using System;

using System.Linq;

using System.Xml.Linq;

public class Program

{

    public static void Main()

    {

        var 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>";

        xml = SetOutlineInfo(xml, 1, "eng", true, DateTime.Parse("2023-09-12"), "1");

        Console.WriteLine(xml);

    }

    public static string SetOutlineInfo(string talksdatabase, int talknumber, string languageCode, bool talkExcluded, DateTime dateExcudedFrom, string excludedNote)

    {

        XDocument doc = XDocument.Parse(talksdatabase);

        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);

            }

​

            return doc.ToString();

        }

        else

            return talksdatabase;

    }

}