XmlSerialize a custom collection with an Attribute

Question

I've got a simple class that inherits from Collection and adds a couple of properties. I need to serialize this class to XML, but the XMLSerializer ignores my additional properties.

I assume this is because of the special treatment that XMLSerializer gives ICollection and IEnumerable objects. What's the best way around this?

Here's some sample code:

using System.Collections.ObjectModel;
using System.IO;
using System.Xml.Serialization;

namespace SerialiseCollection
{
    class Program
    {
        static void Main(string[] args)
        {
            var c = new MyCollection();
            c.Add("Hello");
            c.Add("Goodbye");

            var serializer = new XmlSerializer(typeof(MyCollection));
            using (var writer = new StreamWriter("test.xml"))
                serializer.Serialize(writer, c);
        }
    }

    [XmlRoot("MyCollection")]
    public class MyCollection : Collection<string>
    {
        [XmlAttribute()]
        public string MyAttribute { get; set; }

        public MyCollection()
        {
            this.MyAttribute = "SerializeThis";
        }
    }
}

This outputs the following XML (note MyAttribute is missing in the MyCollection element):

<?xml version="1.0" encoding="utf-8"?>
<MyCollection xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;
    <string>Hello</string>
    <string>Goodbye</string>
</MyCollection>

What I want is

<MyCollection MyAttribute="SerializeThis" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;
    <string>Hello</string>
    <string>Goodbye</string>
</MyCollection>

Any ideas? The simpler the better. Thanks.

Answer 1

Collections generally don't make good places for extra properties. Both during serialization and in data-binding, they will be ignored if the item looks like a collection (IList, IEnumerable, etc - depending on the scenario).

If it was me, I would encapsulate the collection - i.e.

[Serializable]
public class MyCollectionWrapper {
    [XmlAttribute]
    public string SomeProp {get;set;} // custom props etc
    [XmlAttribute]
    public int SomeOtherProp {get;set;} // custom props etc
    public Collection<string> Items {get;set;} // the items
}

The other option is to implement IXmlSerializable (quite a lot of work), but that still won't work for data-binding etc. Basically, this isn't the expected usage.

Answer 2

I've been fighting with the same issue Romaroo is (wanting to add properties to the xml serialization of a class that implements ICollection). I have not found any way to expose properties that are in the collection class. I even tried using the XmlAttribute tag and making my properties show up as attributes of the root node, but no luck there either. I was however able to use the XmlRoot tag on my class to rename it from "ArrayOf...". Here are some references in case you're interested:

• MilkCarton.com
• Microsoft Forum Posting
• Diranieh - look halfway down

Answer 3

If you do encapsulate, as Marc Gravell suggests, the beginning of this post explains how to get your XML to look exactly like you describe.

http://blogs.msdn.com/youssefm/archive/2009/06/12/customizing-the-xml-for-collections-with-xmlserializer-and-datacontractserializer.aspx

That is, instead of this:

<MyCollection MyAttribute="SerializeThis" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;
  <Items>
    <string>Hello</string>
    <string>Goodbye</string>
  <Items>
</MyCollection>

You can have this:

<MyCollection MyAttribute="SerializeThis" 
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"&gt;      
  <string>Hello</string>
  <string>Goodbye</string>
</MyCollection>