From 0175e3f80af34d6b7db154c47a129986be7c5c6a Mon Sep 17 00:00:00 2001
From: Steve Molloy <smolloy@microsoft.com>
Date: Tue, 16 Jun 2026 15:14:53 -0700
Subject: [PATCH] Implement support for XmlAttribute in WriteAttribute method
 and add corresponding test

---
 .../ReflectionXmlSerializationReader.cs       |  9 +++++---
 .../XmlSerializerTests.Internal.cs            | 21 -------------------
 .../tests/XmlSerializer/XmlSerializerTests.cs | 20 ++++++++++++++++++
 3 files changed, 26 insertions(+), 24 deletions(-)

diff --git a/src/libraries/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationReader.cs b/src/libraries/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationReader.cs
index 0c08561ba20ced..c02671e03fd570 100644
--- a/src/libraries/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationReader.cs
+++ b/src/libraries/System.Private.Xml/src/System/Xml/Serialization/ReflectionXmlSerializationReader.cs
@@ -1981,9 +1981,12 @@ private void WriteAttribute(Member member, object? attr = null)
                 }
                 else if (special.TypeDesc.CanBeAttributeValue)
                 {
-                    // https://github.com/dotnet/runtime/issues/1398:
-                    // To Support special.TypeDesc.CanBeAttributeValue == true
-                    throw new NotImplementedException("special.TypeDesc.CanBeAttributeValue");
+                    if (attr is not XmlAttribute xmlAttribute)
+                    {
+                        return;
+                    }
+
+                    value = xmlAttribute;
                 }
                 else
                     throw new InvalidOperationException(SR.XmlInternalError);
diff --git a/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.Internal.cs b/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.Internal.cs
index 733b5ae83c1952..14eec7cd191383 100644
--- a/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.Internal.cs
+++ b/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.Internal.cs
@@ -11,27 +11,6 @@
 
 public static partial class XmlSerializerTests
 {
-    // Move this test to XmlSerializerTests.cs once #1398 is fixed for the ReflectionOnly serializer.
-    [Fact]
-    public static void Xml_CustomDocumentWithXmlAttributesAsNodes()
-    {
-        var customDoc = new CustomDocument();
-        var customElement = new CustomElement() { Name = "testElement" };
-        customElement.AddAttribute(customDoc.CreateAttribute("regularAttribute", "regularValue"));
-        customElement.AddAttribute(customDoc.CreateCustomAttribute("customAttribute", "customValue"));
-        customDoc.CustomItems.Add(customElement);
-        var element = customDoc.Document.CreateElement("regularElement");
-        var innerElement = customDoc.Document.CreateElement("innerElement");
-        innerElement.InnerXml = "<leafElement>innerText</leafElement>";
-        element.InnerText = "regularText";
-        element.AppendChild(innerElement);
-        element.Attributes.Append(customDoc.CreateAttribute("regularElementAttribute", "regularElementAttributeValue"));
-        customDoc.AddItem(element);
-        var actual = SerializeAndDeserialize(customDoc,
-            WithXmlHeader(@"<customElement name=""testElement"" regularAttribute=""regularValue"" customAttribute=""customValue""/>"), skipStringCompare: true);
-        Assert.NotNull(actual);
-    }
-
     // Move this test to XmlSerializerTests.cs once #1401 is fixed for the ReflectionOnly serializer.
     [Fact]
     public static void Xml_DerivedIXmlSerializable()
diff --git a/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs b/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs
index 1451e7407d8148..808120428c809a 100644
--- a/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs
+++ b/src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs
@@ -625,6 +625,26 @@ public static void Xml_XmlAnyAttributeTest()
         }
     }
 
+    [Fact]
+    public static void Xml_CustomDocumentWithXmlAttributesAsNodes()
+    {
+        var customDoc = new CustomDocument();
+        var customElement = new CustomElement() { Name = "testElement" };
+        customElement.AddAttribute(customDoc.CreateAttribute("regularAttribute", "regularValue"));
+        customElement.AddAttribute(customDoc.CreateCustomAttribute("customAttribute", "customValue"));
+        customDoc.CustomItems.Add(customElement);
+        var element = customDoc.Document.CreateElement("regularElement");
+        var innerElement = customDoc.Document.CreateElement("innerElement");
+        innerElement.InnerXml = "<leafElement>innerText</leafElement>";
+        element.InnerText = "regularText";
+        element.AppendChild(innerElement);
+        element.Attributes.Append(customDoc.CreateAttribute("regularElementAttribute", "regularElementAttributeValue"));
+        customDoc.AddItem(element);
+        var actual = SerializeAndDeserialize(customDoc,
+            WithXmlHeader(@"<customElement name=""testElement"" regularAttribute=""regularValue"" customAttribute=""customValue""/>"), skipStringCompare: true);
+        Assert.NotNull(actual);
+    }
+
     [Fact]
     public static void Xml_Struct()
     {