Add child elements to XML

Question 1

you're adding the fields element every time. This isn't an XML issue, but a logic issue. Your problem is that you're adding hte "Fields" element inside the for loop. build all of your elements and then add them all to a single "Fields" element. Or, build the fields element first and then add the sub-elements to it. Here's a sample console app showing the difference.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml.Linq;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(Sum2().ToString());
            Console.WriteLine();
            Console.WriteLine(Sum2Working().ToString());
            Console.Read();
        }

        public static XElement Sum2()
        {

            XNamespace defaultNamespace = XNamespace.Get("http://appserver.weevio.se/schema/SDKr1/Fields.xsd");
            XElement test = new XElement(new XElement(defaultNamespace + "FieldsRoot"));

            string value = "1694.152344;1694.092285;1693.972168;1693.852051";
            string datum = "2013-07-10 20:00:00;2013-07-10 19:00:00;2013-07-10 18:00:00;2013-07-10 17:00:00";

            string[] valueA = value.Split(';');
            string[] datumA = datum.Split(';');

            int d = 0;
            foreach (var customer in valueA)
            {
                XElement xElement = new XElement(defaultNamespace + "Numeric",
                        new XAttribute("value", valueA[d]),
                        new XAttribute("datum", datumA[d]),
                        new XAttribute("status", 16)
                );
                test.Add(new XElement(defaultNamespace + "Fields", new XAttribute("ok", "true"), new XAttribute("nodeId", "TEST.HW"), xElement));
                d = d + 1;
            }
            return test;
        }

        public static XElement Sum2Working()
        {

            XNamespace defaultNamespace = XNamespace.Get("http://appserver.weevio.se/schema/SDKr1/Fields.xsd");
            XElement test = new XElement(new XElement(defaultNamespace + "FieldsRoot"));

            string value = "1694.152344;1694.092285;1693.972168;1693.852051";
            string datum = "2013-07-10 20:00:00;2013-07-10 19:00:00;2013-07-10 18:00:00;2013-07-10 17:00:00";

            string[] valueA = value.Split(';');
            string[] datumA = datum.Split(';');

            int d = 0;

            var fields = new List<XElement>();

            foreach (var customer in valueA)
            {
                XElement xElement = new XElement(defaultNamespace + "Numeric",
                        new XAttribute("value", valueA[d]),
                        new XAttribute("datum", datumA[d]),
                        new XAttribute("status", 16)
                );
                fields.Add(xElement);
                d = d + 1;
            }

            test.Add(new XElement(defaultNamespace + "Fields", new XAttribute("ok", "true"), new XAttribute("nodeId", "TEST.HW"), fields));

            return test;
        }
    }
}

As you can see, Sum2Working() adds the elements to a list and then adds the list to the larger element.

Question 2

You need to move the

test.Add(new XElement(defaultNamespace + "Fields", new XAttribute("ok", "true"), new XAttribute("nodeId", "TEST.HW"), xElement));

Outside the foreach loop in order to achieve that.

Try

[WebMethod]
public XElement Sum2()
{
XNamespace defaultNamespace = XNamespace.Get("http://appserver.weevio.se/schema/SDKr1/Fields.xsd");
XElement test = new XElement(new XElement(defaultNamespace + "FieldsRoot"));

string value = "1694.152344;1694.092285;1693.972168;1693.852051";
string datum = "2013-07-10 20:00:00;2013-07-10 19:00:00;2013-07-10 18:00:00;2013-07-10 17:00:00";

string[] valueA = value.Split(';');
string[] datumA = datum.Split(';');

int d = 0;

Xelement numericElement = new Xelement(); 
foreach (var customer in valueA)
{
    XElement xElement = new XElement(defaultNamespace + "Numeric",
            new XAttribute("value", valueA[d]),
            new XAttribute("datum", datumA[d]),
            new XAttribute("status", 16)
    );
    numericElement.Add(xElement);
    d = d + 1;
}
test.Add(new XElement(defaultNamespace + "Fields", new XAttribute("ok", "true"), new XAttribute("nodeId", "TEST.HW"), numericElement));
return test;

}