Selectively Overriding XML Serialisation Attributes

Introduction

As I mentioned in my last post, although you can override XML serialisation attributes by passing an XmlAttributeOverrides instance to an XmlSerializer, the attributes you provide for a given type and member replace all the existing XML serialisation attributes - you can't simply tweak one or two and leave the rest intact.

If you're thinking that type and members only tend to have one or two serialistion attributes, then take a look at this set attributes from an auto-generated EquityDerivativeBase class:

[System.Xml.Serialization.XmlIncludeAttribute(typeof(EquityDerivativeShortFormBase))]
[System.Xml.Serialization.XmlIncludeAttribute(typeof(EquityOptionTransactionSupplement))]
[System.Xml.Serialization.XmlIncludeAttribute(typeof(BrokerEquityOption))]
[System.Xml.Serialization.XmlIncludeAttribute(typeof(EquityDerivativeLongFormBase))]
[System.Xml.Serialization.XmlIncludeAttribute(typeof(EquityOption))]
[System.Xml.Serialization.XmlIncludeAttribute(typeof(EquityForward))]
[System.CodeDom.Compiler.GeneratedCodeAttribute("xsd", "4.0.30319.1")]
[System.SerializableAttribute()]
[System.Diagnostics.DebuggerStepThroughAttribute()]
[System.ComponentModel.DesignerCategoryAttribute("code")]
[System.Xml.Serialization.XmlTypeAttribute(Namespace="http://www.fpml.org/FpML-5/confirmation")]

Let's suppose I want to use XmlAttributeOverrides to alter the value of the XmlTypeAttribute at run-rime, to place the element in a different namespace. Well I can. But the XmlAttributeOverrides instance I supply is used to replace all the existing attributes. So I lose each of the XmlIncludeAttribute attributes which define the classes which use this class as a base class.

Book and Genre classes (with Xml Attributes)

To demonstrate how to override the attributes selectively I'm going to use the same Book class as in my last post to demonstrate selectively overriding these attributes. I've added a lot more attributes to the members of the Book class to demonstrate that they all get retained.

[XmlType(TypeName="Book")]
[XmlRoot("book", Namespace="http://tempuri.org")]
public class Book
{
  [XmlIgnore]
  public int InternalId { get; set; }

  [XmlElement("title")]
  public string Title { get; set; }

  [DefaultValue("Anonymous")]
  [XmlArray("authors")]
  [XmlArrayItem("author")]
  public string[] Authors { get; set; }

  [XmlElement("isbn13")]
  public string Isbn13 { get; set; }
 
  [XmlText]
  public string Extract { get; set; }

  [XmlAttribute("genre")]
  public Genre Genre { get; set; }

  [XmlNamespaceDeclarations]
  public XmlSerializerNamespaces XmlNamespaces { get; set; }

  [XmlAnyAttribute]
  public XmlAttribute[] OtherAttributes { get; set; }

  [XmlAnyElement]
  public XmlElement[] OtherElements { get; set; }

  public Book()
  {
    XmlNamespaces = new XmlSerializerNamespaces();
    XmlNamespaces.Add("ns", "http://tempuri.org");
  }
}

public enum Genre
{
  [XmlEnum("unknown")]
  Unknown,
  [XmlEnum("autobiography")]
  Autobiography,
  [XmlEnum("computing-text")]
  ComputingText,
  [XmlEnum("classic")]
  Classic
}

Solution

The solution is to copy all the existing attributes into an XmlAttributeOverrides instance (modifying them as they're copied), and then apply the XmlAttributeOverrides to the XmlSerializer. That way the XmlAttributeOverrides object retains all of the original attributes (with the exception of any changes made in transit). Let me show you what I mean:

Continue reading this post here »

Default Values Don't Get Serialised

The Problem

I came across some unusual behaviour during XML Serialisation recently and thought I'd share both the problem and my solution to it.

To demonstrate the behaviour I've created a small console application which serialises two Book objects to the Console. I've marked the default value of the Author property as "Anonymous", which seems reasonable enough. The second of the two Book objects specifically has its Author property set to "Anonymous", which it doesn't really need to have - as that's the default value.

Here's the code:

using System;
using System.ComponentModel;
using System.IO;
using System.Xml.Serialization;

namespace ConsoleApplication
{
  public class Program
  {
    static void Main(string[] args)
    {
      // create some Books
      Book[] books = new Book[]
      {
        new Book { Title = "The Road Ahead", Author = "Bill Gates", Isbn13 = "978-0670859139" },
        new Book { Title = "Beowulf", Author = "Anonymous", Isbn13 = "978-1588278296" },
      };

      // serialise it into a MemoryStream
      XmlSerializer xmlSerializer = new XmlSerializer(typeof(Book[]));
      MemoryStream memoryStream = new MemoryStream();
      xmlSerializer.Serialize(memoryStream, books);
    
      // write the contents of the MemoryStream to the Console
      memoryStream.Position = 0L;
      StreamReader streamReader = new StreamReader(memoryStream);
      Console.WriteLine(streamReader.ReadToEnd());

      // wait for user to hit ENTER
      Console.ReadLine();
    }
  }

  public class Book
  {
    [XmlElement("title")]
    public string Title { get; set; }

    [XmlElement("author")]
    [DefaultValue("Anonymous")]
    public string Author { get; set; }
    
    [XmlElement("isbn13")]
    public string Isbn13 { get; set; }
  }
}

When I run the above, something odd happens. The Author property is not serialised for the second book, as you'll see below. It turns out that this is by design. The theory is that if the default value has been used, then there's no need to explicitly output it - a consumer of the data will simply see that the element is missing and infer the default value.

<?xml version="1.0"?>
<ArrayOfBook xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Book>
    <title>The Road Ahead</title>
    <author>Bill Gates</author>
    <isbn13>978-0670859139</isbn13>
  </Book>
  <Book>
    <title>Beowulf</title>
    <isbn13>978-1588278296</isbn13>
  </Book>
</ArrayOfBook>

The problem, obviously, is that this approach assumes that consumer is aware that a default value exists. Can you see any indication from the serialised XML that a default value exists, or that an element has been suppressed because it's value was equal to the default value? No - that information is hidden from the consumer. We're simply left to hope that the consumer is aware of this default value.

Continue reading this post here »