Cannot convert from 'System.Xml.XmlNode' to 'Google.GData.Client.IExtensionElementFactory

I'm trying to invite guest to google calendar using GData. I found

http://www.mail-archive.com/google-calendar-help-dataapi@googlegroups.com/msg01759.html

But on the line:

entry.ExtensionElements.Add(fragment.FirstChild);

I get a compile error saying:

Error 3 Argument 1: cannot convert from 'System.X开发者_开发问答ml.XmlNode' to 'Google.GData.Client.IExtensionElementFactory' 

What am I missing?

I solved it myself:

entry.ExtensionElements.Add(new XmlExtension(fragment.FirstChild));