using System;

using System.Text;

using System.Text.Json;

using System.IO;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var sourceJson = @"

{

  ""Title"": ""Startpage"",

  ""Links"": [

    {

      ""Id"": 10,

      ""Text"": ""Start"",

      ""Link"": ""/index""

    },

    {

      ""Id"": 11,

      ""Text"": ""Info"",

      ""Link"": ""/info""

    }

  ]

}";

        var updateJson = @"

{

  ""Head"": ""Latest news"",

  ""Links"": [

    {

      ""Id"": 11,

      ""Text"": ""News"",

      ""Link"": ""/news""

    },

    {

      ""Id"": 21,

      ""Text"": ""More News"",

      ""Link"": ""/morenews""

    }

  ]

}

";

        using var source = JsonDocument.Parse(sourceJson);

        using var update = JsonDocument.Parse(updateJson);

        using var stream = new MemoryStream();

        using var writer = new Utf8JsonWriter(stream);

        writer.WriteStartObject();

        // write non existing properties

        foreach (var prop in update.RootElement.EnumerateObject().Where(prop => !source.RootElement.TryGetProperty(prop.Name, out _)))

        {

            prop.WriteTo(writer);

        }

​

        // make updates for existing

        foreach (var prop in source.RootElement.EnumerateObject())

        {

            if (update.RootElement.TryGetProperty(prop.Name, out var overwrite))

            {

                writer.WritePropertyName(prop.Name);

                overwrite.WriteTo(writer);

            }

            else

            {

                prop.WriteTo(writer);

            }

        }

​

        writer.WriteEndObject();

        writer.Flush();

        var resultJson = Encoding.UTF8.GetString(stream.ToArray());

        Console.WriteLine(resultJson);

    }

}