C# XmlWriter namespace issues

Question

I'm using XmlWriter and I'm struggling to create the following XML tag.

<mzML xmlns="http://psi.hupo.org/ms/mzml" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://psi.hupo.org/ms/mzml http://psidev.info/files/ms/mzML/xsd/mzML1.1.0_idx.xsd" version="1.1">

I have the following:

XmlWriterSettings settings = new XmlWriterSettings();
settings.Indent = true;

XmlWriter xmlWriter = XmlWriter.Create(xmlFilePath, settings);

xmlWriter.WriteStartDocument();

xmlWriter.WriteStartElement("mzML", "xmlns", @"http://psi.hupo.org/ms/mzml");

xmlWriter.WriteAttributeString("xsi", "xmlns", @"http://www.w3.org/2001/XMLSchema-instance");

xmlWriter.WriteAttributeString("schemaLocation", "xsi", @"http://psi.hupo.org/ms/mzml http://psidev.info/files/ms/mzML/xsd/mzML1.1.0.xsd");

xmlWriter.WriteAttributeString("version", "1.1");

xmlWriter.WriteEndElement();

xmlWriter.WriteEndDocument();

xmlWriter.Close();

which results in the following:

<mzML:xmlns p1:xsi="http://www.w3.org/2001/XMLSchema-instance" p2:schemaLocation="http://psi.hupo.org/ms/mzml http://psidev.info/files/ms/mzML/xsd/mzML1.1.0.xsd" version="1.1.0" xmlns:p2="xsi" xmlns:p1="xmlns" xmlns:mzML="http://psi.hupo.org/ms/mzml">

The documentation is confusing me; I've tried lots of variations of the above code but can't seem to get anywhere close to my target XML tag.

Can anybody please help?

(P.S. I need to use XmlWriter due to the size of the XML files I need to create.)

Charles Mager · Accepted Answer · 2016-09-01T09:48:40.147

This seems quite confused. If we go through each line in turn:

xmlWriter.WriteStartElement("mzML", "xmlns", @"http://psi.hupo.org/ms/mzml");

This specifies your element has the prefix mzML and the local name xmlns and the namespace of http://.... Your element has no prefix, and the local name should be mzML.

xmlWriter.WriteAttributeString("xsi", "xmlns", 
    @"http://www.w3.org/2001/XMLSchema-instance");

This writes an attribute with the prefix xsi and the namespace xmlns. This is a namespace declaration: it has a prefix of xmlns, a local name of xsi and the namespace should be null. I'd also note that the writing of namespace declaration attributes will be automatically handled by XmlWriter - you'd generally only write them explicitly if you wanted to control which element they occur in and/or in what order.

xmlWriter.WriteAttributeString("schemaLocation", "xsi",
    @"http://psi.hupo.org/ms/mzml http://psidev.info/files/ms/mzML/xsd/mzML1.1.0.xsd");

This writes an attribute with the name schemaLocation (correct) and the namespace xsi. This is not correct, the namespace is http://www.w3.org/2001/XMLSchema-instance.

xmlWriter.WriteAttributeString("version", "1.1");

This is correct.

Putting all of these changes together:

xmlWriter.WriteStartElement("mzML", @"http://psi.hupo.org/ms/mzml");

// these two lines are optional - the namespace declarations are 
// automatically inserted as the *last* attributes when omitted
xmlWriter.WriteAttributeString("xmlns", null, null, "http://psi.hupo.org/ms/mzml");

xmlWriter.WriteAttributeString("xmlns", "xsi", null, 
    "http://www.w3.org/2001/XMLSchema-instance");

xmlWriter.WriteAttributeString("xsi", "schemaLocation", 
    "http://www.w3.org/2001/XMLSchema-instance",
    "http://psi.hupo.org/ms/mzml http://psidev.info/files/ms/mzML/xsd/mzML1.1.0.xsd");

xmlWriter.WriteAttributeString("version", "1.1");

See this fiddle for a working demo. This leaves out the optional namespace declaration attributes to show they still get added.

I hope you're still out there and this ~6yo message pings you... I am using variations on the above to try to build a similar element as the OP. I have code similar to your first line and third line, I skip the optional xmlns attribute as it will be added as you note. However, when the third line runs, it renames the original namespace to x, and adds a x: to the front of all the elements. Do you need to use the "optional" bit to avoid this? — Maury Markowitz, Nov 11 '22 at 17:26

C# XmlWriter namespace issues

1 Answers1