Remove unreachable NotImplementedException array-with-choice

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

@@ -1139,13 +1139,10 @@ private static bool IsWildcard(SpecialMapping mapping)
                     Type collectionType = memberMapping.TypeDesc!.Type!;
                     o = ReflectionCreateObject(memberMapping.TypeDesc.Type!);
 
-                    if (memberMapping.ChoiceIdentifier != null)
-                    {
-                        // https://github.com/dotnet/runtime/issues/1400:
-                        // To Support ArrayMapping Types Having ChoiceIdentifier
-                        throw new NotImplementedException("memberMapping.ChoiceIdentifier != null");
-                    }
-
+                    // When this array is the value of a member that carries an [XmlChoiceIdentifier]
+                    // (e.g. one of the choice element types is itself an array), the parallel choice
+                    // value is recorded by the calling WriteElement against the owning member after
+                    // this method returns, so no additional choice handling is needed here.
                     var arrayMember = new Member(memberMapping);
                     arrayMember.Collection = new CollectionMember();
                     arrayMember.ArraySource = arrayMember.Collection.Add;

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

@@ -1927,6 +1927,33 @@ public static void Xml_TypeWithArrayPropertyHavingComplexChoice()
         Assert.True(Enumerable.SequenceEqual(value.ManyChoices, actual.ManyChoices));
     }
 
+    [Fact]
+    public static void Xml_TypeWithArrayLikeChoiceElement()
+    {
+        // Exercises an [XmlChoiceIdentifier] member where one of the choice element types is
+        // itself an array, so that element's mapping is an ArrayMapping. Deserializing such a
+        // value drives the reflection-based reader's array-reading path while the owning member
+        // carries a choice identifier.
+        var value = new TypeWithArrayLikeChoiceElement()
+        {
+            ManyChoices = new object[] { "hello", new int[] { 1, 2, 3 } },
+            ChoiceArray = new ArrayLikeChoice[] { ArrayLikeChoice.Word, ArrayLikeChoice.Numbers }
+        };
+
+        var actual = SerializeAndDeserialize(value, WithXmlHeader("<TypeWithArrayLikeChoiceElement xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">\r\n  <Word>hello</Word>\r\n  <Numbers>\r\n    <int>1</int>\r\n    <int>2</int>\r\n    <int>3</int>\r\n  </Numbers>\r\n</TypeWithArrayLikeChoiceElement>"));
+
+        Assert.NotNull(actual);
+        Assert.NotNull(actual.ManyChoices);
+        Assert.Equal(2, actual.ManyChoices.Length);
+        Assert.Equal("hello", actual.ManyChoices[0]);
+        int[] numbers = Assert.IsType<int[]>(actual.ManyChoices[1]);
+        Assert.Equal(new int[] { 1, 2, 3 }, numbers);
+
+        // The [XmlIgnore] choice array is populated during deserialization to mirror, per item,
+        // which element each value was read from.
+        Assert.Equal(new ArrayLikeChoice[] { ArrayLikeChoice.Word, ArrayLikeChoice.Numbers }, actual.ChoiceArray);
+    }
+
     [Fact]
     public static void XML_TypeWithTypeNameInXmlTypeAttribute_WithValue()
     {
@@ -3272,3 +3299,23 @@ private static string GuessCachedName(string name)
         throw new ArgumentException($"Cannot guess cached field name from switch name '{name}'");
     }
 }
+
+public class TypeWithArrayLikeChoiceElement
+{
+    // One of the choice element types (Numbers) is an array, so its element mapping is an
+    // ArrayMapping. Each item in ManyChoices is matched to an item in ChoiceArray.
+    [XmlChoiceIdentifier("ChoiceArray")]
+    [XmlElement("Word", typeof(string))]
+    [XmlElement("Numbers", typeof(int[]))]
+    public object[] ManyChoices;
+
+    [XmlIgnore]
+    public ArrayLikeChoice[] ChoiceArray;
+}
+
+public enum ArrayLikeChoice
+{
+    None,
+    Word,
+    Numbers
+}