Automatically selecting parameterized constructor while JsonCovertor Deserialization

Question

I using json .NET to deserialize my json string to my model. below is what I am trying to achieve, please advice what is best way ..

When there is no data my response looks like below json string = "{\"message\":\"SUCCESS\",\"result\":null}"

the result eventually is tied to a view. So when the response is null I would like to intialize my view with default model values. And so would like to call Default constructor on deserialization. Default constructor looks like below.

    public ProfileModel()
    {
        this.DefaultTab = DefaultTabOption.PROFILE;
        this.DataLoadPosition = new DataLoadPositionOptionsModel();
        this.DataLayout = new DataLayoutOptionsModel();
        this.NAData = new NADataOptionsModel();
        this.DataTable = new DataDisplayOptionsModel();
    }

But when there is data, the reponse looks like below.

{"message":"SUCCESS","result":{"dataLayout":{"vertical":false},"dataLoadPosition":{"cell":"B2","cursorLocation":false},"dataTable":{"decimalPts":1},"defaultTab":"BROWSE","naData":{"custom":"","naDataOption":"FORWARDFILL"}}}

In this case, I would like to call my parameterized constructor so that models are initialized correctly.

Deserialization code:

        using (StreamReader reader = new StreamReader(responseStream))
        {
           var t = JsonConvert.DeserializeObject<T>(reader.ReadToEnd());                
           return t;
        }

Where T is my main Model which initilialises multiple models. Below is the parameterized contructor.

public ProfileModel(DefaultTabOption defaultTabModel,
                    DataLoadPositionOptionsModel dataLoadPositionOption ,
                    DataLayoutOptionsModel dataLayoutOptios ,
                    NADataOptionsModel naDataOptions ,
                    DataDisplayOptionsModel dataTableOptions)
{
    this.DefaultTab = defaultTabModel;
    this.DataLoadPosition = dataLoadPositionOption;
    this.DataLayout = dataLayoutOptios;
    this.NAData = naDataOptions;
    this.DataTable = dataTableOptions;
}

What is they best way to deserialize so that default constructor is called when null and parameterized is call when not null. I tried the ConstructorHandling, NullValueHandling but I am not able to achieve desired results.

GSerjo · Accepted Answer · 2014-05-17 00:53:37Z

I've a little simplified your model

public sealed class ProfileModel
{
    public ProfileModel()
    {
        DataLayout = new DataLayoutOptionsModel();
    }

    public ProfileModel(DataLayoutOptionsModel dataLayout)
    {
        DataLayout = dataLayout;
    }

    public DataLayoutOptionsModel DataLayout { get; private set; }
}

public sealed class DataLayoutOptionsModel
{
    public bool Vertical { get; set; }
}

public class ResultModel
{
    public string Message { get; set; }
    public ProfileModel Result { get; set; }
}

To choose concrete constructor, you have to implement custom JsonConverter, for example

public sealed class MyJsonConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return typeof(ProfileModel).IsAssignableFrom(objectType);
    }

    public override object ReadJson(JsonReader reader,
        Type objectType,
        object existingValue,
        JsonSerializer serializer)
    {
        ProfileModel target;
        JObject jObject = JObject.Load(reader);

        JToken resultToken = jObject["Result"];
        //This is result null check
        if (resultToken.Type == JTokenType.Null)
        {
            target = new ProfileModel();
        }
        else
        {
            var optionsModel = resultToken["DataLayout"].ToObject<DataLayoutOptionsModel>();
            target = new ProfileModel(optionsModel);
        }

        serializer.Populate(jObject.CreateReader(), target);
        return target;
    }

    public override void WriteJson(JsonWriter writer,
        object value,
        JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

So, result serialization looks like:

[Fact]
public void Test()
{
    string tinyJsonl = "{\"Message\":\"SUCCESS\",\"Result\":null}";
    var defaultProfile = JsonConvert.DeserializeObject<ProfileModel>(tinyJsonl, new MyJsonConverter());
    Assert.False(defaultProfile.DataLayout.Vertical);

    string fullJson = "{\"Message\":\"SUCCESS\",\"Result\":{\"DataLayout\":{\"Vertical\":true}}}";
    var customProfile = JsonConvert.DeserializeObject<ProfileModel>(fullJson, new MyJsonConverter());
    Assert.True(customProfile.DataLayout.Vertical);
}

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