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 XmlNodeList xnList = doc.SelectNodes("/Setting/RS232Gerät/RS232");					

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XDocument xDoc = XDocument.Load(/*MyFilePath*/);
var xEle = xDoc.Root.AddElement(/*stuff*/);
//...					

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<Setting>
  <IECBus>
    <IECGerät>
      <Name>xyz</Name>
      <Adresse>17</Adresse>
    </IECGerät>
    <IECGerät>
      <Name>abc</Name>
      <Adresse>10</Adresse>
    </IECGerät>
    <RS232>
      <Port>COM1</Port>
      <Nummer>1</Nummer>
    </RS232>
    <RS232>
      <Port>COM7</Port>
      <Nummer>7</Nummer>
    </RS232>
  </IECBus>
</Setting>					

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string str = comboBox2.Text;
            if (str == "") return;

            alterName = str;
            expr = nav.Compile("/Setting/IECBus/IECGerät[@Typ='" + str + "']");
            iterator = nav.Select(expr);//wählt knoten

            if (iterator.MoveNext())
            {
                XPathNavigator nav2 = iterator.Current.Clone();
                nav2.MoveToFirstChild();

                nav2.MoveToNext();
                txtName.Text = nav2.Value;
                nav2.MoveToNext();
                txtAdresse.Text = nav2.Value;
            }					

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<Setting>
  <IECBus>
    <IECGerät>
      <Typ>IEC</Typ>
      <Name>NI</Name>
      <Adresse>17</Adresse>					

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 <IECGerät Name="ABC" Adresse="123">
    </IECGerät>					

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XmlDocumentFragment docFrag = doc.CreateDocumentFragment();
                docFrag.InnerXml = "<IECGerät Name=\"" + txtName.Text + "Adresse=\"" + txtAdresse.Text +"\">" +
                 "</IECGerät>";					

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 XmlDocumentFragment docFrag = doc.CreateDocumentFragment();
                docFrag.InnerXml = "<IECGerät Name=\"" + txtName.Text + "\" Adresse=\"" + txtAdresse.Text + "\">" +
                 "</IECGerät>";					

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try 
      { 
        XmlTextReader reader = new XmlTextReader(FILE_NAME);
        XmlDocument doc = new XmlDocument(); 
        doc.Load(reader);
        reader.Close();
        
        //Typ wählen
        XmlNode oldCd;
        XmlElement root = doc.DocumentElement;
        oldCd = root.SelectSingleNode("Setting/IECBus/IECGerät[Typ='" + alterName + "']");

        XmlElement newCd = doc.CreateElement("IECGerät");
        newCd.SetAttribute("Name",txtName.Text + "Adresse", txtAdresse.Text);

               
        root.ReplaceChild(newCd, oldCd);
        //Speichern 
        doc.Save(FILE_NAME); 
        this.DialogResult = DialogResult.OK;
      } 
      catch (Exception ex) 
      { 
        MessageBox.Show("Exception: {0}", ex.ToString());
        this.DialogResult = DialogResult.Cancel;
      }					

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    public class Settings
    {
        public List<Device> Devices { get; set; }

        public Settings()
        {
            Devices = new List<Device>();
        }
    }					

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    public abstract class Device
    {
    }					

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    public class RS232Device : Device
    {
        public string Port { get; set; }

        public int PortNumber { get; set; }
    }

    public class IECDevice : Device
    {
        public string Name { get; set; }

        public string Adresse { get; set; }
    }					

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        static void Main(string[] args)
        {
            Settings settings = new Settings();

            settings.Devices.Add(new IECDevice
            {
                Adresse = "17",
                Name = "xyz"
            });

            settings.Devices.Add(new IECDevice
            {
                Adresse = "10",
                Name = "abc"
            });

            settings.Devices.Add(new RS232Device
            {
                Port = "COM1",
                PortNumber = 1
            });

            settings.Devices.Add(new RS232Device
            {
                Port = "COM7",
                PortNumber = 7
            });

            XmlSerializer serializer = new XmlSerializer(typeof(Settings));

            serializer.Serialize(File.OpenWrite(@"C:\settings.xml"), settings);
}					

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    [XmlInclude(typeof(IECDevice))]
    [XmlInclude(typeof(RS232Device))]
    public abstract class Device
    {
    }					

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    public class RS232Device : Device
    {
        [XmlElement(ElementName = "Port")]
        public string Port { get; set; }

        [XmlElement(ElementName = "Nummer")]
        public int PortNumber { get; set; }
    }					

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 XmlNodeList xnList = doc.SelectNodes("/Setting/RS232Gerät/RS232");