using System;

using Newtonsoft.Json;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        var json = @"{""operation"": {

            ""type"": ""ADD"",

            ""args"": [

                {

                    ""type"": ""ADD"",

                    ""args"": [

                        {

                            ""columnName"": ""SomeColumnB"",

                            ""row"": 12

                        },

                        {

                            ""columnName"": ""SomeColumnC"",

                            ""row"": 18

                        }

                    ]

                }

            ]

        }}";

        dynamic data = JsonConvert.DeserializeObject(json);

        Console.WriteLine(data.operation.type.ToString());

        Console.WriteLine(data.operation.args[0].args[0].columnName.ToString());

        Console.WriteLine((int)data.operation.args[0].args[0].row);

    }

}

​