i think i will do it so
internal System.Xml.XmlNodeList GetElementsByQuery(string sqlQuery)
{
// Festlegen Verlauf der ProgressBar
string str = this.repository.SQLQuery(sqlQuery);
System.Xml.XmlDocument doc = new System.Xml.XmlDocument();
doc.Load(new StringReader(str));
System.Xml.XmlElement document = doc.DocumentElement;
return doc.SelectNodes("//Object_ID");
}