How to pass arguments to a non-default constructor?

Question

I have approximately the following picture:

public class Foo
{
   public Foo(Bar bar, String x, String y)
   {
       this.Bar = bar;
       this.X = x;
             


        

Answer 1

Here are my thoughts regarding problem solution:

The problem:

Json.Net's custom deserialization api is not transparent, i.e. affects my class hierarchy.

Actually it's not a problem in case when you have 10-20 classes in your project, though if you have huge project with thousands of classes, you are not particularly happy about the fact that you need comply your OOP design with Json.Net requirements.

Json.Net is good with POCO objects which are populated (initialized) after they are created. But it's not truth in all cases, sometimes you get your objects initialized inside constructor. And to make that initialization happen you need to pass 'correct' arguments. These 'correct' arguments can either be inside serialized text or they can be already created and initialized some time before. Unfortunately Json.Net during deserialization passes default values to arguments that he doesn't understand, and in my case it always causes ArgumentNullException.

The solution:

Here is approach that allows real custom object creation during deserialization using any set of arguments either serialized or non-serialized, the main problem is that the approach sub-optimal, it requires 2 phases of deserialization per object that requires custom deserialization, but it works and allows deserializing objects the way you need it, so here goes:

First we reassemble the CustomCreationConverter class the following way:

public class FactoryConverter<T> : Newtonsoft.Json.JsonConverter
{
    /// <summary>
    /// Writes the JSON representation of the object.
    /// </summary>
    /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
    /// <param name="value">The value.</param>
    /// <param name="serializer">The calling serializer.</param>
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotSupportedException("CustomCreationConverter should only be used while deserializing.");
    }

    /// <summary>
    /// Reads the JSON representation of the object.
    /// </summary>
    /// <param name="reader">The <see cref="JsonReader"/> to read from.</param>
    /// <param name="objectType">Type of the object.</param>
    /// <param name="existingValue">The existing value of object being read.</param>
    /// <param name="serializer">The calling serializer.</param>
    /// <returns>The object value.</returns>
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
            return null;

        T value = CreateAndPopulate(objectType, serializer.Deserialize<Dictionary<String, String>>(reader));

        if (value == null)
            throw new JsonSerializationException("No object created.");

        return value;
    }

    /// <summary>
    /// Creates an object which will then be populated by the serializer.
    /// </summary>
    /// <param name="objectType">Type of the object.</param>
    /// <returns></returns>
    public abstract T CreateAndPopulate(Type objectType, Dictionary<String, String> jsonFields);

    /// <summary>
    /// Determines whether this instance can convert the specified object type.
    /// </summary>
    /// <param name="objectType">Type of the object.</param>
    /// <returns>
    ///     <c>true</c> if this instance can convert the specified object type; otherwise, <c>false</c>.
    /// </returns>
    public override bool CanConvert(Type objectType)
    {
        return typeof(T).IsAssignableFrom(objectType);
    }

    /// <summary>
    /// Gets a value indicating whether this <see cref="JsonConverter"/> can write JSON.
    /// </summary>
    /// <value>
    ///     <c>true</c> if this <see cref="JsonConverter"/> can write JSON; otherwise, <c>false</c>.
    /// </value>
    public override bool CanWrite
    {
        get
        {
            return false;
        }
    }
}

Next we create the factory class that will create our Foo:

public class FooFactory : FactoryConverter<Foo>
{
    public FooFactory(Bar bar)
    {
        this.Bar = bar;
    }

    public Bar Bar { get; private set; }

    public override Foo Create(Type objectType, Dictionary<string, string> arguments)
    {
        return new Foo(Bar, arguments["X"], arguments["Y"]);
    }
}

Here is sample code:

var bar = new Bar("BarObject");

var fooSrc = new Foo
(
    bar,
    "A", "B"
);

var str = JsonConvert.SerializeObject(fooSrc);

var foo = JsonConvert.DeserializeObject<Foo>(str, new FooFactory(bar));

Console.WriteLine(str);

In this case foo contains an argument that we needed to pass to a Foo constructor during deserialization.

Answer 2

I'm not an expert on Json.NET, but AFAIK that simply is not possible. If I were you, I would look at options to fixup this after deserialization.

Very few serialization APIs will allow you to control construction to that degree; the four most typical approaches are (most common first):

• invoke the parameterless constructor
• skip the constructor completely
• use a constructor that has an obvious 1:1 mapping to the members being serialized
• use a user-supplied factory method

It sounds like you want the last, which is pretty rare. You may have to settle for doing this outside the constructor.

Some serialization APIs offer "serialization/deserialization callbacks", which allow you to run a method on the object at various points (typically before and after both serialize and deserialize), including passing some context information into the callback. IF Json.NET supports deserialization callbacks, that might be the thing to look at. This question suggests that the [OnDeserialized] callback pattern may indeed be supported; the context comes from the JsonSerializerSettings's .Context property that you can optionally supply to the deserialize method.

Otherwise, just run it manually after deserialization.

My rough pseudo-code (completely untested):

// inside type: Foo
[OnDeserialized]
public void OnDeserialized(StreamingContext ctx) {
    if(ctx != null) {
        Bar bar = ctx.Context as Bar;
        if(bar != null) this.Bar = bar; 
    }
}

and

var ctx = new StreamingContext(StreamingContextStates.Other, bar);
var settings = new JsonSerializerSettings { Context = ctx };
var obj = JsonConvert.DeserializeObject<Foo>(fooJsonString, settings);

Answer 3

If you have a constructor whose only parameters are your non-serialized values, create your instance first and then populate your object instead of deserializing. The JsonConvert class has a PopulateObject method, defined as follows:

public static void PopulateObject(
    string value,                      // JSON string
    object target)                     // already-created instance

If you have particular serialization settings, there's an overload that also includes a JsonSerializerSettings parameter.

Add a Foo constructor that has a single Bar parameter, you could do something like:

var bar = new Bar("Hello World");
var foo = new Foo(bar);
JsonConvert.PopulateObject(fooJsonString, foo);

You may need to adjust your class to use fields for mapping, or make adjustments to NHibernate to allow writing to private setters (use of a custom IProxyValidator class).