        [XmlChoiceIdentifier("ChoiceType")]
+        [XmlElement("Word", typeof(string))]
+        [XmlElement("Number", typeof(int))]
+        public object Item;
+
+        [XmlIgnore]
+        public AliasedChoiceType ChoiceType;
+    }
+
    internal class MyFileStreamSurrogateProvider : ISerializationSurrogateProvider
    {
        static MyFileStreamSurrogateProvider()