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.
DeserializeObject Method. Deserializes the JSON to a . NET object.
Specifies the settings on a JsonSerializer object. Newtonsoft.Json.
Converts an Enum to and from its name string value. Newtonsoft.Json. JsonConverter. Newtonsoft.Json.Converters.
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 (archived link).
What's required is to subclass JsonConverter
, overriding the ReadJson
method and creating a new abstract Create
method which accepts a JObject
.
The JObject class provides a means to load a JSON object and provides access to the data within this object.
The overridden ReadJson
method creates a JObject
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 existence 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 bool CanWrite { get { return false; } } 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; } }
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