Duplicate elements when adding XElement to XDocument

Question

I'm writing a program in C# that will go through a bunch of config.xml files and update certain elements, or add them if they don't exist. I have the portion down that updates an element if it exists with this code:

XDocument xdoc = XDocument.Parse(ReadFile(_file));
XElement element = xdoc.Elements("project").Elements("logRotator")
                        .Elements("daysToKeep").Single();
element.Value = _DoRevert;

But I'm running into issues when I want to add an element that doesn't exist. Most of the time part of the tree is in place and when I use my code it adds another identical tree, and that causes the program reading the xml to blow up.

here is how I am attempting to do it

xdoc.Element("project").Add(new XElement("logRotator", new XElement("daysToKeep", _day)));

and that results in a structure like this(The numToKeep tag was already there):

<project>
  <logRotator>
    <daysToKeep>10</daysToKeep>
  </logRotator>
  <logRotator>
    <numToKeep>13</numToKeep>
  </logRotator>
</project>

but this is what I want

<project>
  <logRotator>
    <daysToKeep>10</daysToKeep>
    <numToKeep>13</numToKeep>
  </logRotator>
</project>

Answer 1

If numToKeep already exists, try this:

xdoc.Element("project")
    .Element("logRotator")
    .Add(new XElement("daysToKeep", _day));

Otherwise, to add the whole line when logRotator doesn't exist, use:

xdoc.Element("project").Add(
    new XElement("logRotator", 
        new XElement("daysToKeep", _day),
        new XElement("numToKeep", _num)
));

See XElement.Add(Object[])

Answer 2

This finds the logRotator element for a given project if it exists and creates the element if it does not.

// project is XElement
XElement logRotator = project.Element("logRotator");

if (logRotator == null)
{
    logRotator = new XElement("logRotator");
    project.Add(logRotator);
}

logRotator.Add(new XElement("daysToKeep", someValue));

For reusability (if you have other elements that could or could not exist), you could extract that into a method.

static void AddToElement(XElement outerParent, string innerParent, string name, object value)
{
    XElement inner = outerParent.Element(innerParent);
    if (inner == null)
    {
        inner = new XElement(innerParent);
        outerParent.Add(inner);
    }

    inner.Add(new XElement(name, value));
}

Simple test

string xml = @"<project> 
                    <logRotator> 
                    <numToKeep>13</numToKeep> 
                    </logRotator> 
                </project>";

XDocument document = XDocument.Parse(xml);
XElement project = document.Element("project");
AddToElement(project, "logRotator", "daysToKeep", 10);

Console.WriteLine(document.ToString());

Answer 3

With the kickstart from everyone here I ended up doing this and it works. It is far from elegant, but it gets the job done

/// <summary>
/// adds an element to an xml tree and builds the tree up if it is needed
/// </summary>
/// <param name="outerParent">root node</param>
/// <param name="innerPath">root/childOne/innerParent/name</param>
/// <param name="name">element to add</param>
/// <param name="value">elements value</param>
static XElement BuildTree(XElement outerParent, string innerParentPath, string name, object value)
{
    List<string> s = innerParentPath.Split('/').ToList(); 
    string str = "";
    XElement prevInner = null;
    if (s.Count != 2)//use 2 since we know the root will always be there
    {
        var t = new List<string>(s);
        t.RemoveRange(s.Count - 1, 1);//remove last element
        string[] sa = t.ToArray();
        str = string.Join("/", sa);
        prevInner = BuildTree(outerParent, str, name, value);//call recursively till we get to root;
    }
    else
    {
        prevInner = outerParent;
    }
    XElement inner = prevInner.Element(s[s.Count - 1]);
    if (inner == null)
    {
        if (s[s.Count - 1] == name)//add actual element if we're at top lvl.
        {
            prevInner.Add(new XElement(name, value));
        }
        else
        {
            inner = new XElement(s[s.Count - 1]);
            prevInner.Add(inner);
        }
    }
    return inner;
}