環境

VisualStudio 2015

xmltojson.cs

using Newtonsoft.Json;
using System.IO;
using System.Xml;

namespace Lib
{
    class XmlToJson
    {
        static public void ToJson(string path_to_xml, string path_to_json)
        {
            XmlDocument doc = new XmlDocument();
            doc.LoadXml(File.ReadAllText(path_to_xml));
            File.WriteAllText(path_to_json, JsonConvert.SerializeXmlNode(doc));
        }
    }
}

Usage

static void Main(string[] args)
{
    XmlToJson.ToJson("hoge.xml", "hoge.json");
}

変換元のxml

<?xml version="1.0" encoding="utf-8"?>
<persons>
  <person id="1">
    <firstname>イチキータ</firstname>
    <lastname>山田</lastname>
    <birthday>2001/10/01</birthday>
  </person>
  <person id="2">
    <firstname>ニキータ</firstname>
    <lastname>山田</lastname>
    <birthday>2004/11/09</birthday>
  </person>
</persons>

変換後のjson

{
  "?xml": {
    "@version": "1.0",
    "@encoding": "utf-8"
  },
  "persons": {
    "person": [
      {
        "@id": "1",
        "firstname": "イチキータ",
        "lastname": "山田",
        "birthday": "2001/10/01"
      },
      {
        "@id": "2",
        "firstname": "ニキータ",
        "lastname": "山田",
        "birthday": "2004/11/09"
      }
    ]
  }

}

所感

C#すっげえ。