Adding a new entry with Linq to XML at the right position (based on numerical attribute)

Question

I have come up with this function to save or update an entry in the XML file.

public void SetOutlineInfo(string talksdatabase, int talknumber, string languageCode, bool talkExcluded, DateTime dateExcudedFrom, string excludedNote)
{
    try
    {
        XDocument doc = XDocument.Load(talksdatabase);
        var ptLang = doc.Descendants(languageCode).FirstOrDefault();
        if (ptLang != null)
        {
            var ptInfo = ptLang
                            .Descendants("PublicTalk")
                            .Where(p => p.Attribute("Number").Value == talknumber.ToString()).FirstOrDefault();
            if (ptInfo != null)
            {
                ptInfo.Attribute("Excluded").Value = talkExcluded ? "true" : "false";
                ptInfo.Attribute("ExcludedFromDate").Value = dateExcudedFrom.ToString("yyyy-MM-dd");
                ptInfo.Attribute("ExcludedNote").Value = excludedNote;
            }
            else
            {
                ptLang.Add(new XElement("PublicTalk",
                    new XAttribute("Number", talknumber),
                    new XAttribute("Excluded", talkExcluded),
                    new XAttribute("ExcludedFromDate", dateExcudedFrom.ToString("yyyy-MM-dd")),
                    new XAttribute("ExcludedNote", excludedNote)
                    ));
            }

            doc.Save(talksdatabase);
        }
    }
    catch (Exception ex)
    {
        SimpleLog.Log(ex);
    }
}

The only issue is that adding a new entry does not add it in the correct numerical position (the Number attribute).

I did some searching and they all show reading in records and sorting with orderby. But I am writing records.

---

As requested in the comments.

Here is an XML:

<?xml version="1.0" encoding="utf-8"?>
<PublicTalkTitles>
  <eng>
    <PublicTalk Number="47" Excluded="true" ExcludedFromDate="2019-04-01" ExcludedNote="S-147 19.03" />
  </eng>
</PublicTalkTitles>

When a add a new entry, say talk number 1:

<?xml version="1.0" encoding="utf-8"?>
<PublicTalkTitles>
  <eng>
    <PublicTalk Number="1" Excluded="true" ExcludedFromDate="2023-09-12" ExcludedNote="1" />
    <PublicTalk Number="47" Excluded="true" ExcludedFromDate="2019-04-01" ExcludedNote="S-147 19.03" />
  </eng>
</PublicTalkTitles>

Answer 1

You need to first filter for all nodes which are <= talknumber then take MaxBy that number. You need to parse it to an int in order to do this filtering and sorting correctly.

Then that node is either:

• the one you want
  • Change it
• or the one before it
  • Use AddBeforeSelf
• or null
  • Use AddFirst on the parent.

---

    var ptLang = doc.Element("PublicTalkTitles").Element(languageCode);
    if (ptLang != null)
    {
        var pt = ptLang.Elements("PublicTalk");
        var ptInfo = pt
            .Where(p => int.Parse(p.Attribute("Number").Value) <= talknumber)
            .MaxBy(p => int.Parse(p.Attribute("Number").Value));
        if (int.Parse(ptInfo?.Attribute("Number").Value ?? "0") == talknumber)
        {
            ptInfo.Attribute("Excluded").Value = talkExcluded ? "true" : "false";
            ptInfo.Attribute("ExcludedFromDate").Value = dateExcudedFrom.ToString("yyyy-MM-dd");
            ptInfo.Attribute("ExcludedNote").Value = excludedNote;
        }
        else
        {
            var newElem = new XElement("PublicTalk",
                                    new XAttribute("Number", talknumber),
                                    new XAttribute("Excluded", talkExcluded),
                                    new XAttribute("ExcludedFromDate", dateExcudedFrom.ToString("yyyy-MM-dd")),
                                    new XAttribute("ExcludedNote", excludedNote)
                                   );
            if (ptInfo != null)
                ptInfo.AddAfterSelf(newElem);
            else
                ptLang.AddFirst(newElem);
        }
    }

dotnetfiddle

If you don't have MaxBy you can instead use OrderByDescending.FirstOrDefault which is less efficient.

var ptInfo = pt
    .Where(p => int.Parse(p.Attribute("Number").Value) <= talknumber)
    .OrderByDescending(p => int.Parse(p.Attribute("Number").Value))
    .FirstOrDefault();