Create XML Nodes based on XPath?

Question

Does anyone know of an existing means of creating an XML hierarchy programatically from an XPath expression?

For example if I have an XML fragment such as:

<feed>
    <entry>
        <data></data>
        <content></content>
    </entry>
</feed>

Given the XPath expression /feed/entry/content/@source I would have:

<feed>
    <entry>
        <data></data>
        <content @source=""></content>
    </entry>
</feed>

I realize this is possible using XSLT but due to the dynamic nature of what I'm trying to accomplish a fixed transformation won't work.

I am working in C# but if someone has a solution using some other language please chime in.

Thanks for the help!

Answer 1

In the example you present the only thing being created is the attribute ...

XmlElement element = (XmlElement)doc.SelectSingleNode("/feed/entry/content");
if (element != null)
    element.SetAttribute("source", "");

If what you really want is to be able to create the hierarchy where it doesn't exist then you could your own simple xpath parser. I don't know about keeping the attribute in the xpath though. I'd rather cast the node as an element and tack on a .SetAttribute as I've done here:

static private XmlNode makeXPath(XmlDocument doc, string xpath)
{
    return makeXPath(doc, doc as XmlNode, xpath);
}

static private XmlNode makeXPath(XmlDocument doc, XmlNode parent, string xpath)
{
    // grab the next node name in the xpath; or return parent if empty
    string[] partsOfXPath = xpath.Trim('/').Split('/');
    string nextNodeInXPath = partsOfXPath.First();
    if (string.IsNullOrEmpty(nextNodeInXPath))
        return parent;

    // get or create the node from the name
    XmlNode node = parent.SelectSingleNode(nextNodeInXPath);
    if (node == null)
        node = parent.AppendChild(doc.CreateElement(nextNodeInXPath));

    // rejoin the remainder of the array as an xpath expression and recurse
    string rest = String.Join("/", partsOfXPath.Skip(1).ToArray());
    return makeXPath(doc, node, rest);
}

static void Main(string[] args)
{
    XmlDocument doc = new XmlDocument();
    doc.LoadXml("<feed />");

    makeXPath(doc, "/feed/entry/data");
    XmlElement contentElement = (XmlElement)makeXPath(doc, "/feed/entry/content");
    contentElement.SetAttribute("source", "");

    Console.WriteLine(doc.OuterXml);
}

Answer 2

If the XPath string is processed from back to front, its easier to process non rooted XPaths eg. //a/b/c... It should support Gordon's XPath syntax too although I have not tried...

static private XmlNode makeXPath(XmlDocument doc, string xpath)
{
    string[] partsOfXPath = xpath.Split('/');
    XmlNode node = null;
    for (int xpathPos = partsOfXPath.Length; xpathPos > 0; xpathPos--)
    {
        string subXpath = string.Join("/", partsOfXPath, 0, xpathPos);
        node = doc.SelectSingleNode(subXpath);
        if (node != null)
        {
            // append new descendants
            for (int newXpathPos = xpathPos; newXpathPos < partsOfXPath.Length; newXpathPos++)
            {
                node = node.AppendChild(doc.CreateElement(partsOfXPath[newXpathPos]));
            }
            break;
        }
    }

    return node;
}

Answer 3

Here is my version. Hope this also would help someone.

    public static void Main(string[] args)
    {

        XmlDocument doc = new XmlDocument();
        XmlNode rootNode = GenerateXPathXmlElements(doc, "/RootNode/FirstChild/SecondChild/ThirdChild");

        Console.Write(rootNode.OuterXml);

    }

    private static XmlDocument GenerateXPathXmlElements(XmlDocument xmlDocument, string xpath)
    {
        XmlNode parentNode = xmlDocument;

        if (xmlDocument != null && !string.IsNullOrEmpty(xpath))
        {
            string[] partsOfXPath = xpath.Split('/');


            string xPathSoFar = string.Empty;

            foreach (string xPathElement in partsOfXPath)
            {
                if(string.IsNullOrEmpty(xPathElement))
                    continue;

                xPathSoFar += "/" + xPathElement.Trim();

                XmlNode childNode = xmlDocument.SelectSingleNode(xPathSoFar);
                if(childNode == null)
                {
                    childNode = xmlDocument.CreateElement(xPathElement);
                }

                parentNode.AppendChild(childNode);

                parentNode = childNode;
            }
        }

        return xmlDocument;
    }

Answer 4

Here's my quick hack that can also create attributes as long as you use a format like /configuration/appSettings/add[@key='name']/@value.

static XmlNode createXPath(XmlDocument doc, string xpath)
{
  XmlNode node=doc;
  foreach (string part in xpath.Substring(1).Split('/'))
  {
    XmlNodeList nodes=node.SelectNodes(part);
    if (nodes.Count>1) throw new ComponentException("Xpath '"+xpath+"' was not found multiple times!");
    else if (nodes.Count==1) { node=nodes[0]; continue; }

    if (part.StartsWith("@"))
    {
      var anode=doc.CreateAttribute(part.Substring(1));
      node.Attributes.Append(anode);
      node=anode;
    }
    else
    {
      string elName, attrib=null;
      if (part.Contains("["))
      {
        part.SplitOnce("[", out elName, out attrib);
        if (!attrib.EndsWith("]")) throw new ComponentException("Unsupported XPath (missing ]): "+part);
        attrib=attrib.Substring(0, attrib.Length-1);
      }
      else elName=part;

      XmlNode next=doc.CreateElement(elName);
      node.AppendChild(next);
      node=next;

      if (attrib!=null)
      {
        if (!attrib.StartsWith("@")) throw new ComponentException("Unsupported XPath attrib (missing @): "+part);
        string name, value;
        attrib.Substring(1).SplitOnce("='", out name, out value);
        if (string.IsNullOrEmpty(value) || !value.EndsWith("'")) throw new ComponentException("Unsupported XPath attrib: "+part);
        value=value.Substring(0, value.Length-1);
        var anode=doc.CreateAttribute(name);
        anode.Value=value;
        node.Attributes.Append(anode);
      }
    }
  }
  return node;
}

SplitOnce is an extension method:

public static void SplitOnce(this string value, string separator, out string part1, out string part2)
{
  if (value!=null)
  {
    int idx=value.IndexOf(separator);
    if (idx>=0)
    {
      part1=value.Substring(0, idx);
      part2=value.Substring(idx+separator.Length);
    }
    else
    {
      part1=value;
      part2=null;
    }
  }
  else
  {
    part1="";
    part2=null;
  }
}

Sample:

public static void Set(XmlDocument doc, string xpath, string value)
{
  if (doc==null) throw new ArgumentNullException("doc");
  if (string.IsNullOrEmpty(xpath)) throw new ArgumentNullException("xpath");

  XmlNodeList nodes=doc.SelectNodes(xpath);
  if (nodes.Count>1) throw new ComponentException("Xpath '"+xpath+"' was not found multiple times!");
  else if (nodes.Count==0) createXPath(doc, xpath).InnerText=value;
  else nodes[0].InnerText=value;
}

e.g.

Set(doc, "/configuration/appSettings/add[@key='Server']/@value", "foobar");

Answer 5

I liked Chris' version because it handled attributes in xpaths and the other solutions didn't (though it doesn't handle "text()" in the path that well which I fixed). I unfortunately had to use this in a VB app, so here's the conversion for that:

        Private Sub SplitOnce(ByVal value As String, ByVal separator As String, ByRef part1 As String, ByRef part2 As String)
        If (value IsNot Nothing) Then
            Dim idx As Integer = value.IndexOf(separator)
            If (idx >= 0) Then
                part1 = value.Substring(0, idx)
                part2 = value.Substring(idx + separator.Length)
            Else
                part1 = value
                part2 = Nothing
            End If
        Else
            part1 = ""
            part2 = Nothing
        End If
    End Sub
    Private Function createXPath(ByVal doc As XmlDocument, ByVal xpath As String) As XmlNode
        Dim node As XmlNode = doc
        Dim part As String
        For Each part In xpath.Substring(1).Split("/")
            Dim nodes As XmlNodeList = node.SelectNodes(part)
            If (nodes.Count > 1) Then
                Throw New Exception("Xpath '" + xpath + "' was not found multiple times!")
            ElseIf (nodes.Count = 1) Then
                node = nodes(0)
                Continue For
            End If

            If (part.EndsWith("text()")) Then
                ' treat this the same as previous node since this is really innertext
                Exit For
            ElseIf (part.StartsWith("@")) Then
                Dim anode As XmlAttribute = doc.CreateAttribute(part.Substring(1))
                node.Attributes.Append(anode)
                node = anode
            Else
                Dim elName As String = Nothing
                Dim attrib As String = Nothing
                If (part.Contains("[")) Then
                    SplitOnce(part, "[", elName, attrib)
                    If (Not attrib.EndsWith("]")) Then
                        Throw New Exception("Unsupported XPath (missing ]): " + part)
                    End If
                    attrib = attrib.Substring(0, attrib.Length - 1)
                Else
                    elName = part
                End If
                Dim nextnode As XmlNode = doc.CreateElement(elName)
                node.AppendChild(nextnode)
                node = nextnode
                If (attrib IsNot Nothing) Then
                    If (Not attrib.StartsWith("@")) Then
                        Throw New Exception("Unsupported XPath attrib (missing @): " + part)
                    End If
                    Dim name As String = ""
                    Dim value As String = ""
                    SplitOnce(attrib.Substring(1), "='", name, value)
                    If (String.IsNullOrEmpty(value) Or Not value.EndsWith("'")) Then
                        Throw New Exception("Unsupported XPath attrib: " + part)
                    End If
                    value = value.Substring(0, value.Length - 1)
                    Dim anode As XmlAttribute = doc.CreateAttribute(name)
                    anode.Value = value
                    node.Attributes.Append(anode)
                End If
            End If
        Next
        Return node
    End Function