In my post about implementing entity object row state using 'is dirty' and 'cloning' I described how to exclude properties during binary serialization using [NonSerialized]. The same can be done with XML serialization using [XmlIgnore]. In addition, properties can be temporarily excluded when serializing to XML. The mechanism behind conditionally excluding properties from XML serialization is not well known, thus this post to shed some light on it.
The bool <property name>Specified mechanism is the .NET serialization framework answer to XSD minOccurs="0" applied by code generators such as XSD.EXE, XsdObjectGen, and others. Most developers actually implicitly use this mechanism when they design and use typed datasets with optional fields (columns).
In short, whenever .NET serializes the properties of a class to XML, it always checks to see if there exists a boolean metod whose name is the name of the property suffixed with Specified. If such a method exists, and it returns false, the property is not included in the serialized XML; if it returns true, the property and value is serialized. The oppsite logic is applied on de-serialization of optional XML elements.
Thus, you can implement and use Specified methods at will to get conditional XML serialization in .NET. The serializer does not discriminate between methods implemented by code generators and methods implemented by you. Partial classes in .NET 2.0 also makes adding such methods to code generated classes much simpler.
I got to know the details of how XSD.EXE handles XSD minOccurs="0" and xsi:nil="true" very well when doing "web-service contract first" in combination with InfoPath last year. The support for doing WSCF in .NET 2.0 has improved with the support for e.g. interface based .ASMX web-services, but Visual Studio 2005 still has a way to go to support WSCF to its maximum extent. I expect that this will be finally solved when WCF (Indigo) is released.