XML generated by XMLWriter in WCF Custom Encoder generating namespace conflict

Question

I'm using a custom encoder in my WCF application to write out the SOAP message XML. When I use the default XML textMessageEncoding built into WCF it's fine, but when I use a custom encoder I get a problem with the namespaces - the xmlns:a tag below (in the element and the element) is defined twice for two different namespaces and this is causing problems on the service side when parsing

<s:Envelope xmlns:a="http://www.w3.org/2005/08/addressing" xmlns:u="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
  <s:Header>
    <Action a:mustUnderstand="1" u:Id="_3" xmlns="http://www.w3.org/2005/08/addressing" **xmlns:a="http://schemas.xmlsoap.org/soap/envelope/"**></Action>
    <MessageID u:Id="_4" xmlns="http://www.w3.org/2005/08/addressing">
      <!--Omitted-->
    </MessageID>
    <ActivityId CorrelationId="1" xmlns="http://schemas.microsoft.com/2004/09/ServiceModel/Diagnostics">
      <!--Omitted-->
    </ActivityId>

Any idea on how to fix this issue? I'm using the C# XmlWriter in the custom encoder to write the XML and that's what seems to be causing the issue.

Also, how do I get the XmlWriter to use a prefix for the <Action> tag above so that it's <a:Action> rather than using the xmlns for every declaration -

<Action a:mustUnderstand="1" u:Id="_3" xmlns="http://www.w3.org/2005/08/addressing"

Here's my XmlWriterSettings

XmlWriterSettings writerSettings = new XmlWriterSettings();
writerSettings.Encoding = Encoding.GetEncoding(factory.CharSet);
writerSettings.OmitXmlDeclaration = true;
writerSettings.NamespaceHandling = NamespaceHandling.OmitDuplicates;  

Answer 1

Fixed this by using a XmlDictionaryWriter instead of an XmlWriter. Luckily I'm using Utf-8 encoding and had the option to do this