using System;

using System.Text.Json;

using System.Text.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var options = new JsonSerializerOptions();

        options.Converters.Add(new EmptyStringToNullConverter());

        options.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;

        Console.WriteLine(JsonSerializer.Serialize(new Test(), options));

    }

}

​

public class Test

{

    public string NameNull {get; set;} = null;

    [JsonIgnore( Condition = JsonIgnoreCondition.WhenWritingDefault)]

    public string NameEmpty {get; set;} = "";

    public string NameOne {get; set;} = "One";

}

​

public class EmptyStringToNullConverter : JsonConverter<string>

{

    public override string? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

    {

        return reader.GetString();

    }

​

    public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)

    {

        if(!string.IsNullOrEmpty(value)) writer.WriteStringValue(value);

    /*

        if (value == string.Empty)

            writer.WriteNullValue();

        else

            writer.WriteStringValue(value);

            */

    }

}