JsonConvert convert to custom object

Question

I made a simple custom JsonConvert class. Right now I'm trying to convert the JSON data to a custom class like so:

public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
    //JObject jo = JObject.Load(reader);
    MyItem instance = serializer.Deserialize<MyItem>(reader);

    // want to do more thing here...
}

Every time it executes the Deserialize<>() method it re-enters the ReadJson method and thus keeping me in a never ending loop.

How can I convert the JSON data to my custom object without ending up in a never ending loop?

Updated

The JSON I'm receiving is generic. I need the custom converter to map it to the right type. The JSON looks like this:

{
    /* Base obj properties, should be mapped to Page.cs */
    "$type": "App.Models.Page, App",
    "title": "Page title",
    "technicalName": "some_key",
    "fields": [

        /* Should be mapped to InputField.cs */
        {
            "$type": "App.Models.InputField, App",
            "type": "input",
            "typeTitle": "Input",
            "title": "Input",
            "technicalName": "input",
            "inputType": "text",
            "defaultValue": "Input default value"
        },

        /* Should be mapped to TextareaField.cs */
        {
            "$type": "App.Models.TextareaField, App",
            "type": "textarea",
            "typeTitle": "Textarea",
            "title": "Textarea",
            "technicalName": "textarea",
            "defaultValue": "default textarea value"
        }
    ]
}

In short my class files look like this:

class Page
{
    public string Title {get;set;}
    public string TechnicalName {get;set;}

    public List<Field> Fields {get;set;} // Note the base class "Field"
}

class InputField : Field // Field has base properties
{
 // InputField has its own properties
}

class TextareaField : Field // Field has base properties
{
 // TextareaField has its own properties
}

TypeNameHandling

I've set the json formatters settings:

jsonOutputFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.Objects;
jsonInputFormatter.SerializerSettings.TypeNameHandling = TypeNameHandling.Auto;

When I output a Page object through Web API then I get a JSON structure exactly like the one above. When I sent back that exact JSON data I expect it to nicely map back to a Page object with the correct types in the Field list property.

But all the Fields are mapped to its base class Field instead of the types that are defined in $type.

So currently the JSON formatter works great for outputting. It shows all the $types. But the input mapping isn't really mapping back to its specific types.

Why did you create a custom JsonConvert in the first place? Why not use the one provided by the library? — Yuval Itzchakov
– Yuval Itzchakov, Commented May 25, 2015 at 11:45
@YuvalItzchakov Because I have a page that posts a JSON array which can be of a generic type. I'm eventually going to use the custom converter to map it back to the right type. — Vivendi
– Vivendi, Commented May 25, 2015 at 11:49
You can try changing the method signature to public new object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) — Jurgen Camilleri
– Jurgen Camilleri, Commented May 25, 2015 at 11:51
@JurgenCamilleri Doesn't work, it's an abstract method that needs to be overridden. — Vivendi
– Vivendi, Commented May 25, 2015 at 11:52

Community · Accepted Answer · 2017-05-23 10:31:28Z

1

Given that you use Json.NET on the Web API side, you don't need a custom JsonConverter. You can explicitly tell the JsonFormatter (which internally uses Json.NET) to use TypeNameHandling.All. That way, it'll know exactly which type to deserialize given an IEnumerable<BaseType>. More on that can be find in how to deserialize JSON into IEnumerable<BaseType> with Newtonsoft JSON.NET

edited May 23, 2017 at 10:31

CommunityBot

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answered May 25, 2015 at 13:58

Yuval Itzchakov

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Jurgen Camilleri · Accepted Answer · 2015-05-25 12:16:11Z

1

public static class ConvertToJson
{ 
    public static T Deserialize<T>(string json)
    {
        using (MemoryStream ms = new MemoryStream(Encoding.Unicode.GetBytes(json)))
        {
            DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(T));
            return (T)serializer.ReadObject(ms);
        }
    }
}

edited May 25, 2015 at 12:16

Jurgen Camilleri

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answered May 25, 2015 at 12:01

Gera

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