XmlSerializer is a great peace of technology. Combined with xsd.exe and friends (XmlSerializerNamespaces, et al.) it's a powerful tool for one to get around XML instance serialization/deserialization.
But, there is a potentially serious bug present, even in 3.5 SP1 version of the .NET Framework.
Suppose we have the following XML structure:
<Envelope xmlns="NamespaceA"
xmlns:B="NamespaceB">
<B:Header></B:Header>
<Body></Body>
</Envelope>
This tells you that Envelope, and Body elements are in the same namespace (namely 'NamespaceA'), while Header is qualified with 'NamespaceB'.
Now suppose we need to programmatically insert <B:Header> element into an empty, core, document.
Core document:
<Envelope xmlns="NamespaceA"
xmlns:B="NamespaceB">
<Body></Body>
</Envelope>
Now do an XmlNode.InsertNode() of the following:
<B:Header>...</B:Header>
We should get:
<Envelope xmlns="NamespaceA"
xmlns:B="NamespaceB">
<B:Header>...</B:Header>
<Body></Body>
</Envelope>
To get the to be inserted part one would serialize (using XmlSerializer) the following Header document:
<B: Header xmlns:B="NamespaceB">
...
</B:Header>
To do this, a simple XmlSerializer magic will do the trick:
XmlSerializerNamespaces xsn = new XmlSerializerNamespaces();
xsn.Add("B", "NamespaceB");
XmlSerializer xSer = new XmlSerializer(typeof(Header));
XmlTextWriter tw = new XmlTextWriter(ms, null);
xSer.Serialize(tw, h, xsn);
ms.Seek(0, SeekOrigin.Begin);
XmlDocument doc = new XmlDocument()
doc.Load(ms);
ms.Close();
This would generate exactly what we wanted. A prefixed namespace based XML document, with the B prefix bound to 'NamespaceB'.
Now, if we would import this document fragment into our core document using XmlNode.ImportNode(), we would get:
<Envelope xmlns="NamespaceA"
xmlns:B="NamespaceB">
<B:Header xmlns:B="NamespaceB">...</B:Header>
<Body></Body>
</Envelope>
Which is valid and actually, from an XML Infoset view, an isomorphic document to the original. So what if it's got the same namespace declared twice, right?
Right - until you involve digital signatures. I have described a specific problem with ambient namespaces in length in this blog entry: XmlSerializer, Ambient XML Namespaces and Digital Signatures.
When importing a node from another context, XmlNode and friends do a resolve against all namespace declarations in scope. So, when importing such a header, we shouldn't get a duplicate namespace declaration.
The problem is, we don't get a duplicate namespace declaration, since XmlSerializer actually inserts a normal XML attribute into the Header element. That's why we seem to get another namespace declaration. It's actually not a declaration but a plain old attribute. It's even visible (in this case in XmlElement.Attributes), and it definitely shouldn't be there.
So if you hit this special case, remove all attributes before importing the node into your core document. Like this:
XmlSerializerNamespaces xsn = new XmlSerializerNamespaces();
xsn.Add("B", "NamespaceB");
XmlSerializer xSer = new XmlSerializer(typeof(Header));
XmlTextWriter tw = new XmlTextWriter(ms, null);
xSer.Serialize(tw, h, xsn);
ms.Seek(0, SeekOrigin.Begin);
XmlDocument doc = new XmlDocument()
doc.Load(ms);
ms.Close();
doc.DocumentElement.Attributes.RemoveAll();
Note that the serialized document representation will not change, since an ambient namespace declaration (B linked to 'NamespaceB') still exists in the XML Infoset of doc XML document.