https://stackoverflow.com/questions/77834272/create-newtonsoft-json-linq-jarray-from-another-array-with-different-structure

CSharp · Console

/Program.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;

using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using Newtonsoft.Json.Converters;
using Newtonsoft.Json.Serialization;	

using NUnit.Framework; // Throws NUnit.Framework.AssertionException

//https://stackoverflow.com/questions/77834272/create-newtonsoft-json-linq-jarray-from-another-array-with-different-structure

    class TestClass
    {
        public static void Test()
        {
			var jsonString = GetJson();
			
			// Parse (or load from a stream) the incoming JSON as a JArray
			var array = JArray.Parse(jsonString);
			
			// For each object in the array, query they Key/Value properties of each Fields[*] object and turn then into a single JPropery, 
			// then add those properties to a new JObject
			var query = array.Select(i => 
				new JObject(i.SelectTokens("Fields[*]").Select(o => new JProperty((string)o["Key"], o["Value"]))));

			// Create a new JArray from the query
			var newArray = new JArray(query);
			
			Console.WriteLine(newArray);
			
			Assert.That(JToken.DeepEquals(JArray.Parse(GetRequiredJson()), newArray));

			Func<JToken, JObject> f = i => new JObject(i.SelectTokens("Fields[*]").Select(o => 
				new JProperty((string)o["Key"], o["Value"])));
		}	
		
		static string GetJson() =>
			"""
			[
			  {
				"Fields": [
				  {
					"Value": "child",
					"Key": "Relationship"
				  },
				  {
					"Value": "Alex",
					"Key": "FirstName"
				  }
				]
			  }
			]
			""";
			
		static string GetRequiredJson() =>
			"""
			[
			  {
				"Relationship": "child",
				"FirstName": "Alex"
			  }
			]			
			""";
	}

	public class Program
	{
		public static void Main()
		{
			Console.WriteLine("Environment version: {0} ({1}), {2}", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , Environment.Version, Environment.OSVersion);
			Console.WriteLine("{0} version: {1}", typeof(JsonSerializer).Namespace, typeof(JsonSerializer).Assembly.FullName);
			Console.WriteLine();

			try
			{
				TestClass.Test();
			}
			catch (Exception ex)
			{
				Console.WriteLine("Failed with unhandled exception: ");
				Console.WriteLine(ex);
				throw;
			}
		}
	}