Adding elements to Lists in the XmlSerializer
I've been here a couple of times before. You have some list of data in xml you want to load into a list. for example
<?xml version="1.0" encoding="utf-8" ?>
<items>
<item id="1" text="some text" />
<item id="2" text="some more text"/>
</tasks>
Now I've blogged a few times about the XmlSerializer, so you may know I like it, and that I've torn it to pieces in the past. So anyway, my immediate reaction to needing to load xml data like this is to write code such as the following
[Serializable, XmlRoot("item"), XmlType("item")]
public class Item
{
[XmlAttribute("text")]
public string Text { get; set; }
[XmlAttribute("id")]
public int Id { get; set; }
}
XmlSerializer itemsSerializer = new XmlSerializer(typeof(Item[]));
Item[] arrItems = (Item[]) itemSerializer.Deserialize(filestream);
and lo and behold, it works perfectly. Well, I wanted to add a property to the file like <items prop="value"> Obviously neither an Item[] nor a List<Item> offers any properties I can use, but this should be easy, right. simply subclass List<T> and add my property, tagging it up for XML, and then swap the XmlSerializer constructor to take my new type.
[Serializable]
[XmlRoot("items"), XmlType("items")]
public class ItemList : List<Item>
{
private string prop = null; [XmlAttribute("prop")]
public string Prop
{
get { return prop; }
set { prop = value; }
}
}
Wrong! My new data in the xml file is never loaded. So a little later in snippet compiler, I find out it's not written either. What's going on?
Well after trying various things out, it looks like anything inheriting from IEnumerable<T> gets intercepted by the XmlSerializer, and uses a built in serializer instead. This means it never honours your extra properties, either for writing or reading, since it doesn't ever generate the code to look for them. Frustrating eh.
Well, imagine my surprise to find out for the first time after 8 years of .NET development that you don't need to implement IEnumerable explicitly to get foreach! Well, since I'm only loading the data and looping over it, this is perfect. We have to be a little clever with the attributes to get the xml to work the same as a straight IEnumerable serializer job, but that's easy enough (XmlElementAttribute against the nested List will do it) and Bingo. The following class[Serializable]
[XmlRoot("items"), XmlType("items")]
public class ItemList
{
private string prop = null;
private List<Item> items = new List<Item>(); [XmlAttribute("prop")]
public string Prop
{
get { return prop; }
set { prop = value; }
} [XmlElement("item")]
public List<Item> Items
{
get { return items; }
set { items = value; }
} public IEnumerator<Item> GetEnumerator()
{
return items.GetEnumerator();
}
}
[XmlRoot("items"), XmlType("items")]
public class ItemList
{
private string prop = null;
private List<Item> items = new List<Item>(); [XmlAttribute("prop")]
public string Prop
{
get { return prop; }
set { prop = value; }
} [XmlElement("item")]
public List<Item> Items
{
get { return items; }
set { items = value; }
} public IEnumerator<Item> GetEnumerator()
{
return items.GetEnumerator();
}
}
will successfully load and save effectively the same XML as if you were working directly off a List or Array, but wanted to add a property.
Now, all we have to work out is how to do this and get an immediate IList implementation on the class as well!
