create a node in a xml document if it does not exist using linq to xml

Question

I am querying a xml document using the XPathSelectElement method.

if the node does not exist I would like to insert a node with that path in the same document. The parent nodes should also be created if they do not exist. Is there an easy way to do this without looping through the parents checking if they exist? (Add a new node using XPath)

Swansdown

Answer 1

No, there is not... this is no different than if you were looking for a Directory on a File System, and had to ensure that all of the parent directories were there to.

Example:

if (Directory.Exists(@":c:\test1\test2\blah blah\blah blah2")) ...

It's true that the Directory.CreateDirectory method will create all parents that need to be there to have the child show up, but there is no equivalent in XML (using .NET classes, including LINQ-to-XML).

You'll have to loop through each one manually. I suggest you make a helper method called "EnsureNodeExists" that does that for you :)

Edinburgh

Good analogy. It would be nice if there existed a method like this though. I actually solved this problem in a alternative way. Looping through each node in the master document and checking if the target document has this node. if not I would add this node and all children to the target doc. Works pretty well – Swansdown

Answer 2

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);
}

Flyaway

Answer 3

This is a bit late, but for the next poor soul to come here and find no help here's the static method I ended up writing. It's not flawless, but should handle a variety of use cases. Essentially I just go from the bottom of the XPath and keep cutting off sections until I find a match, then go back down the path and create what's missing. It will fail if you use a path like /root/test//someNode if someNode doesn't exist, but for a path ../someNode//foo/bar/zoot, if ../someNode//foo exists it should work fine.

public static XElement GetOrCreateNodeAtXPath(XDocument doc, string key)
{
    //you don't really need this, but it makes throwing errors easier when you're 10 levels deep in the recursion
    return GetOrCreateNodeAtXPath(doc, key, key);
}
private static XElement GetOrCreateNodeAtXPath(XDocument doc, string key, string originalKey)
{
    var node = doc.XPathSelectElement(key);
    if (node != null)
        return node;
    if (!key.Contains('/')) //we've reached the root, and couldn't find anything
        throw new Exception($"Could not create node at path {originalKey}, no part of path matches document");
    var slashIndex = key.LastIndexOf('/');
    var newKey = key.Substring(0, slashIndex);
    var newNodeName = key.Substring(slashIndex + 1);
    var parentNode = GetOrCreateNodeAtXPath(doc, newKey, originalKey);
    var childNode = new XElement(newNodeName);
    parentNode.Add(childNode); 
    return childNode;
}

You may wanna swap XDocument with XElement or XNode or something in the signature, I was just dealign with docs so I used docs.

Threnode