using System;

using System.Text.Json;

using System.Text.Encodings.Web;

​

public class Program

{

    public static void Main()

    {

        JsonElement element = JsonDocument.Parse("{\"key1\": \n \"foo+bar\"}").RootElement;

        JsonSerializerOptions options = new JsonSerializerOptions()

        {

            Encoder = JavaScriptEncoder.UnsafeRelaxedJsonEscaping, 

        };

​

        Console.WriteLine(element.ToString());

        Console.WriteLine(JsonSerializer.Serialize(element));

        Console.WriteLine(JsonSerializer.Serialize(element, options));

    }

}