using System;

using System.Collections.Generic;

using Newtonsoft.Json;

​

public class Program

{

    public static void Main()

    {

        var source = new Source();

        source.Items.Add(("a", "b"));

        source.Items.Add(("c", "d"));

        var settings = new JsonSerializerSettings{Converters = {new TupleConverter()}};

        var json = JsonConvert.SerializeObject(source, settings);

        Console.WriteLine(json);

    }

}

​

public class Source

{

    public List<(string, string)> Items { get; } = new List<(string, string)>();

}

​

public class TupleConverter : JsonConverter<(string, string)>

{

    public override (string, string) ReadJson(JsonReader reader, Type objectType, (string, string) existingValue, bool hasExistingValue, JsonSerializer serializer)

    {

        throw new NotImplementedException();

    }

​

    public override void WriteJson(JsonWriter writer, (string, string) value, JsonSerializer serializer)

    {

        writer.WriteStartObject();

        writer.WritePropertyName(value.Item1);

        writer.WriteValue(value.Item2);

        writer.WriteEndObject();

    }

}