How to set the default XML namespace for an XDocument
<p>How can I set the default namespace of an existing XDocument (so I can deserialize it with DataContractSerializer). I tried the following:</p>

<pre><code>var doc = XDocument.Parse("&lt;widget/&gt;");
var attrib = new XAttribute("xmlns",
                            "http://schemas.datacontract.org/2004/07/Widgets");
doc.Root.Add(attrib);
</code></pre>

<p>The exception I get is is <code>The prefix '' cannot be redefined from '' to 'http://schemas.datacontract.org/2004/07/Widgets' within the same start element tag.</code> </p>

<p>Any ideas?</p>
