Program.cs

using System;
using System.Linq;
using System.Xml.Linq;


public class Program
{
    static void Main()
    {    
        
        XNamespace ns = "urn:iso:std:iso:20022:tech:xsd:camt.054.001.04";
        XDocument doc = XDocument.Load("camt054.xml"); // FILE NAME. 
        var notifications = doc
            .Descendants(ns + "Ntfctn")
            .Select(n => new
            {
                Account = n.Element(ns + "Acct")?
                             .Element(ns + "Id")?
                             .Element(ns + "IBAN")?.Value,
                Transactions = n.Elements(ns + "Ntry")
                                .Select(entry => new
                                {
                                    Amount = entry.Element(ns + "Amt")?.Value,
                                    Currency = entry.Element(ns + "Amt")?.Attribute("Ccy")?.Value,
                                    CreditOrDebit = entry.Element(ns + "CdtDbtInd")?.Value,
                                    Date = entry.Element(ns + "BookgDt")?.Element(ns + "Dt")?.Value,
                                    Message = entry.Descendants(ns + "Ustrd").FirstOrDefault()?.Value, // Här kan vi läsa ut fakturanummret om det skickas med.
                                    CurrencyRate = entry.Descendants(ns + "NtryDtls").Descendants(ns + "TxDtls").Descendants(ns + "AmtDtls").Descendants(ns + "TxAmt").Descendants(ns + "CcyXchg").Elements(ns + "XchgRate").FirstOrDefault()?.Value,                                     
                                }).ToList()
                //CustomerDetails = n.Element(ns + "")    
            });

        foreach (var n in notifications)
        {
            Console.WriteLine($"Acccount: {n.Account}");
            Console.WriteLine("Transactions:");
            foreach (var t in n.Transactions)
            {
                Console.WriteLine($"  Date: {t.Date}");
                Console.WriteLine($"  Payment type: {t.CreditOrDebit}");
                Console.WriteLine($"  Amount: {t.Amount} {t.Currency}");
                Console.WriteLine($"  Currency rate: {t.CurrencyRate}");
                Console.WriteLine($"  Message: {t.Message}");                
                Console.WriteLine();

                if (t.Message != null)
                {
                    var possibleInvoice = SearchForInvoiceInMessage(t.Message);
                    if(possibleInvoice != null)
                    {
                        Console.WriteLine($"    Possible invoice: {possibleInvoice}");
                    }
                }
            }
        }


    }

    private static object SearchForInvoiceInMessage(string message)
    {
        int letterCount = 0;
        var index = 0;
        var startindex = 0;
        foreach (char c in message)
        {
            // letterCount++;
            if (!char.IsWhiteSpace(c))
            {
                letterCount++;
            }
            else
            {                
                letterCount = 0;
                startindex = index + 1;
            }
            if (letterCount == 13)
            {
                var candidate = message.Substring(startindex, 13);
                var isInvoiceNumber = CheckInvoiceNumber(candidate);
                if (isInvoiceNumber)
                {
                    return candidate;
                }
            }
            index++;
        }
        return null;
    }

    private static bool CheckInvoiceNumber(string candidate)
    {
        // EXAMPLE INVOICE NUMBER: D25BCF99-0005
        if (!candidate.Contains('-')) return false;
        if(candidate.ElementAt(8) != '-') //

        for (int i = 0; i < 8; i++)
        {
            if (!char.IsLetterOrDigit(candidate[i]))
                return false;
        }
        for (int i = 9; i < 13; i++)
        {
            if (!char.IsDigit(candidate[i]))
                return false;
        }
        return true;
    }
}