Specifying a namespace is the same whether for an element or an attribute:
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Xml;
using System.Xml.Linq;
using System.Text;
using System.IO;
class Program
{
static void Main()
{
XNamespace ns = "http://domain.com/xmlspace";
var data = new XElement(ns + "data",
new XAttribute(ns + "id", 123)
);
Console.WriteLine(data);
}
}