Using XML decorations to specify default values during de-serialization

Question

I have a problem deserializing some XML; the XML supplied by a third party is quite verbose, so if there is no value set for an particular element, it will supply and empty element (e.g. <element1 />).

This is a problem for certain elements, for example, those that are meant to store integers. I have control over the third party, so I could either get them to specify a default value (<myinteger>0</myinteger>) or I can get them to omit these elements entirely. Both of these should avoid the problem.

However, there may be situations in future, where we don't have so much control - in which case, is there a way of specifying, perhaps via a decoration, a default value?

    [XmlElement("myinteger")=0???]
    public int MyInteger
    {
        get
        {
            return myInteger;
        }
        set
        {
            myInteger= value;
        }
    }

Solution

XmlSerializer does support [DefaultValue], but it uses it during serialization. During deserialization, it simply runs the constructor, then takes incoming values and applies them. A common approach, then, is to use the constructor (or field-initializers):

public Foo() {
    Bar = 4;
}
[DefaultValue(4), XmlAttribute("bar")]
public int Bar {get;set;}

However; XmlSerializer's interpretation of this is not "supply an empty element" - but rather "omit the element(/attribute)". Even int? doesn't map to "empty". To handle empty elements, you would have to handle it as a string. Which is ugly.

Comments

[CJM]

Hmmmm... simply getting the third party to omit empty elements of a description seems the easiest solution. I was just curious what I could do where I didn't have the luxury of that choice. I did consider the string option, but it is just an big can of worms...

[Matthew Flaschen]

There is at least one important effect during deserialization. If the element is empty, without DefaultValue there may be a parsing error. However, with it, the generated code will skip the empty element.