Question

I’m implementing IXMLSerializable in one of my classes. It contains a few numeric properties that are nullable (int? double? etc..)

What is the correct way to serialize/serialize these through IXMLSerializable? Here’s what I’m doing now, which works, but obviously does not seem like the correct way to do it.

void IXmlSerializable.WriteXml(XmlWriter writer)
{
    ...

    if (this._PropName == null)
    {
        writer.WriteElementString("PropName", "NULL");
    }
    else
    {
        writer.WriteElementString("PropName", this._PropName.ToString());
    }
    ...
}

void IXmlSerializable.ReadXml(XmlReader reader)    
{
    string tempStr;
    ...

    reader.ReadStartElement("PropName"); 

    if (tempStr != "NULL")
    {
        this._PropName = double.Parse(tempStr);
    }
    else 
    {
        this._PropName = null;
    }
    ...
}

Update:
It was asked that I give a little background as to why I am implementing IXmlSerializable. I’m working on a program for architectural design, where I need a class that represents a collection of Floors. Each Floor has properties such as Floor.Area area, Floor.Height etc. The elevation of the floor, however, is defined by the sum of the floor heights below it. So whenever a Floor.Height property changes or the FloorCollection is modified the elevations of the Floors are re-cacluated.

My FloorCollection class, which I need to serialize, inherits from BindingList. If I try to serialize this class directly it will serialize the collection of floors, but not any properties or fields in the class. See my previous post on this.

Now I’m trying to add the ability to restrict the maximum height, maximum top elevation, and minimum bottom elevation of the building floors in the collection. So I’m using nullable doubles to represent these restrictions, where a null value means unrestricted. The elevation properties can be positive, negative, or zero. So there needs to be an alternate state, null, that identifies when there is no restriction.

Now that I think about it might be easier overall to just have a separate Boolean value that identifies if there is a elevation/height restriction, and then a regular double property that identifies what the restriction is if it is enabled.

Answer 1

You want to always write the XML for the property, but if the property value is null you want to include an xsi:nil=”true” attribute.

void IXmlSerializable.WriteXml(XmlWriter writer)
{
    ...

    if (this._PropName == null)
    {
        writer.WriteStartElement("PropName");
        writer.WriteAttributeString("xsi", "nil", "http://www.w3.org/2001/XMLSchema-instance", "true");
        writer.WriteEndElement();
    }
    else
    {
        writer.WriteElementString("PropName", this._PropName.ToString());
    }
    ...
}

You are also probably going to want to write an xsi:type=”xsd:datatype” attribute, where xsd is the http://www.w3.org/2001/XMLSchema namespace. This will allow you to read the data type back in during deserialization to know if (and how) to convert the value.