using System;

using System.Collections.Generic;   

using Newtonsoft.Json;

​

public class Program

{

    public static void Main()

    {

        Dictionary<(A, B), C> dictionary = new Dictionary<(A, B), C>

        {

            { (new A { Name = "A" }, new B { Number = 10 }), new C { Value = 1 } }, 

            { (new A { Name = "A" }, new B { Number = 10 }), new C { Value = 1 } },

        };

        var json = JsonConvert.SerializeObject(dictionary);

        Console.WriteLine(json);

    }

    public class A

    {

        public string Name { get; set; }

    }

    public class B

    {

        public int Number { get; set; } 

    }

    public class C

    {

        public double Value {get; set;}

    }

}