using System.Collections.Generic;

using System;

using System.Linq;

using System.Xml.Linq;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var source = new List<string>();

        source.Add("fid|sid|pd|pt");

        //2. comment this in and see 

        //source.Add("fid2|sid2|pd2|pt2");

        //1. when you do a FROM, you receive collections back, not single objects

        var headerElements = from str in source

            let fields = str.Split('|')

            select new XElement("Header",

                new XElement("FileId", fields[1]),//da calcolare

                new XElement("SenderId", fields[2]),

                new XElement("ProcessingDate", DateTime.Now.ToString("yyyyMMdd")),

                new XElement("ProcessingTime", DateTime.Now.ToString("HHmm"))

        );

        Console.WriteLine(JsonConvert.SerializeObject(headerElements));

        //now, if you want only one element, you could assume that source has only one entry, and 

        var headerElement = headerElements.FirstOrDefault();

        Console.WriteLine(JsonConvert.SerializeObject(headerElement));

        //BUT: in that case, solving that via LINQ makes absolutely no sense

    }

}