Either decorate your root entity with the XmlRoot attribute which will be used at compile time.
[XmlRoot(Namespace = "www.contoso.com", ElementName = "MyGroupName", DataType = "string", IsNullable=true)]
Or specify the root attribute when de serializing at runtime.
XmlRootAttribute xRoot = new XmlRootAttribute();
xRoot.ElementName = "user";
// xRoot.Namespace = "http://www.cpandl.com";
xRoot.IsNullable = true;
XmlSerializer xs = new XmlSerializer(typeof(User),xRoot);