We can add serializer settings to allow customization of this.

Code I have used before is along the lines of below. It sets the default namespace to "" which clears the ambient namespaces... but if the object has an XmlRoot attribute with a namespace, it uses that. The resultant XML has the correct default namespace and no ambient namespaces.

var objectType = toBeSerialised.GetType();
var defaultNamespace = string.Empty;
var xmlRootAttributes = toBeSerialised.GetType().GetCustomAttributes(typeof(XmlRootAttribute), true) as XmlRootAttribute[];
if (xmlRootAttributes != null && xmlRootAttributes.Length > 0)
{

defaultNamespace = xmlRootAttributes[0].Namespace;

}

// Use this object to prevent the serilaizer from adding extra "Ambient" namespaces
var xmlSerializerNamespaces = new XmlSerializerNamespaces();
xmlSerializerNamespaces.Add(String.Empty, defaultNamespace);

var xmlSerializer = new XmlSerializer(objectType);

xmlSerializer.Serialize(xmlWriter, toBeSerialised, xmlSerializerNamespaces);

True it is "mostly harmless" but

A) It is a waste of space
B) It confuses people consuming the service
C) It reveals to attackers extra details about the implementation of the service

Since it is so easy to fix, I'm disappointed.

Sorry, my last comment was harsh! I'll look into it myself and see if I can't supply a potential fix.

Thanks for looking at it.

With all the posts and people searching on how to do this I'd hardly call it harmless. Obviously it causes a lot of issues for people. Suppressing it should be a lot easier that having to right custom serializers. I find myself having to remove it a lot more than needing it there.

I hope to submit a pull request for this in the next few days.