How to use private setters with NewtonSoftJson

Writing software with OOP language like c# requires good encapsulation in order to have healthy code base and extensible system. To achieve this you have to use the internal, private, protected, public access modifiers properly. However, when you want to deserialize to an object which has private set properties using the default NewtonSoft.Json settings you are out of luck. It is pretty easy to achieve this.

This is our class which we want to deserialize:

public class UserBook
{
    UserBook()
    {
        Notes = new List<string>();
    }

    public UserBook(string username) : this()
    {
        if (string.IsNullOrEmpty(username)) throw new System.ArgumentNullException(nameof(username));

        Username = username;
    }

    public string Username { get; private set; }

    public IEnumerable<string> Notes { get; private set; }

    public void TakeNote(string note) { ... }
}

And this is the json:

{
	"username": "mynkow ",
	"notes": [
		"Today was sunny day.",
		"Today was rainy day."
	]
}

Now we need a JsonSerializer instance so we could deserialize. To configure the serializer you need to set the proper JsonSerializerSettings and pass them to the static factory JsonSerializer.Create(settings). In previous versions of Newtonsoft.Json this was done like this:

public JsonSerializer GetSerializer()
{
    var settings = new JsonSerializerSettings();
    settings.ConstructorHandling = ConstructorHandling.AllowNonPublicDefaultConstructor;
    var contractResolver = new Newtonsoft.Json.Serialization.CamelCasePropertyNamesContractResolver();
    contractResolver.DefaultMembersSearchFlags = contractResolver.DefaultMembersSearchFlags | System.Reflection.BindingFlags.NonPublic;
    settings.ContractResolver = contractResolver;

    serializer = JsonSerializer.Create(settings);
}

There are two things happening here. The first is .ConstructorHandling which allows the serializer to use non-public constructors and instantiate the Notes collection. The second is .ContractResolver which hints the serializer to consider also non-public properties.

With the introduction of DotNet Core the Newtonsoft.Json library got some updates. One of which was the DefaultMembersSearchFlags being marked as obsolete. The good part is that you could continue using them with the full dot net framework and you will just see the following warning:

DefaultMembersSearchFlags is obsolete. To modify the members serialized inherit from DefaultContractResolver and override the GetSerializableMembers method instead.

It is easy to get rid of this warning. Here is the code:

public JsonSerializer GetSerializer()
{
    var settings = new JsonSerializerSettings();
    settings.ConstructorHandling = ConstructorHandling.AllowNonPublicDefaultConstructor;
    settings.ContractResolver = new ContractResolverWithPrivates();

    serializer = JsonSerializer.Create(settings);
}

public class ContractResolverWithPrivates : Newtonsoft.Json.Serialization.CamelCasePropertyNamesContractResolver
{
    protected override Newtonsoft.Json.Serialization.JsonProperty CreateProperty(System.Reflection.MemberInfo member, MemberSerialization memberSerialization)
    {
        var prop = base.CreateProperty(member, memberSerialization);

        if (!prop.Writable)
        {
            var property = member as System.Reflection.PropertyInfo;
            if (property != null)
            {
                var hasPrivateSetter = property.GetSetMethod(true) != null;
                prop.Writable = hasPrivateSetter;
            }
        }

        return prop;
    }
}

UserBook userBook = GetSerializer().Deserialize<UserBook>(json);

Software is fun! Happy coding!

Written on May 3, 2018