In case it helps anyone, further to meda's reply. Here's how you make this work with XElement rather than xmlTextWriter and XDocument

XNamespace ns = "http://james.newtonking.com/projects/json";
var items = new XElement("items",new XAttribute(XNamespace.Xmlns+"json",ns));

   items.Add(new XElement("item",new XAttribute(ns+"Array",true),
                        new XElement("name", "name"),
                        new XElement("Detail", "detail")));

then to convert it

 XmlDocument doc = new XmlDocument();
            doc.LoadXml(items.ToString());
            var converted JsonConvert.SerializeXmlNode(doc);

If anybody looking for converting the XML to Json, the below method can be used. It encloses all JSONObject inside a JSONArray.

public JSONObject getJSONFromXML(String xml) {
    JSONObject jsonObject = null;
    try {
        jsonObject = XML.toJSONObject(xml);
        convertJsonObjectToArray(jsonObject);
    } catch (JSONException e) {
        System.out.println("Error in parsing JSON From xml: " + e);
    }
    return jsonObject;
}

private void convertJsonObjectToArray(JSONObject jsonObject) throws JSONException {
    for (Object key : jsonObject.keySet()) {
        Object val = jsonObject.get(key.toString());
        if (val instanceof JSONObject) {
            convertJsonObjectToArray((JSONObject) val);
            JSONArray jsonArray = new JSONArray();
            jsonArray.put(val);
            jsonObject.put(key.toString(), jsonArray);
        } else if (val instanceof JSONArray) {
            JSONArray jsonArray = (JSONArray) val;
            for (int i = 0; i < jsonArray.length(); i++) {
                JSONObject obj = (JSONObject) jsonArray.get(i);
                convertJsonObjectToArray(obj);
            }
        }
    }
}

It shouldn't impact too much in your performance if you create an array of objects in your code from your XML doc (you need to know the content structure and use POCO objects) and then serialize that list with json. Makes it sense in your case?

https://github.com/abdmob/x2js - my own library (updated URL from http://code.google.com/p/x2js/):

This library provides XML to JSON (JavaScript Objects) and vice versa javascript conversion functions. The library is very small and doesn't require any other additional libraries.

API functions

• new X2JS() - to create your instance to access all library functionality. Also you could specify optional configuration options here
• X2JS.xml2json - Convert XML specified as DOM Object to JSON
• X2JS.json2xml - Convert JSON to XML DOM Object
• X2JS.xml_str2json - Convert XML specified as string to JSON
• X2JS.json2xml_str - Convert JSON to XML string

Online Demo on http://jsfiddle.net/abdmob/gkxucxrj/1/

var x2js = new X2JS();
function convertXml2JSon() {
    $("#jsonArea").val(JSON.stringify(x2js.xml_str2json($("#xmlArea").val())));
}

function convertJSon2XML() {
    $("#xmlArea").val(x2js.json2xml_str($.parseJSON($("#jsonArea").val())));
}

convertXml2JSon();
convertJSon2XML();
$("#convertToJsonBtn").click(convertXml2JSon);
$("#convertToXmlBtn").click(convertJSon2XML);

Based on Mario's solution, I made some changes, specially for nested children in the Json object. The xmlJSONObj is the same, but the obj param is a List that contains the path to the JSON child, and the index param is only required for the recursion (it should start at 0 in the first execution).

/**
       * Forces a JSON Object to be an Array. When converting from XML to JSON, There may be cases in
       * which we want a list of elements in the final JSON but there is only one element in the XML
       * file, so the XML.toJSONObject method will fail. This methods forces a JSON element to be an
       * array instead of just a JSON Object.
       * 
       * @param xmlJSONObj obtained by passing the XML through the method toJSONObject
       * @param obj list of String that contains the path to the JSON child to convert to Array
       * @param index required for recursion, starts at 0
       * @throws org.json.JSONException
       */
      public static void forceToJSONArray(JSONObject xmlJSONObj, List<String> obj, int index)
          throws org.json.JSONException {
        Object myObj = xmlJSONObj.opt(obj.get(index));
        if (myObj instanceof JSONObject && obj.size() == index + 1) {
          JSONObject myJSONObj = (JSONObject) myObj;
          JSONArray jsonArray = new JSONArray();
          jsonArray.add(myJSONObj);
          xmlJSONObj.put(obj.get(index), jsonArray);
        } else if (myObj instanceof JSONObject) {
          forceToJSONArray((JSONObject) myObj, obj, index + 1);
        }
      }

Example:

We have this JSON:

{
   "person":{
      "name":"Tony",
      "data":{
         "car":"Toyota"
      }
   }
}

And we want this JSON:

{
   "person":{
      "name":"Tony",
      "data":{
         "car":["Toyota"]
      }
   }
}

We should call the method like this:

forceToJSONArray(xmlJSONObj, Arrays.asList("person", "data", "car"), 0);