using System;

using System.Collections.Generic;

using System.Linq;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"

            {

              ""schoolConfig"": [

                {

                  ""schoolTypeCode"": ""C1"",

                  ""schools"": [

                    {

                      ""schoolId"": 456,

                      ""config"": [

                        {

                          ""name"": ""Classes"",

                          ""value"": [

                            {

                              ""id"": 1

                            },

                            {

                              ""id"": 2

                            }

                          ]

                        }

                      ]

                    },

                    {

                      ""schoolId"": 123,

                      ""config"": [

                        {

                          ""name"": ""Classes"",

                          ""value"": [

                            {

                              ""id"": 11

                            }

                          ]

                        }

                      ]

                    }

                  ]

                },

                {

                  ""schoolTypeCode"": ""C2"",

                  ""schools"": [

                    {

                      ""schoolId"": 50,

                      ""config"": [

                        {

                          ""name"": ""Classes"",

                          ""value"": [

                            {

                              ""id"": 12

                            }

                          ]

                        }

                      ]

                    },

                    {

                      ""schoolId"": 10,

                      ""config"": [

                        {

                          ""name"": ""Classes"",

                          ""value"": [

                            {

                              ""id"": 10

                            }

                          ]

                        }

                      ]

                    }

                  ]

                }

              ]

            }"; 

        dynamic jsonObj = Newtonsoft.Json.JsonConvert.DeserializeObject(json);

        JToken classes = jsonObj.SelectToken("$.schoolConfig[?(@.schoolTypeCode == 'C1')].schools[?(@.schoolId == 456)].config[?(@.name == 'Classes')]");

​

        JArray appList = (JArray)classes["value"];   // cast to JArray

​

        var itemToAdd = new JObject();

        itemToAdd["id"] = 5;

        appList.Add(itemToAdd);

​

        //classes["value"] = Newtonsoft.Json.JsonConvert.SerializeObject(appList).ToString();

​

        string output = Newtonsoft.Json.JsonConvert.SerializeObject(jsonObj, Newtonsoft.Json.Formatting.Indented);

        Console.WriteLine(output);      

    }

}