Handle nullable primitives before passing them to WritePrimitive…

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

@@ -397,6 +397,17 @@ private void WriteElement(object? o, ElementAccessor element, bool writeAccessor
                 {
                     WriteQualifiedNameElement(name, ns!, element.Default, (XmlQualifiedName)o!, element.IsNullable, mapping.IsSoap, mapping);
                 }
+                else if (o == null && element.IsNullable)
+                {
+                    if (mapping.IsSoap)
+                    {
+                        WriteNullTagEncoded(element.Name, ns);
+                    }
+                    else
+                    {
+                        WriteNullTagLiteral(element.Name, ns);
+                    }
+                }
                 else
                 {
                     WritePrimitiveMethodRequirement suffixNullable = mapping.IsSoap ? WritePrimitiveMethodRequirement.Encoded : WritePrimitiveMethodRequirement.None;

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

@@ -42,6 +42,19 @@ public static void Xml_ByteArrayAsRoot()
         Assert.Equal(x, y);
     }
 
+    [Fact]
+    public static void Xml_ByteArrayNull()
+    {
+        Assert.Null(SerializeAndDeserialize<byte[]>(null,
+@"<?xml version=""1.0""?>
+<base64Binary d1p1:nil=""true"" xmlns:d1p1=""http://www.w3.org/2001/XMLSchema-instance"" />"));
+        byte[] x = new byte[] { 1, 2 };
+        byte[] y = SerializeAndDeserialize<byte[]>(x,
+@"<?xml version=""1.0""?>
+<base64Binary>AQI=</base64Binary>");
+        Assert.Equal(x, y);
+    }
+
     [Fact]
     public static void Xml_CharAsRoot()
     {
@@ -1279,6 +1292,19 @@ public static void XML_TypeWithByteArrayAsXmlAttribute()
         Assert.True(Enumerable.SequenceEqual(value.XmlAttributeForms, actual.XmlAttributeForms));
     }
 
+    [Fact]
+    public static void XML_TypeWithNullableByteArray()
+    {
+        var value = new TypeWithNullableByteArray(); // XmlAttributeForms == null
+
+        var actual = SerializeAndDeserialize(value,
+            "<?xml version=\"1.0\"?>\r\n<MyXmlType xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\">\r\n  <XmlAttributeForms xsi:nil=\"true\" />\r\n</MyXmlType>");
+
+        Assert.NotNull(actual);
+        Assert.Null(value.XmlAttributeForms);
+        Assert.Null(actual.XmlAttributeForms);
+    }
+
     [Fact]
     public static void XML_TypeWithByteArrayArrayAsXmlAttribute()
     {

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

@@ -1987,6 +1987,13 @@ public class TypeWithByteArrayAsXmlAttribute
         public byte[] XmlAttributeForms;
     }
 
+    [XmlType(TypeName = "MyXmlType")]
+    public class TypeWithNullableByteArray
+    {
+        [XmlElement(DataType = "base64Binary", IsNullable = true)]
+        public byte[] XmlAttributeForms { get; set; }
+    }
+
     [XmlType(TypeName = "MyXmlType")]
     public class TypeWithByteArrayArrayAsXmlAttribute
     {