public class Program
{
    class A { }
    class B { }

    public static void Main()
    {
        Dictionary<string, object> dict = new Dictionary<string, object>();
        dict["Foo"] = new A();
        dict["Bar"] = new B();
        dict["Baz"] = new object();
        dict["Rainbow"] = Color.FromArgb(20, 20, 20, 20);

        var ser = new JsonSerializer
        {
            TypeNameHandling = TypeNameHandling.Auto
        };
        var w = new JsonTextWriter(Console.Out)
        {
            Formatting = Formatting.Indented
        };
        ser.Serialize(w, dict);
    }
}

{
  "Foo": {
    "$type": "Program+A, mi1i2eqo"
  },
  "Bar": {
    "$type": "Program+B, mi1i2eqo"
  },
  "Baz": {},
  "Rainbow": "20, 20, 20, 20"
}