Develop Reference

Using Newtonsoft.JSON custom converters to read json with different input

I am using NewtonSoft.Json to read/write our data as json. One (very simplified) example of this is:
{
"$type": "MyNamespace.LandingEvent, MyAssembly",
"TimeOfLanding": "2021-04-11T15:00:00.000Z",
"AirportName": "KLAX",
"AirportRunway": "25L"
}
With a C# DTO class that mimicks the properties. Note that we use TypeNameHandling.
We want to change our C# class to a more complex setup:
class Airport
{
public string Name { get; set; }
public string Runway { get; set; }
}
class LandingEvent
{
public DateTime TimeOfLanding { get; set; }
public Airport Airport { get; set; }
}
which will result in, that new data will be written to JSON as:
{
"$type": "MyNamespace.LandingEvent, MyAssembly",
"TimeOfLanding": "2021-04-11T15:00:00.000Z",
"Airport": {
"Name": "KLAX",
"Runway": "25L"
}
}
But we still need to be able to read the old JSON data and parse into the new class structure. And this is what I currently struggle with.
I know that the way to go is probably a specialized JsonConverter. I have a couple of questions in this regard:
How do I read the $type property and instantiate the right type? (my overriden CanConvert() method is fed the name of a base-class (due to the real context being more complex than this example).
I only want to do custom read, if the property AirportName exsist. How do I fall-back to default deserialization, if this is not the case?
Edit: Some clarification is in order. If I create a custom JsonConverter, then CanConvert will receive the type EventBase, but the $type can actually contain either "MyNamespace.LandingEvent, MyAssembly" or "MyNamespace.TakeoffEvent, MyAssembly". Therefore I will probably need to instantiate the returned object myself based on this value. I am not sure how, though.

You can use a custom JsonConverter to do double duty in handling both the polymorphic event types and the varying JSON formats. Below is an example. It works by loading the data into a JObject, where it can read the $type property and instantiate the correct event type. From there, it will try to populate the event object from the JSON. If the Airport fails to deserialize, it will then attempt to read the legacy airport proprties and populate a new Airport instance from that.
class EventConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return typeof(BaseEvent).IsAssignableFrom(objectType);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
JObject obj = JObject.Load(reader);
string type = (string)obj["$type"];
BaseEvent baseEvent;
if (type.Contains(nameof(TakeoffEvent)))
{
baseEvent = new TakeoffEvent();
}
else
{
baseEvent = new LandingEvent();
}
serializer.Populate(obj.CreateReader(), baseEvent);
if (baseEvent.Airport == null)
{
baseEvent.Airport = new Airport
{
Name = (string)obj["AirportName"],
Runway = (string)obj["AirportRunway"]
};
}
return baseEvent;
}
public override bool CanWrite => false;
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
Note: this assumes your class structure actually looks like this:
class Airport
{
public string Name { get; set; }
public string Runway { get; set; }
}
class BaseEvent
{
public Airport Airport { get; set; }
}
class TakeoffEvent : BaseEvent
{
public DateTime TimeOfTakeoff { get; set; }
}
class LandingEvent : BaseEvent
{
public DateTime TimeOfLanding { get; set; }
}
To use the converter, add it to the Converters collection in the JsonSerializerSettings, and pass the settings to DeserializeObject():
var settings = new JsonSerializerSettings
{
TypeNameHandling = TypeNameHandling.Objects,
Converters = new List<JsonConverter> { new EventConverter() }
};
var baseEvent = JsonConvert.DeserializeObject<BaseEvent>(json, settings);
Here is a working demo: https://dotnetfiddle.net/jSaq4T
See also: Adding backward compatibility support for an older JSON structure

Classes change, this kind of Json strings change and will get extra features in future versions. You'll keep adjusting your declarations. With Newtonsoft, you can add custom handlers for varying class inheritance and keep using deserialize, but you'll have to maintain that code.
For dynamic Json, I find it easier to use JObject, JArray and JToken instead, to freely parse a Json string. Especially if you're only interested in some of the fields.
I can only give you an example, I think this is (a little) related to your project, but not the same part (smiley)
I use below code to decode part of a glTF 3d-object file produced by Blender in MSFS-converted format. This Json-like format consists of sections. Each Json section looks something like this,
"asset" : {
"extensions" : {
"ASOBO_normal_map_convention" : {
"tangent_space_convention" : "DirectX"
}
},
"generator" : "Extended Khronos glTF Blender I/O v1.0.0",
"version" : "2.0"
},
.. but these sections and their fields are mostly optional and in some GLtf's they are not filled in. It is not "serializable or deserializable" to classes.
I declare some
public JObject AssetObject;
.. filling it in from Json string sJson as follows:
dynamic stuff = JObject.Parse(sJson);
var pp = stuff.Children();
Dictionary<string, bool> d = new Dictionary<string, bool>();
foreach (JProperty jo in pp) d[jo.Name] = true; // all sections
string cSection= "asset";
if (!d.ContainsKey(cSection)) { LogLine(98, "Warning: BPG Json has no " + cSection + " section."); return false; }
else
{
AssetObject = (JObject)stuff[cSection];
ParseGLBAsset();
}
Notice the use of a dynamic declaration at first, a section will land in JObject via cast. I store the various parts of the section into string properties. The parse itself takes place in ParseGLBAsset(), this function looks as follows:
public void ParseGLBAsset()
{
foreach (JProperty jbp in AssetObject.Children())
if (jbp.Name == "generator")
{ GLBGenerator = jbp.Value.ToString(); }
else
if (jbp.Name == "extensions")
{
GLBAssetExtensions = jbp.Value.ToString();
LogLine(0, "Asset extensions: " + GLBAssetExtensions);
}
else
if (jbp.Name == "version")
{ GLBVersion = jbp.Value.ToString(); }
LogLine(1, "Found asset.generator=" + GLBGenerator);
LogLine(1, "Found asset.version=" + GLBVersion);
}

How to create C# classes to deserialize a JSON string which starts and ends with square brackets

I am receiving a JSON string back from an API and want to deserialize it into C# objects but cannot get the classes correct.
I have tried creating the classes using http://json2csharp.com/ but it can't parse the JSON, however https://jsonlint.com/ says that the JSON is valid.
I also tried running JsonClassGeneratorLib but that says
Unable to cast object of type Newtonsoft.Json.Linq.JValue to Newtonsoft.Json.Linq.JObject
It seems to be an issue because the JSON is enclosed in [] square brackets. I believe this is valid but makes it into an array. I think I need an array somewhere in my class.
string Json = #"[""error"",{""code"":2,""msg"":""This API Key is invalid""}]";
var obj = JsonConvert.DeserializeObject<RootObject>(Json);
public class CodeMsg
{
[JsonProperty("code")]
public long Code { get; set; }
[JsonProperty("msg")]
public string Msg { get; set; }
}
public class Result
{
[JsonProperty("error")]
public string String { get; set; }
public CodeMsg cm { get; set; }
}
public class RootObject
{
public Result Result { get; set; }
}
I always get the error
Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'ConsoleApp1.RootObject' because the type requires a JSON object (e.g. {"name":"value"}) to deserialize correctly.
To fix this error either change the JSON to a JSON object (e.g. {"name":"value"}) or change the deserialized type to an array or a type that implements a collection interface (e.g. ICollection, IList) like List<T> that can be deserialized from a JSON array

Try this:
var obj = JsonConvert.DeserializeObject<List<Result>>(Json);
Explanation: If the JSON is an Array, the C# RootObject has to be either deriving from List/IEnumerable itself, or you deserialize it to a List/Array of the Type.
You can dump your RootObject class. If you wanted to use the RootObject type, make it derive from List. But this is not worth the hassle.

Your JSON is a heterogeneous array containing a string and an object. This will not deserialize cleanly into a strongly-typed class structure without a little help. One possible solution is to use a custom JsonConverter like this:
public class ResultConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return objectType == typeof(Result);
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
JArray array = JArray.Load(reader);
Result result = new Result
{
String = array[0].Value<string>(),
cm = array[1].ToObject<CodeMsg>(serializer)
};
return result;
}
public override bool CanWrite
{
get { return false; }
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
Then tie the converter to your Result class with a [JsonConverter] attribute like this:
[JsonConverter(typeof(ResultConverter))]
public class Result
{
public string String { get; set; }
public CodeMsg cm { get; set; }
}
Finally, deserialize the JSON into the Result class like this:
var result = JsonConvert.DeserializeObject<Result>(Json);
Working demo: https://dotnetfiddle.net/RLpm5W
Note: You can delete the RootObject class; it is not needed here.

Just make sure, your json string must have same properties as JsonHolder class.
public class JsonHolder
{
public string Id {get;set;}
public string Name {get;set;}
public string Gender {get;set;}
}
var jsonHolderList = new JsonConvert.DeserializeObject<List<JsonHolder>>(jsonString);
var jsonHolder = jsonHolderList.Single()
You can also convert json string into c# object as dynamic object. Just make sure, your json string must have same properties as JsonHolder class.
dynamic obj= new JsonConver.DeserializeObject<List<JsonHolder>>(StrJson);

Deserialize string by class name

Let's say I have a Value that is deserialized from a class.
public class MyValue
{
public string MyPropertyA { get; set; }
public string MyPropertyB { get; set; }
public string DeserializationClass { get; } = typeof(MyValue).Name;
}
I serialize this using JsonConvert class. MyValue class has a property DeserializationClass that should be used as info from which class the string was serialized from. In other words, when I deserialize the string into an object, this property serves as info which class should be used to deserialize the string. However, I am kinda stuck here as I am not sure how to get back the class from the string. Can anybody help me here?
public class Program
{
void Main()
{
var serialized = Serialize();
var obj = Deserialize(serialized);
}
string Serialize()
{
var objValue = new MyValue { MyPropertyA="Something", MyPropertyB="SomethingElse" };
return JsonConvert.SerializeObject<MyClass>(value);
}
object Deserialize(string serialized)
{
//How to deserialize based on 'DeserializationClass' property in serialized string?
return = JsonConvert.Deserialize<???>(serialized);
}
}
EDIT: Modified example to make it more clear what I need as I don't have access to objValue when I need to deserialize the string.

probably you might need to use JsonSerializerSettings.
What you might need to do is
JsonSerializerSettings setting = new JsonSerializerSettings
{
TypeNameHandling = TypeNameHandling.All,
};
and then while serializing use this setting.
var serialized = JsonConvert.SerializeObject(objValue,setting);
this will give you Json like this
{"$type":"WPFDatagrid.MyValue, WPFDatagrid","MyPropertyA":"Something","MyPropertyB":"SomethingElse","DeserializationClass":"MyValue"}
from this you can find the name of the class used it to actually get your type.
Hope this helps !!

There is an overload
If your Type is in form of a Namespace, you can obtain the type from a string representation:
Type objValueType = Type.GetType("Namespace.MyValue, MyAssembly");
object deserialized = JsonConvert.Deserialize(objValueType, serialized);

JSON.NET Deserialization - Single Result vs Array

I am having trouble trying to determine how to make my Serialization Properly be able to access a single result, as well as an array.
When I make a REST call looking for something on a server, sometimes it will return an Array of models, but if the search results only have a single model, it will not be returned as an error. This is when I get an exception that I cannot deserialize because the Object Property is expecting an array, but is instead receiving a single object.
Is there a way to define my class so that it can handle a single object of type ns1.models when that is returned instead of an array of objects?
[JsonObject]
public class Responses
{
[JsonProperty(PropertyName = "ns1.model")]
public List<Model> Model { get; set; }
}
Response that can be deserialized:
{"ns1.model":[
{"#mh":"0x20e800","ns1.attribute":{"#id":"0x1006e","$":"servername"}},
{"#mh":"0x21a400","ns1.attribute":{"#id":"0x1006e","$":"servername2"}}
]}
Response that cannot be serialized (because JSON includes only a singe "ns1.model"):
{"ns1.model":
{"#mh":"0x20e800","ns1.attribute":{"#id":"0x1006e","$":"servername"}}
}
Exception:
Newtonsoft.Json.JsonSerializationException was unhandled HResult=-2146233088 Message=Cannot deserialize the current JSON object (e.g. {"name":"value"}) into type 'System.Collections.Generic.List`1[ConsoleApplication1.Model]' because the type requires a JSON array (e.g. [1,2,3]) to deserialize correctly. To fix this error either change the JSON to a JSON array (e.g. [1,2,3]) or change the deserialized type so that it is a normal .NET type (e.g. not a primitive type like integer, not a collection type like an array or List<T>) that can be deserialized from a JSON object. JsonObjectAttribute can also be added to the type to force it to deserialize from a JSON object. Path '['ns1.model-response-list'].['ns1.model-responses'].['ns1.model'].#mh', line 1, position 130

To handle this you have to use a custom JsonConverter. But you probably already had that in mind.
You are just looking for a converter that you can use immediately. And this offers more than just a solution for the situation described.
I give an example with the question asked.
How to use my converter:
Place a JsonConverter Attribute above the property. JsonConverter(typeof(SafeCollectionConverter))
public class Response
{
[JsonProperty("ns1.model")]
[JsonConverter(typeof(SafeCollectionConverter))]
public List<Model> Model { get; set; }
}
public class Model
{
[JsonProperty("#mh")]
public string Mh { get; set; }
[JsonProperty("ns1.attribute")]
public ModelAttribute Attribute { get; set; }
}
public class ModelAttribute
{
[JsonProperty("#id")]
public string Id { get; set; }
[JsonProperty("$")]
public string Value { get; set; }
}
And this is my converter:
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using System;
namespace stackoverflow.question18994685
{
public class SafeCollectionConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return true;
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
//This not works for Populate (on existingValue)
return serializer.Deserialize<JToken>(reader).ToObjectCollectionSafe(objectType, serializer);
}
public override bool CanWrite => false;
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
throw new NotImplementedException();
}
}
}
And this converter uses the following class:
using System;
namespace Newtonsoft.Json.Linq
{
public static class SafeJsonConvertExtensions
{
public static object ToObjectCollectionSafe(this JToken jToken, Type objectType)
{
return ToObjectCollectionSafe(jToken, objectType, JsonSerializer.CreateDefault());
}
public static object ToObjectCollectionSafe(this JToken jToken, Type objectType, JsonSerializer jsonSerializer)
{
var expectArray = typeof(System.Collections.IEnumerable).IsAssignableFrom(objectType);
if (jToken is JArray jArray)
{
if (!expectArray)
{
//to object via singel
if (jArray.Count == 0)
return JValue.CreateNull().ToObject(objectType, jsonSerializer);
if (jArray.Count == 1)
return jArray.First.ToObject(objectType, jsonSerializer);
}
}
else if (expectArray)
{
//to object via JArray
return new JArray(jToken).ToObject(objectType, jsonSerializer);
}
return jToken.ToObject(objectType, jsonSerializer);
}
public static T ToObjectCollectionSafe<T>(this JToken jToken)
{
return (T)ToObjectCollectionSafe(jToken, typeof(T));
}
public static T ToObjectCollectionSafe<T>(this JToken jToken, JsonSerializer jsonSerializer)
{
return (T)ToObjectCollectionSafe(jToken, typeof(T), jsonSerializer);
}
}
}
What does it do exactly?
If you place the converter attribute the converter will be used for this property. You can use it on a normal object if you expect a json array with 1 or no result. Or you use it on an IEnumerable where you expect a json object or json array. (Know that an array -object[]- is an IEnumerable)
A disadvantage is that this converter can only be placed above a property because he thinks he can convert everything. And be warned. A string is also an IEnumerable.
And it offers more than an answer to the question:
If you search for something by id you know that you will get an array back with one or no result.
The ToObjectCollectionSafe<TResult>() method can handle that for you.
This is usable for Single Result vs Array using JSON.net
and handle both a single item and an array for the same property
and can convert an array to a single object.
I made this for REST requests on a server with a filter that returned one result in an array but wanted to get the result back as a single object in my code. And also for a OData result response with expanded result with one item in an array.
Have fun with it.

I think your question has been answered already. Please have a look at this thread:
How to handle both a single item and an array for the same property using JSON.net .
Basically the way to do it is to define a custom JsonConvertor for your property.

There is not an elegant solution to your problem in the current version of JSON.NET. You will have to write custom parsing code to handle that.
As #boyomarinov said you can develop a custom converter, but since your JSON is pretty simple you can just parse your JSON into an object and then handle the two cases like this:
var obj = JObject.Parse(json);
var responses = new Responses { Model = new List<Model>() };
foreach (var child in obj.Values())
{
if (child is JArray)
{
responses.Model = child.ToObject<List<Model>>();
break;
}
else
responses.Model.Add(child.ToObject<Model>());
}

Use JRaw type proxy property ModelRaw:
public class Responses
{
[JsonIgnore]
public List<Model> Model { get; set; }
[JsonProperty(PropertyName = "ns1.model")]
public JRaw ModelRaw
{
get { return new JRaw(JsonConvert.SerializeObject(Model)); }
set
{
var raw = value.ToString(Formatting.None);
Model = raw.StartsWith("[")
? JsonConvert.DeserializeObject<List<Model>>(raw)
: new List<Model> { JsonConvert.DeserializeObject<Model>(raw) };
}
}
}

c# JavaScriptConverter - how to deserialize custom property?

I've got a class which has been serialized into JSON, and which I'm trying to deserialize into an object.
e.g.
public class ContentItemViewModel
{
public string CssClass { get; set; }
public MyCustomClass PropertyB { get; set; }
}
the simple property (CssClass) will deserialize with:
var contentItemViewModels = ser.Deserialize<ContentItemViewModel>(contentItems);
But PropertyB gets an error...
We added a JavaScriptConverter:
ser.RegisterConverters(new List<JavaScriptConverter>{ publishedStatusResolver});
But when we added 'MyCustomClass' as a 'SupportedType', the Deserialize method was never called. However when we have ContentItemViewModel as the SupportedType, then Deserialize is called.
We've got a current solution which looks something like this:
class ContentItemViewModelConverter : JavaScriptConverter
{
public override object Deserialize(IDictionary<string, object> dictionary, Type type, JavaScriptSerializer serializer)
{
var cssClass = GetString(dictionary, "cssClass"); //I'm ommitting the GetString method in this example...
var propertyB= GetString(dictionary, "propertyB");
return new ContentItemViewModel{ CssClass = cssClass ,
PropertyB = new MyCustomClass(propertyB)}
}
public override IDictionary<string, object> Serialize(object obj, JavaScriptSerializer serializer)
{
throw new Exception("Only does the Deserialize");
}
public override IEnumerable<Type> SupportedTypes
{
get
{
return new List<Type>
{
typeof(ContentItemViewModel)
};
}
}
}
But we'd prefer a simpler solution of only deserializing MyCustomClass, as there are a number of other fields which are on the ViewModel, and it seems a waste to have to edit this converter every time we change/add a property....
Is there a way to Deserialize JUST PropertyB of type MyCustomClass?
Thanks for your help!

Have you considered using DatacontractJsonSerializer
[DataContract]
public class MyCustomClass
{
[DataMember]
public string foobar { get; set; }
}
[DataContract]
public class ContentItemViewModel
{
[DataMember]
public string CssClass { get; set; }
[DataMember]
public MyCustomClass PropertyB { get; set; }
}
class Program
{
static void Main(string[] args)
{
ContentItemViewModel model = new ContentItemViewModel();
model.CssClass = "StackOver";
model.PropertyB = new MyCustomClass();
model.PropertyB.foobar = "Flow";
//Create a stream to serialize the object to.
MemoryStream ms = new MemoryStream();
// Serializer the User object to the stream.
DataContractJsonSerializer ser = new DataContractJsonSerializer(typeof(ContentItemViewModel));
ser.WriteObject(ms, model);
byte[] json = ms.ToArray();
ms.Close();
string s= Encoding.UTF8.GetString(json, 0, json.Length);
Console.ReadLine();
}
}
Add all possible classes to DatacontractJsonSerializer.KnownTypes if MyCustomClass has derivations.

For whatever it may be worth after all this time, but I stumbled over the same problem and the solution is that the Deserializer hasn't got a clue about the classes you are deserializing unless you give him the necessary information.
On the top level, it knows the type from the type parameter of Deserialize<>(). That's why your converter for ContentItemViewModel works. For nested objects, it needs __type properties and a JavaScriptTypeResolver.
var ser = new JavaScriptSerializer(new SimpleTypeResolver());
ser.RegisterConverters(myconverters);
MyClass myObject = new MyClass();
string json = ser.Serialize(myObject);
// set a breakpoint here to see what has happened
ser.Deserialize<MyClass>(json);
A TypeResolver adds a __type property to each serialized object. You can write a custom type resolver that uses short names. In this sample, I use the SimpleTypeResolver from .net that "simply" stores the fully qualified type name as __type. When deserializing, the JavaScriptDeserializer finds __type and asks the TypeResolver for the correct type. Then it knows a type and can call a registered JavaScriptConverter.Deserialize method.
Without a TypeResolver, objects are deserialized to a Dictionary because JavaScriptSerializer doesn't have any type information.
If you can't provide a __type property in your json string, I think you'll need to deserialize to Dictionary first and then add a "guessing-step" that interprets the fields to find the right type. Then, you can use the ConvertToType method of JavaScriptSerializer to copy the dictionary into the object's fields and properties.
If you need to use the JavaScriptSerializer that is provides by ASP.NET and can't create your own, consider this section from the .ctor help of JavaScriptSerializer:
The instance of JavaScriptSerializer that is used by the asynchronous communication layer for invoking Web services from client script uses a special type resolver. This type resolver restricts the types that can be deserialized to those defined in the Web service’s method signature, or the ones that have the GenerateScriptTypeAttribute applied. You cannot modify this built-in type resolver programmatically.
Perhaps the GenerateScriptType Attribute can help you. But I don't know what kind of __type Properties are be needed here.