C#•4mo ago

✅ JSON Deserialize Derived Class

Hi, i'm struggling to deserialize my object with the Newtonsoft.Json Librairy. I've a collection of Step named Steps which can be a Stepspecial or Stepnormal. I'm trying to write my JsonConverter to deserialize the collection of Step but when i try to deserialize i keep getting the stack overflow error because jObject.ToObject call back ReadJson :

public class StepConverterJSON : JsonConverter<Step>
{
    #region Properties
    public override bool CanWrite => false;
    #endregion

    #region Override Methods
    public override void WriteJson(JsonWriter writer, Step? value, JsonSerializer serializer) { throw new NotImplementedException(); }

    public override Step? ReadJson(JsonReader reader, Type objectType, Step? existingValue, bool hasExistingValue, JsonSerializer serializer)
    {
        // Load a JObject from the JsonReader
        JObject jObject = JObject.Load(reader);

        if(jObject["Message"] != null)
        {
            // Use the new serializer to deserialize the object
            StepSpecial? stepSpecial = serializer.Deserialize<StepSpecial>(reader);
            return stepSpecial;
        }
        else
        {
            StepNormal? stepNormal = jObject.ToObject<StepNormal>(serializer);
            //StepNormal? stepNormal = serializer.Deserialize<StepNormal>(reader);
            return stepNormal;
        }
    }
    #endregion
}

public class StepConverterJSON : JsonConverter<Step>
{
    #region Properties
    public override bool CanWrite => false;
    #endregion

    #region Override Methods
    public override void WriteJson(JsonWriter writer, Step? value, JsonSerializer serializer) { throw new NotImplementedException(); }

    public override Step? ReadJson(JsonReader reader, Type objectType, Step? existingValue, bool hasExistingValue, JsonSerializer serializer)
    {
        // Load a JObject from the JsonReader
        JObject jObject = JObject.Load(reader);

        if(jObject["Message"] != null)
        {
            // Use the new serializer to deserialize the object
            StepSpecial? stepSpecial = serializer.Deserialize<StepSpecial>(reader);
            return stepSpecial;
        }
        else
        {
            StepNormal? stepNormal = jObject.ToObject<StepNormal>(serializer);
            //StepNormal? stepNormal = serializer.Deserialize<StepNormal>(reader);
            return stepNormal;
        }
    }
    #endregion
}

Any idea how to solve this and write a good ReadJson method ? i've thought about the populate method but not sure this is the good way of doing it.

45 Replies

Elio•4mo ago

test.json

Jimmacle•4mo ago

can you $paste the json?

MODiX•4mo ago

If your code is too long, you can post to https://paste.mod.gg/ and copy the link into chat for others to see your shared code!

Elio•4mo ago

https://paste.mod.gg/cgqvpondvcnu/0

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Elio•4mo ago

well it seem that i solved my problem just by adding this setting .... JsonSerializerSettings settings = new JsonSerializerSettings { TypeNameHandling = TypeNameHandling.All }; but i do not really uderstand why and if it's the good solution

leowest•4mo ago

you should avoid Newtonsoft unless you have an edge case that forces u to use it.

leowest•4mo ago

System.Text.Json is the way to go these days.... with your data your modesl would look like this for STJ https://paste.mod.gg/wifpvbzpkwvj/0

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leowest•4mo ago

var result = JsonSerializer.Deserialize<Width>(json);

var result = JsonSerializer.Deserialize<Width>(json);

no need to write any corverters either unless I missed out something from your json

Elio•4mo ago

why do we avoid Newtonsoft ? thanks for the solution i'll try to do it this way

Jimmacle•4mo ago

it's pretty much obsolete with STJ and doesn't perform as well

leowest•4mo ago

because its inefficient, slow, the same people that created newtonsoft made STJ

Elio•4mo ago

Given the number of its popularity, I thought it was a good librairy

leowest•4mo ago

newtonsoft was made in the acient era where there was no json reason reader and there was a wide range of dirty json in the market that needed handling

Elio•4mo ago

ok good to know

leowest•4mo ago

its popular because it existed when nothing existed so that is years until .net core came out

Elio•4mo ago

do i really need to specify the name of each property for the json ?

leowest•4mo ago

but the performances alone should speak for it self. u dont if they are the same except casing I do because if I need to change it later for whaterver reason I dont need to change the underlying variable name or vice-verse if the naming in the json

Elio•4mo ago

i understand thanks i will quiclky try this and let u know if everything work well 🙂

leowest•4mo ago

I was trying to find the benchmark they did comparing both but I couldn't find lol

JC•4mo ago

If you're already deep into Newtonsoft with things like custom converters switching to STJ might not be trivial

Elio•4mo ago

i've got the same problem as before, when i deserialize the array of step, it only use the class step and ignore the subclass stepspecial and stepnormal that's good i've started yesterday so i can still change from newton to STJ

JC•4mo ago

STJ can have some behavior thats unexpected, but generally reasonable. If you're serializing to an interface, only the members of that interface will be serialized.

Elio•4mo ago

here is the good class : https://paste.mod.gg/pymisjadpkel/0

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A tool for sharing your source code with the world!

Somgör from Human Resources•4mo ago

You should totally use the built in JSON class. Pretty sure someone else already said that tho

JC•4mo ago

I think the problem, as I understand it, is that you have Step[] as your target, but want to include derived classes as a part of the result

Elio•4mo ago

yep that's it

JC•4mo ago

I don't know if that's something trivially done by System.Text.Json. If you try to deserialize to Step[], you're going to get an array of Step only.

Elio•4mo ago

i want to have stepnormal and stepspecial instead of only step

JC•4mo ago

Now, I'll assume you are not planning to allow typenamehandling

Elio•4mo ago

yup and i can't really figure it out how to say to STJ to select the good class when i want to deserialize my object which contains this list

JC•4mo ago

JsonDerivedTypeAttribute would probably be what you're looking for, if .NET 7+ is your target

Jimmacle•4mo ago

it seems like the issue with this is the data itself, it does not include a type discriminator field

JC•4mo ago

You could take a look at https://github.com/dmitry-bym/JsonKnownTypes if you're going to stick with netwonsoft

GitHub

GitHub - dmitry-bym/JsonKnownTypes: Simple way to serialize and des...

Simple way to serialize and deserialize polymorphic types for Json.NET - dmitry-bym/JsonKnownTypes

Jimmacle•4mo ago

so you have to infer what C# type it is by the presence/absence of properties or simply make a single model with all the potential properties but marked optional

JC•4mo ago

but you probably want to add a descriminator field As far as your current converter is concerned take a look at changing StepSpecial? stepSpecial = serializer.Deserialize<StepSpecial>(reader); to StepSpecial? stepSpecial = jObject.ToObject<StepSpecial>(serializer);

Elio•4mo ago

i'm open to switch to STJ, i just need to figure it out how to handle the desesrialization of my polymorphic object

Jimmacle•4mo ago

do you have control over the json schema?

JC•4mo ago

I think you're recursively entering the same serializer by trying to deserialize the derived type.

Elio•4mo ago

what do you mean ? yup and with newtonsoft i've found the solution how to do it, rn i'm looking to find how to do it with STJ

Jimmacle•4mo ago

are you creating this json or is it coming from a program you can't change?

Elio•4mo ago

y i'm creating the json

Jimmacle•4mo ago

https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism?pivots=dotnet-8-0

How to serialize properties of derived classes with System.Text.Jso...

Learn how to serialize polymorphic objects while serializing to and deserializing from JSON in .NET.

Elio•4mo ago

to resume i want to save programbending as a json in order to export/import it from a usb

Jimmacle•4mo ago

do this

Elio•4mo ago

perfect that was what i was looking for ! 😍 Well i should be careful about my vocabulary it would have save some time i think 😅 polymorphic object/derived classes Thank you all for helping me !

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✅ JSON Deserialize Derived Class