Xml Element not present and list null and Object reference not set to an instance of an object

Question

I am retireving data from xml field, there are nodes that may not exist (because the xml file is generated dynamically) . the problem is when I look for a node and it is not pre

Answer 1

From code you provided I can say, that you are passing null to getXmlComptes method (or to getXmlTransactions if it is called elsewhere).

I suggest you to cast elements:

public static List<Transaction> getXmlTransactions(XElement n)
{
   if (n == null)
       throw new ArgumentNullException("Konto is null");

    var transactions = from t in n.Elements("Transaktion")
                       select new Transaction {
                           TransID = (string)t.Element("TransID"),
                           TypeTransaction = (string)t.Element("TransArt"),
                           DateEntree = (string)t.Element("BuchDat"),
                           Montant = (string)t.Element("BetragWAE"),
                           Devise = (string)t.Element("BuchDat"),
                           Pays = (string)t.Element("GegenLandText"),
                           AbreviationPays = (string)t.Element("GegenLand"),
                           autresinfo = (string)t.Element("Kommentar")
                       };

    return transactions.ToList();
}

And second method:

public static List<Compte> getXmlComptes(XElement n)
{
   if (n == null)
       throw new ArgumentNullException("Something is null");

    var comptes = from k in n.Elements("Konto")
                  select new Compte
                  {
                      NumCompte = (string)k.Element("KtoNr"),
                      typeCompte = (string)k.Element("KontoArt"),
                      DateOuverture = (string)k.Element("KtoOeff"),
                      IBAN = (string)k.Element("IBAN"),
                      Devise = (string)k.Element("Waehrung"),
                      CommentairesCompte = (string)k.Element("Kommentar"),
                      Trans = getXmlTransactions(k)
                  };

    return comptes.ToList();
}

Run this code and you'll see were you are passing null. If you need to assign empty string to some property if element is missing in xml, then use null-coalescing operator:

TypeTransaction = (string)t.Element("TransArt") ?? ""

But I would keep property value as null to show that it was not in xml.