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/Program.cs

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
					
public class Program
{
	public static void Main()
	{
		var source = new List<TreeModel>
		{
			new TreeModel
			{
				ChangeFlowsFromParent = false,
				ChangeFlowsToParent = true,
				StreamType = "mainine",
				StreamName = "ArgOS_2_0",
				Parent = null,
				Compliance = "Released",
			},
			new TreeModel
			{
				ChangeFlowsFromParent = true,
				ChangeFlowsToParent = true,
				StreamType = "Release",
				StreamName = "ArgOS_2_0_DHAL1",
				Parent = "ArgOS_2_0",
				Compliance = "Released",
			},
			new TreeModel
			{
				ChangeFlowsFromParent = false,
				ChangeFlowsToParent = true,
				StreamType = "Release",
				StreamName = "ArgOS_2_0_child_DHAL2",
				Parent = "ArgOS_2_0_DHAL1",
				Compliance = "Released",
			},
			new TreeModel
			{
				ChangeFlowsFromParent = false,
				ChangeFlowsToParent = true,
				StreamType = "Release",
				StreamName = "ArgOS_child_Gen2",
				Parent = "ArgOS_2_0_child_DHAL2",
				Compliance = "Released",
			},
			new TreeModel
			{
				ChangeFlowsFromParent = true,
				ChangeFlowsToParent = true,
				StreamType = "Release",
				StreamName = "ArgOS_2_0_DHAL2",
				Parent = "ArgOS_2_0",
				Compliance = "Released",
			},
			new TreeModel
			{
				ChangeFlowsFromParent = true,
				ChangeFlowsToParent = true,
				StreamType = "Release",
				StreamName = "ArgOS_2_0_DHAL3",
				Parent = "ArgOS_2_0",
				Compliance = "Released",
			},
		};
		var output = MapToTreeModelJsonCollection(source);
		var json = JsonConvert.SerializeObject(output,Formatting.Indented);
		Console.WriteLine(json);
	}
	
	static ICollection<TreeModelJson> MapToTreeModelJsonCollection(ICollection<TreeModel> source)
	{
		// map all items
		var allItems = source.Select(e => new TreeModelJson
									 {
										 ChangeFlowsFromParent = e.ChangeFlowsFromParent.ToString().ToLower(),
										 ChangeFlowsToParent = e.ChangeFlowsToParent.ToString().ToLower(),
										 Compliance = e.Compliance,
										 Parent = e.Parent ?? "none",
										 StreamName = e.StreamName,
										 StreamType = e.StreamType,
									 }).ToList();
		
		// build tree structure
		foreach (var item in allItems)
		{
			var children = allItems.Where(e => e.Parent == item.StreamName).ToList();
			if (children.Any())
			{
				item.Children = children;
			}
		}
		
		// return only root items
		return allItems.Where(e => e.Parent == "none").ToList();
	}

	
    class TreeModel
    {
        public string StreamName { get; set; }
        public string ParentName { get; set; }
        public string StreamType { get; set; }
        public bool ChangeFlowsFromParent { get; set; }
        public bool ChangeFlowsToParent { get; set; }
        public string Compliance { get; set; }
        public string Parent { get; set; }
    }

    class TreeModelJson
    {
        [JsonProperty("ChangeFlowsFromParent")]
        public string ChangeFlowsFromParent { get; set; }
        [JsonProperty("ChangeFlowsToParent")]
        public string ChangeFlowsToParent { get; set; }
        [JsonProperty("StreamType")]
        public string StreamType { get; set; }
        [JsonProperty("streamName")]
        public string StreamName { get; set; }
        [JsonProperty("Parent")]
        public string Parent { get; set; }
        [JsonProperty("Compliance")]
        public string Compliance { get; set; }
        [JsonProperty("children", NullValueHandling = NullValueHandling.Ignore)]
        public ICollection<TreeModelJson> Children { get; set; }
    }
	
}