using System;

using System.Linq;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"

            {

              ""dir"": {

                ""nested_dir"": {

                  ""file.txt"": ""foo.bar""

                }

              }

            }";

        JObject root = JObject.Parse(json);

        JToken file = root["dir"]["nested_dir"]["file.txt"];

        Console.WriteLine(string.Join("/", file.PathAsArray()));

    }

}

​

public static class JsonExtensions

{

    public static string[] PathAsArray (this JToken token)

    {

        return token.AncestorsAndSelf()

                    .OfType<JProperty>()

                    .Select(p => p.Name)

                    .Reverse()

                    .ToArray();

    }

}