Rewrite XMLDocument to use namespace prefix

Question

I have a XMLDocument which, when I save to file, repeats a namespace on most of the elements, as in

<Test>
    <Test xmlns="http://example.com/schema1">

      <Name xmlns="http://example.com/schema2">xyz</Name>
      <AddressInfo xmlns="http://example.com/schema2">
        <Address>address</Address>
        <ZipCode>zzzz</ZipCode>
      </AddressInfo>
       ...

Is it possible to amend this file so that it uses a namespace prefix throughout the document, ie something like

<Test xmlns="http://example.com/schema1" xmlns:p="http://example.com/schema2"  >

 <p:Name>xyz</p:Name>
 <p:AddressInfo">
   <p:Address>address</p:Address>
   <p:ZipCode>zzzz</p:ZipCode>
 </p:AddressInfo>        
 ...

I have tried adding

   doc.DocumentElement.SetAttribute("xmlns:p", "http://example.com/schema2");

but whilst this adds the namespace to the header, the main body of the file is unchanged.

Answer 1

You can simply change XmlElement.Prefix property value :

doc.DocumentElement.SetAttribute("xmlns:p", "http://example.com/schema2");
//xpath for selecting all elements in specific namespace :
var xpath = "//*[namespace-uri()='http://example.com/schema2']";
foreach(XmlElement node in doc.SelectNodes(xpath))
{
    node.Prefix = "p";
}
doc.Save("path_to_file.xml");

Answer 2

One way to do this using LINQ to XML is to add the namespace declarations into the root and then remove all the existing ones, so something like:

var doc = XDocument.Parse(xml);

var existingDeclarations = doc.Descendants()
    .SelectMany(e => e.Attributes().Where(a => a.IsNamespaceDeclaration))
    .ToList();

doc.Root.Add(new XAttribute(XNamespace.Xmlns + "p", "http://example.com/schema2"));

existingDeclarations.Remove();

I'm not aware of such an easy solution using XmlDocument, but I'd always recommend using LINQ to XML unless you have a good reason not to.

Answer 3

I found this question and it addresses a problem I was trying to solve, but I needed a more general solution, so I developed the extension method below. It was inspired by Charles Mager's answer, but as you can see, it goes well beyond that. Truthfully, I regret that I was sucked into this problem, since it was such a pain to figure out, but someone else might benefit from it.

The business of the 'valueChangingAttribNames' parameter is likely something you can ignore. I had to deal with it because System.Runtime.Serialization.DataContractSerializer generates i:type attributes that embed namespace prefixes within the attribute value.

Here is a code snippet that shows usage of the extension method:

XName valueChangingAttribName = "{http://www.w3.org/2001/XMLSchema-instance}type";
var xDoc = XDocument.Load(stream);
xDoc.Root.ConsolidateNamespaces(new List<XName> { valueChangingAttribName });

One small caveat: for the case of prefix collision, I devised the method of appending letters of the alphabet to the existing prefix. It strikes me as unlikely that you'd have a case where there are more than 26 different namespaces, all of which had been assigned the same prefix. Nonetheless, if you have cases like that, then you'd need to modify my method of generating a unique prefix.

    /// <summary>
    /// This method finds all namespace declarations on the descendants of the given root XElement
    /// and moves them from the decendant elements to the root element. It is thus possible to 
    /// make the XML string much smaller and more human-readable in cases when there are many
    /// redundant declarations of the same namespace.
    /// </summary>
    /// <param name="xRoot">The element whose descendants are to have their namespaces declarations
    /// removed. Also, the element that is to contain the consolidated namespace declarations</param>
    /// <param name="valueChangingAttribNames">A list of the names of XAttributes whose value
    /// must be updated to reflect changes to the namespace prefixes.
    /// This is designed to handle cases like d7p1 in the example:
    ///     xmlns:d7p1="http://www.w3.org/2001/XMLSchema" i:type="d7p1:int"
    /// generated by System.Runtime.Serialization.DataContractSerializer. In order to treat this
    /// example, the XName of the i:type attribute should be included in the list.  If the value
    /// of the 'valueChangingAttribNames' parameter is null, then no such changes are made to the
    /// values of XAttributes.  The default is null.
    /// </param>
    public static void ConsolidateNamespaces(this XElement xRoot,
                            List<XName> valueChangingAttribNames = null)
    {
        String letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        // Find all XAttributes that represent namespace declarations
        var nsAttributes = xRoot.Descendants().SelectMany(e => e.Attributes())
                    .Where(a => a.IsNamespaceDeclaration).ToList();
        // create groupings by common prefix
        var prefixGroups = nsAttributes.GroupBy(a => a.Name.LocalName);

        // Each time an XAttribute is resolved, we remove it from the list.
        // Loop until all XAttributes are resolved and removed from the list.
        while (nsAttributes.Any())
        {
            // Inner loop repeats until no XAttributes can be resolved without dealing with conflicting
            // prefixes (i.e. same prefix refers to different namespace in different XElements)
            while (nsAttributes.Any())
            {
                // Loop through each XAttribute
                foreach (var attr in nsAttributes.ToList())
                {
                    // If the root already contains a declaration of the same namespace
                    // (regardless of whether the prefix matches between the root and the descendant)
                    if (xRoot.Attributes().Any(a => a.Value == attr.Value))
                    {
                        // We have only to remove the XAttribute from the descendant, and Xml.Linq
                        // framework automatically rectifies prefix on the descendant if necessary.
                        TransferNamespaceToRoot(nsAttributes, xRoot, null, attr,
                                                    valueChangingAttribNames);
                    }
                }
                // Take the first remaining prefix group where there is no prefix name conflict
                var prefixGroup = prefixGroups
                          .FirstOrDefault(g => g.Select(a => a.Value).Distinct().Count() == 1);
                // If no such groups remain, then we're done with the inner loop
                if (prefixGroup == null) break;
                // The list of XAttributes that have matching prefix & namespace
                var attribs = prefixGroup.ToList();
                for (int j = 0; j < attribs.Count; j++)
                    // The first XAttribute must be added to the root, and the rest simply need
                    // to be removed from the descendant.
                    TransferNamespaceToRoot(nsAttributes, xRoot,
                                 (j == 0 ? attribs[j] : null), attribs[j], valueChangingAttribNames);
            }
            // Take the first remaining prefix group. We know there is a prefix name conflict since
            // this group was not processed in the above loop.
            var conflictGroup = prefixGroups.FirstOrDefault();
            if (conflictGroup == null) break;
            // The processing of the previous loop should guarantee that all namespaces in
            // this group are not yet declared in the root.  Each of the conflicting prefixes
            // must be renamed.  If we try to add the existing prefix to the root for any one
            // of the namespaces in this group, then the Xml.Linq framework will incorrectly match
            // the other prefixes in this group to that namespace.
            foreach (var attr in conflictGroup.ToList())
            {
                // If the root already contains a declaration of the same namespace
                // (could only be a previously-processed XAttribute from this same conflict group)
                if (xRoot.Attributes().Any(a => a.Value == attr.Value))
                {
                    // We have only to remove the XAttribute from the descendant, and Xml.Linq
                    // framework automatically rectifies prefix on the descendant if necessary.
                    TransferNamespaceToRoot(nsAttributes, xRoot, null, attr, valueChangingAttribNames);
                    continue;
                }
                // Find a new prefix that doesn't conflict with any existing prefixes
                String newPrefix = attr.Name.LocalName + "_A";
                for (int i = 1; xRoot.Attributes().Any(a => a.Name.LocalName == newPrefix); i++)
                    newPrefix = attr.Name.LocalName + "_" + letters[i];
                // Add the namespace declaration to the root, using the new prefix
                XNamespace ns = attr.Value;
                var newAttr = new XAttribute(XNamespace.Xmlns + newPrefix, attr.Value);
                TransferNamespaceToRoot(nsAttributes, xRoot, newAttr, attr, valueChangingAttribNames);
            }
        }

    }
    /// <summary>
    /// This private method is designed as a helper to public method ConsolidateNamespaces.
    /// The method is designed to remove a given XAttribute from a descendant XElement,
    /// and add a given XAttribute to the root XElement.  The XAttribute to add to the root
    /// may be the same as the item to remove, different than the item to remove, or none,
    /// as appropriate.
    /// </summary>
    /// <param name="nsAttributes">The list of XAttributes that the caller is to process.
    /// This method is designed to remove 'attrToRemove' from the list.</param>
    /// <param name="xRoot">The root XElement to which 'attrToAdd' is added.</param>
    /// <param name="attrToAdd">The XAttribute that is to be added to 'xRoot'.  This may be
    /// the same as or different than 'attrToRemove', or it may be null if the caller does
    /// not need to add anything to the root.</param>
    /// <param name="attrToRemove">The XAttribute that is to be removed from a descendant XElement
    /// and removed from 'nsAttributes'</param>
    /// <param name="valueChangingAttribNames">A list of the names of XAttributes whose value
    /// must be updated to reflect changes to the namespace prefixes.
    /// This is designed to handle cases like d7p1 in the example:
    ///     xmlns:d7p1="http://www.w3.org/2001/XMLSchema" i:type="d7p1:int"
    /// generated by System.Runtime.Serialization.DataContractSerializer. In order to treat this
    /// example, the XName of the i:type attribute should be included in the list.  If the value
    /// of the 'valueChangingAttribNames' parameter is null, then no such changes are made to the
    /// values of XAttributes.  The default is null.
    /// </param>
    private static void TransferNamespaceToRoot(List<XAttribute> nsAttributes, XElement xRoot,
                            XAttribute attrToAdd, XAttribute attrToRemove,
                            List<XName> valueChangingAttribNames)
    {
        if (valueChangingAttribNames != null)
        {
            // Change the value of any 'value changing attributes' that incorporate the prefix.
            // This is designed to handle cases like d7p1 in the example:
            //     <d2p1:Value xmlns:d7p1="http://www.w3.org/2001/XMLSchema" i:type="d7p1:int">
            // that are generated by System.Runtime.Serialization.DataContractSerializer.
            // The Xml.Linq framework does not rectify such cases where the prefix is
            // within the XAttribute vaue.

            String oldPrefix = attrToRemove.Name.LocalName;
            String newPrefix = oldPrefix;
            // If no XAttribute is to be added to the root
            if (attrToAdd == null)
            {
                // find the existing XAttribute in the root for the namespace,
                // and use the prefix that it declares.
                var srcAttr = xRoot.Attributes()
                        .FirstOrDefault(a => a.IsNamespaceDeclaration && a.Value == attrToRemove.Value);
                if (srcAttr != null)
                    newPrefix = srcAttr.Name.LocalName;
            }
            else
                // If a new XAttribute is to be added to the root, then use the prefix it declares
                newPrefix = attrToAdd.Name.LocalName;

            if (oldPrefix != newPrefix)
            {
                foreach (XName attribName in valueChangingAttribNames)
                {
                    // Do string replacement of the prefix in the XAttribute value
                    var vcAttrib = attrToRemove.Parent.Attribute(attribName);
                    vcAttrib.Value = vcAttrib.Value.Replace(attrToRemove.Name.LocalName, newPrefix);
                }
            }
        }

        // Add the XAttribute to the root element
        if (attrToAdd != null)
            xRoot.Add(attrToAdd);
        // Remove the namespace declaration from the descendant
        attrToRemove.Remove();
        // remove the XAttribute from the list of XAttributes to be processed
        nsAttributes.Remove(attrToRemove);
    }