using System;

using System.ComponentModel;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var settings =  new JsonSerializerSettings

        {

            DefaultValueHandling = DefaultValueHandling.Populate,

            NullValueHandling = NullValueHandling.Ignore

        };

        string json = @"{""MyString"": null}";

        string json = @"{""MyString"": null}";

        var obj = JsonConvert.DeserializeObject<MyClass>(json, settings);

        Console.WriteLine(obj.MyString);

    }

    public class MyClass

    {

        [DefaultValue(null)]

        public string MyString { get; set; } = "Mes amis";

    }

}