using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Runtime.Serialization.Formatters;

using System.IO;

using System.Data;

using System.ComponentModel.DataAnnotations;

using System.Globalization;

using System.Reflection;

using System.Text.RegularExpressions;

​

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/58931339/change-property-name-in-json-string

​

    public class NameRemappingJsonWriter : JsonTextWriter

    {

        readonly Func<string, int, string, string> map;

        public NameRemappingJsonWriter(TextWriter textWriter, Func<string, int, string, string> map) : base(textWriter)

        {

            if (map == null)

                throw new ArgumentNullException(nameof(map));

            this.map = map;

        }

        public override void WritePropertyName(string name)

        {

            base.WritePropertyName(map(name, Top, Path));

        }       

        public override void WritePropertyName(string name, bool escape)

        {

            base.WritePropertyName(map(name, Top, Path), escape);

        }       

    }

​

    public static partial class JsonExtensions

    {

        public static void StreamRenameJsonProperties(string inFilePath, string outFilePath, string oldName, Regex parentPathRegex, string newName, Formatting formatting = Formatting.Indented)

        {

            Func<string, int, string, string> map = (name, depth, parentPath) =>

            {

                if (name == oldName && parentPathRegex.IsMatch(parentPath))

                    return newName;

                return name;

            };

            StreamRenameJsonProperties(inFilePath, outFilePath, map, formatting);

        }

        public static void StreamRenameJsonProperties(string inFilePath, string outFilePath, Func<string, int, string, string> map, Formatting formatting = Formatting.Indented)

        {

            using (var textReader = new StreamReader(inFilePath))

            using (var jsonReader = new JsonTextReader(textReader) { DateParseHandling = DateParseHandling.None })

            {

                using (var textWriter = new StreamWriter(outFilePath))

                using (var jsonWriter = new NameRemappingJsonWriter(textWriter, map) { Formatting = formatting })

                {

                    jsonWriter.WriteToken(jsonReader);

                }

            }

        }

    }

​

    class TestClass

    {

        public static void Test()

        {

            var inPath = "Question58931339.Input.json";

            var outPath = "Question58931339.Output.json";

​

            try

            {

                File.WriteAllText(inPath, GetJson());

                Test(inPath, outPath);

                Console.WriteLine("Output JSON:");

                Console.WriteLine(File.ReadAllText(outPath));

            }

            finally

            {

                File.Delete(inPath);

                File.Delete(outPath);

            }

        }   

​

        static void Test(string inFilePath, string outFilePath, Formatting formatting = Formatting.Indented)

        {

            JsonExtensions.StreamRenameJsonProperties(inFilePath, outFilePath, "name", new Regex("profiles\\[[0-9]+\\]$"), "profileName");

        }

        static string GetJson()

        {

            var json = @"[

    {

        ""name"": ""name 1"",

        ""profiles"":[{

            ""name"": ""profile 1.1""

            },{

            ""name"": ""profile 1.2""

        }]

    },{

        ""name"": ""name 2"",

        ""profiles"":[{

            ""name"": ""profile 2.1""

            }]

    }

]

";

            return json;

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Environment version: {0} ({1})", System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription , GetNetCoreVersion());

            Console.WriteLine("Json.NET version: " + typeof(JsonSerializer).Assembly.FullName);

            Console.WriteLine();

​

            try

            {

                TestClass.Test();

            }

            catch (Exception ex)

            {

                Console.WriteLine("Failed with unhandled exception: ");

                Console.WriteLine(ex);

                throw;

            }

        }

        public static string GetNetCoreVersion()

        {

            //https://techblog.dorogin.com/how-to-detect-net-core-version-in-runtime-ecd65ad695be

            //

            var assembly = typeof(System.Runtime.GCSettings).GetTypeInfo().Assembly;

            var assemblyPath = assembly.CodeBase.Split(new[] { '/', '\\' }, StringSplitOptions.RemoveEmptyEntries);

            int netCoreAppIndex = Array.IndexOf(assemblyPath, "Microsoft.NETCore.App");

            if (netCoreAppIndex > 0 && netCoreAppIndex < assemblyPath.Length - 2)

                return assemblyPath[netCoreAppIndex + 1];

            return null;

        }

    }

​

#if false

So is your question, *How can I rename the properties `..profiles[*].name` to `..profiles[*].profileName`using some sort of streaming transformation, without loading the entire JSON into memory?*  

Because you can do it easily with [tag:json.net] if you load the entire thing into memory, see https://dotnetfiddle.net/W46243

#endif

​