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23

I am trying to extend the JSON.net example given here http://james.newtonking.com/projects/json/help/CustomCreationConverter.html

I have another sub class deriving from base class/Interface

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

public class Employee : Person
{
    public string Department { get; set; }
    public string JobTitle { get; set; }
}

public class Artist : Person
{
    public string Skill { get; set; }
}

List<Person> people  = new List<Person>
{
    new Employee(),
    new Employee(),
    new Artist(),
};

How do I deserialize following Json back to List< Person >

[
  {
    "Department": "Department1",
    "JobTitle": "JobTitle1",
    "FirstName": "FirstName1",
    "LastName": "LastName1"
  },
  {
    "Department": "Department2",
    "JobTitle": "JobTitle2",
    "FirstName": "FirstName2",
    "LastName": "LastName2"
  },
  {
    "Skill": "Painter",
    "FirstName": "FirstName3",
    "LastName": "LastName3"
  }
]

I don't want to use TypeNameHandling JsonSerializerSettings. I am specifically looking for custom JsonConverter implementation to handle this. The documentation and examples around this are pretty sparse on the net. I can't seem to get the the overridden ReadJson() method implementation in JsonConverter right.

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2 Answers

up vote 71 down vote accepted

Using the standard CustomCreationConverter, I was struggling to work how to generate the correct type (Person or Employee), because in order to determine this you need to analyse the JSON and there is no built in way to do this using the Create method.

I found a discussion thread pertaining to type conversion and it turned out to provide the answer. Here is a link: Type converting.

What's required is to subclass JsonConverter, overriding the ReadJson method and creating a new abstract Create method which acceps a JObject object.

The JObject class provides a means to load a JSON object and provdes access to the data within this object.

The overridden ReadJson method creates a JObject object and invokes the Create method (implemented by our derived converter class), passing in the JObject instance.

This JObject instance can then be analysed to determine the correct type by checking existance of certain fields.

Example

string json = "[{\"Department\": \"Department1\",\"JobTitle\": \"JobTitle1\",\"FirstName\": \"FirstName1\",\"LastName\": \"LastName1\"},{\"Department\": \"Department2\",\"JobTitle\": \"JobTitle2\",\"FirstName\": \"FirstName2\",\"LastName\": \"LastName2\"},{\"Skill\": \"Painter\",\"FirstName\": \"FirstName3\",\"LastName\": \"LastName3\"}]";
List<Person> persons = JsonConvert.DeserializeObject<List<Person>>(json, new PersonConverter());

...

public class PersonConverter : JsonCreationConverter<Person>
{
    protected override Person Create(Type objectType, JObject jObject)
    {
        if (FieldExists("Skill", jObject))
        {
            return new Artist();
        }
        else if (FieldExists("Department", jObject))
        {
            return new Employee();
        }
        else
        {
            return new Person();
        }
    }

    private bool FieldExists(string fieldName, JObject jObject)
    {
        return jObject[fieldName] != null;
    }
}

public abstract class JsonCreationConverter<T> : JsonConverter
{
    /// <summary>
    /// Create an instance of objectType, based properties in the JSON object
    /// </summary>
    /// <param name="objectType">type of object expected</param>
    /// <param name="jObject">contents of JSON object that will be deserialized</param>
    /// <returns></returns>
    protected abstract T Create(Type objectType, JObject jObject);

    public override bool CanConvert(Type objectType)
    {
        return typeof(T).IsAssignableFrom(objectType);
    }

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

        // Create target object based on JObject
        T target = Create(objectType, jObject);

        // Populate the object properties
        serializer.Populate(jObject.CreateReader(), target);

        return target;
    }

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

Hope this helps.

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7  
Thank you so much for providing the solution. Works gr8!. I think the class JsonCreationConverter should be added to Json.NET framework. – Snakebyte Nov 7 '11 at 0:45
I agree. I'm glad it helped. – jdavies Nov 8 '11 at 9:08
up vote 1 down vote

Just thought i would share a solution also based on this that works with the Knowntype attribute using reflection , had to get derived class from any base class, solution can benefit from recursion to find the best matching class though i didn't need it in my case, matching is done by the type given to the converter if it has KnownTypes it will scan them all until it matches a type that has all the properties inside the json string, first one to match will be chosen.

usage is as simple as:

 string json = "{ Name:\"Something\", LastName:\"Otherthing\" }";
 var ret  = JsonConvert.DeserializeObject<A>(json, new KnownTypeConverter());

in the above case ret will be of type B.

JSON classes:

[KnownType(typeof(B))]
public class A
{
   public string Name { get; set; }
}

public class B : A
{
   public string LastName { get; set; }
}

Converter code:

/// <summary>
    /// Use KnownType Attribute to match a divierd class based on the class given to the serilaizer
    /// Selected class will be the first class to match all properties in the json object.
    /// </summary>
    public  class KnownTypeConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return System.Attribute.GetCustomAttributes(objectType).Any(v => v is KnownTypeAttribute);
        }

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

            // Create target object based on JObject
            System.Attribute[] attrs = System.Attribute.GetCustomAttributes(objectType);  // Reflection. 

                // Displaying output. 
            foreach (System.Attribute attr in attrs)
            {
                if (attr is KnownTypeAttribute)
                {
                    KnownTypeAttribute k = (KnownTypeAttribute) attr;
                    var props = k.Type.GetProperties();
                    bool found = true;
                    foreach (var f in jObject)
                    {
                        if (!props.Any(z => z.Name == f.Key))
                        {
                            found = false;
                            break;
                        }
                    }

                    if (found)
                    {
                        var target = Activator.CreateInstance(k.Type);
                        serializer.Populate(jObject.CreateReader(),target);
                        return target;
                    }
                }
            }
            throw new ObjectNotFoundException();


            // Populate the object properties

        }

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