using System;

using System.Collections.Generic;

using System.Diagnostics.CodeAnalysis;

using System.Linq;

using Newtonsoft.Json;

​

    public class Section

    {

        public int SectionID { get; set; }

        public string SectionName { get; set; }

        public int? SubSectionID { get; set; }

        public string SubSectionName { get; set; }

        public int? TagID { get; set; }

        public string TagName { get; set; }

    }

​

​

public class Test

{

    public static void Main()

    {

            var sections = new List<Section>

            {

                new Section { SectionID = 1, SectionName = "Section 1", SubSectionID = null, SubSectionName = null        ,   TagID = 1, TagName = "Tag 1"},

                new Section { SectionID = 2, SectionName = "Section 2", SubSectionID = null, SubSectionName = null        ,   TagID = 2, TagName = "Tag 2"},

                new Section { SectionID = 3, SectionName = "Section 3", SubSectionID = 1   , SubSectionName = "SubSection 1", TagID = 3, TagName = "Tag 3"},

                new Section { SectionID = 3, SectionName = "Section 3", SubSectionID = 2   , SubSectionName = "SubSection 2", TagID = 3, TagName = "Tag 3"},

                new Section { SectionID = 3, SectionName = "Section 3", SubSectionID = 3   , SubSectionName = "SubSection 3", TagID = 3, TagName = "Tag 3"},

                new Section { SectionID = 4, SectionName = "Section 4", SubSectionID = null, SubSectionName = null        ,   TagID = 2, TagName = "Tag 2"},

                new Section { SectionID = 5, SectionName = "Section 5", SubSectionID = null, SubSectionName = null        ,   TagID = 1, TagName = "Tag 1"},

            };

​

            var result = sections

                .GroupBy(s => new {s.SectionID, s.SectionName, s.TagID, s.TagName})

                .Select(x => new

                {

                    x.Key.SectionID,

                    x.Key.SectionName,

                    x.Key.TagID,

                    x.Key.TagName,

                    SubSection = x.Count() == 1 ? null : x.Select(y => new

                    {

                        y.SubSectionID,

                        y.SubSectionName

                    }).ToArray()

                });

​

            var settings = new JsonSerializerSettings

            {

                NullValueHandling = NullValueHandling.Ignore,

                Formatting = Formatting.Indented

            };

            var json = JsonConvert.SerializeObject(result, settings);

            Console.WriteLine(json);

    }

}