using System;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"

        {

            ""Foo"": {

                ""P1"": null,

                ""P2"": ""hello world"",

                ""P3"": null,

                ""P4"": {

                    ""P1"": 1,

                    ""P2"": null,

                    ""P3"": null

                },

                ""FooArray"": [

                    {

                        ""F1"": null,

                        ""F2"": null,

                        ""F3"": null

                    }

                ]

            },

            ""Bar"": null

        }";

        JToken token = JsonHelper.RemoveEmptyChildren(JToken.Parse(json));

        Console.WriteLine(token.ToString(Formatting.Indented));

    }

}

​

​

public static class JsonHelper

{

    public static JToken RemoveEmptyChildren(JToken token)

    {

        if (token.Type == JTokenType.Object)

        {

            JObject copy = new JObject();

            foreach (JProperty prop in token.Children<JProperty>())

            {

                JToken child = prop.Value;

                if (child.HasValues)

                {

                    child = RemoveEmptyChildren(child);

                }

                if (!IsEmpty(child))

                {

                    copy.Add(prop.Name, child);

                }

            }

            return copy;

        }

        else if (token.Type == JTokenType.Array)

        {

            JArray copy = new JArray();

            foreach (JToken item in token.Children())

            {

                JToken child = item;

                if (child.HasValues)

                {

                    child = RemoveEmptyChildren(child);

                }

                if (!IsEmpty(child))

                {

                    copy.Add(child);

                }

            }

            return copy;

        }

        return token;

    }

​

    public static bool IsEmpty(JToken token)

    {

        return (token.Type == JTokenType.Null) ||

               (token.Type == JTokenType.Array && !token.HasValues) ||

               (token.Type == JTokenType.Object && !token.HasValues);

    }

}