Fix reflection-based XmlSerializer to support XmlSchemaObject-derived types

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

@@ -1576,9 +1576,7 @@ private static XmlSerializationCollectionFixupCallback GetCreateCollectionOfObje
             {
                 if (structMapping.TypeDesc.Type != null && typeof(XmlSchemaObject).IsAssignableFrom(structMapping.TypeDesc.Type))
                 {
-                    // https://github.com/dotnet/runtime/issues/1399:
-                    // To Support Serializing XmlSchemaObject
-                    throw new NotImplementedException(nameof(XmlSchemaObject));
+                    DecodeName = false;
                 }
 
                 object? o = ReflectionCreateObject(structMapping.TypeDesc.Type!)!;
@@ -1812,10 +1810,24 @@ private static XmlSerializationCollectionFixupCallback GetCreateCollectionOfObje
                     {
                         MemberInfo[] memberInfos = o!.GetType().GetMember(member.Mapping.Name);
                         MemberInfo memberInfo = memberInfos[0];
-                        object? collection = null;
-                        SetCollectionObjectWithCollectionMember(ref collection, member.Collection, member.Mapping.TypeDesc!.Type!);
-                        var setMemberValue = GetSetMemberValueDelegate(o, memberInfo.Name);
-                        setMemberValue(o, collection);
+
+                        // For read-only collection properties (no setter), add items directly to the existing
+                        // collection instance returned by the getter. This matches what the IL-based serializer does.
+                        if (memberInfo is PropertyInfo pi && pi.GetSetMethod(nonPublic: true) == null)
+                        {
+                            object? existingCollection = pi.GetValue(o);
+                            if (existingCollection != null)
+                            {
+                                AddObjectsIntoTargetCollection(existingCollection, member.Collection, member.Mapping.TypeDesc!.Type!);
+                            }
+                        }
+                        else
+                        {
+                            object? collection = null;
+                            SetCollectionObjectWithCollectionMember(ref collection, member.Collection, member.Mapping.TypeDesc!.Type!);
+                            var setMemberValue = GetSetMemberValueDelegate(o, memberInfo.Name);
+                            setMemberValue(o, collection);
+                        }
                     }
 
                     member.EnsureCollection?.Invoke(o!);

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

@@ -76,39 +76,4 @@ public static void XML_TypeWithFieldsOrdered()
         Assert.Equal(value.StringField1, actual.StringField1);
         Assert.Equal(value.StringField2, actual.StringField2);
     }
-
-    // TODO - Move this test to XmlSerializerTests.RuntimeOnly.cs once #1399 is fixed for the ReflectionOnly serializer.
-    [Fact]
-    public static void Xml_XmlSchema()
-    {
-        var expectedXml = WithXmlHeader("<xsd:schema xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" elementFormDefault=\"qualified\" targetNamespace=\"http://example.com/my-schema\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">\r\n  <xsd:element name=\"MyElement\" type=\"xsd:string\" />\r\n  <xsd:group name=\"MyGroup\">\r\n    <xsd:sequence>\r\n      <xsd:element name=\"Item1\" />\r\n      <xsd:element name=\"Item2\" />\r\n    </xsd:sequence>\r\n  </xsd:group>\r\n</xsd:schema>");
-
-        XmlSchema schema = new XmlSchema
-        {
-            TargetNamespace = "http://example.com/my-schema",
-            ElementFormDefault = XmlSchemaForm.Qualified
-        };
-        schema.Items.Add(new XmlSchemaElement
-        {
-            Name = "MyElement",
-            SchemaTypeName = new XmlQualifiedName("string", "http://www.w3.org/2001/XMLSchema")
-        });
-        schema.Items.Add(new XmlSchemaGroup
-        {
-            Name = "MyGroup",
-            Particle = new XmlSchemaSequence
-            {
-                Items = { new XmlSchemaElement { Name = "Item1" }, new XmlSchemaElement { Name = "Item2" } }
-            }
-        });
-        schema.Namespaces.Add("xsd", "http://www.w3.org/2001/XMLSchema");
-        schema.Namespaces.Add("xsi", "http://www.w3.org/2001/XMLSchema-instance");
-
-        var actual = SerializeAndDeserialize(schema, expectedXml, () => new XmlSerializer(typeof(XmlSchema)));
-
-        Assert.Equal(schema.TargetNamespace, actual.TargetNamespace);
-        Assert.Equal(schema.ElementFormDefault, actual.ElementFormDefault);
-        Assert.Equal(schema.Items.Count, actual.Items.Count);
-    }
-
 }

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

@@ -861,6 +861,81 @@ public static void XML_XmlSchemaWithNamespacesWriteWithNamespaceManager()
             schema.Write(memStream, new XmlNamespaceManager(new NameTable()));
     }
 
+    [Fact]
+    public static void Xml_XmlSchema()
+    {
+        var expectedXml = WithXmlHeader("<xsd:schema xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" elementFormDefault=\"qualified\" targetNamespace=\"http://example.com/my-schema\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">\r\n  <xsd:element name=\"MyElement\" type=\"xsd:string\" />\r\n  <xsd:group name=\"MyGroup\">\r\n    <xsd:sequence>\r\n      <xsd:element name=\"Item1\" />\r\n      <xsd:element name=\"Item2\" />\r\n    </xsd:sequence>\r\n  </xsd:group>\r\n</xsd:schema>");
+
+        XmlSchema schema = new XmlSchema
+        {
+            TargetNamespace = "http://example.com/my-schema",
+            ElementFormDefault = XmlSchemaForm.Qualified
+        };
+        schema.Items.Add(new XmlSchemaElement
+        {
+            Name = "MyElement",
+            SchemaTypeName = new XmlQualifiedName("string", "http://www.w3.org/2001/XMLSchema")
+        });
+        schema.Items.Add(new XmlSchemaGroup
+        {
+            Name = "MyGroup",
+            Particle = new XmlSchemaSequence
+            {
+                Items = { new XmlSchemaElement { Name = "Item1" }, new XmlSchemaElement { Name = "Item2" } }
+            }
+        });
+        schema.Namespaces.Add("xsd", "http://www.w3.org/2001/XMLSchema");
+        schema.Namespaces.Add("xsi", "http://www.w3.org/2001/XMLSchema-instance");
+
+        var actual = SerializeAndDeserialize(schema, expectedXml, () => new XmlSerializer(typeof(XmlSchema)));
+
+        Assert.Equal(schema.TargetNamespace, actual.TargetNamespace);
+        Assert.Equal(schema.ElementFormDefault, actual.ElementFormDefault);
+        Assert.Equal(schema.Items.Count, actual.Items.Count);
+    }
+
+    [Fact]
+    public static void XmlSchemaObject_RoundTrip()
+    {
+        // Verify that XmlSchemaObject-derived types can be deserialized with the reflection-based
+        // serializer. Previously a NotImplementedException was thrown during deserialization.
+        var element = new XmlSchemaElement
+        {
+            Name = "TestElement",
+            SchemaTypeName = new XmlQualifiedName("string", "http://www.w3.org/2001/XMLSchema")
+        };
+        var serializer = new XmlSerializer(typeof(XmlSchemaElement));
+        using var ms = new MemoryStream();
+        serializer.Serialize(ms, element);
+        ms.Position = 0;
+
+        var result = (XmlSchemaElement?)serializer.Deserialize(ms);
+        Assert.NotNull(result);
+        Assert.Equal("TestElement", result.Name);
+        Assert.Equal(new XmlQualifiedName("string", "http://www.w3.org/2001/XMLSchema"), result.SchemaTypeName);
+    }
+
+    [Fact]
+    public static void XmlSchemaObject_RoundTrip_WithConstraintInReadOnlyCollection()
+    {
+        // Verify that items in read-only collection properties (like XmlSchemaElement.Constraints)
+        // are correctly round-tripped using the reflection-based serializer.
+        var element = new XmlSchemaElement { Name = "TestElement" };
+        element.Constraints.Add(new XmlSchemaKey { Name = "PrimaryKey" });
+
+        var serializer = new XmlSerializer(typeof(XmlSchemaElement));
+        using var ms = new MemoryStream();
+        serializer.Serialize(ms, element);
+        ms.Position = 0;
+
+        var result = (XmlSchemaElement?)serializer.Deserialize(ms);
+        Assert.NotNull(result);
+        Assert.Equal("TestElement", result.Name);
+        Assert.Equal(1, result.Constraints.Count);
+        var key = Assert.IsType<XmlSchemaKey>(result.Constraints[0]);
+        Assert.Equal("PrimaryKey", key.Name);
+    }
+
     [Fact]
     public static void Xml_XmlElementAsRoot()
     {