https://twitter.com/markhibberd/status/469296280213020672

gistfile1.cs

using System;
using Newtonsoft.Json;

class Foo {
    public override string ToString() {
        return "Foo";
    }
}

class Bar<T> where T : Foo {
    public readonly T foo;

    public Bar(T foo) {
        this.foo = foo;
    }
}

class Baz : Foo {
    public override string ToString() {
        return "Baz";
    }
}

class Program {
    static void Main(string[] args) {
        var settings = new JsonSerializerSettings {TypeNameHandling = TypeNameHandling.Auto};
        var json = JsonConvert.SerializeObject(new Bar<Foo>(new Baz()), settings);
        Console.WriteLine(json); // {"foo": {"$type": "Baz, ConsoleApplication13"}}
        var bar = JsonConvert.DeserializeObject<Bar<Foo>>(json, settings);
        Console.WriteLine(bar.foo.ToString()); // prints "Baz"
    }
}