title: wcf return an XmlDocument?
body: <p>I have a WCF service where Im building up a block of XML using an XmlWriter.  Once complete I want to have the WCF return it as an XmlDocument.</p>
  
  <p>But if I have XmlDocument in the [OperationContract] it doesnt work:</p>
  
  <pre><code>[OperationContract]
  XmlDocument GetNextLetter();
  </code></pre>
  
  <p>The WCF test utility gives:</p>
  
  <blockquote>
    <p>System.Runtime.Serialization.InvalidDataContractException: Type 'System.Xml.XmlDocument' cannot be serialized.</p>
  </blockquote>