JsonUtility.FromJson keeps throwing ArgumentException

Question

I am trying to receive data from my backend in my unity project, the data looks like this:

{"subjectid":98,"name":"test23","first_name":"test23","date_of_birth":"1998-02-16","age":23}

I am using the following line to get the data into an object:

PatientBackend patient = JsonUtility.FromJson<PatientBackend>(responseBody);

The object looks like this:

  using System.Collections;
    using System.Collections.Generic;
    using UnityEngine;
    
    [System.Serializable]
    public class PatientBackend : MonoBehaviour
    {
        public int subjectid;
        public string name;
        public string first_name;
        public string date_of_birth;
        public int age;
    
       public PatientBackend(string name, string first_name, string date_of_birth)
        {
            this.name = name;
            this.first_name = first_name;
            this.date_of_birth = date_of_birth;
        }
}

But everytime this is called it thows the following exception:

System.ArgumentException: Cannot deserialize JSON to new instances of type 'PatientBackend.'
  at UnityEngine.JsonUtility.FromJson (System.String json, System.Type type) [0x00056] in <5070e0347dee4c9faba7201166fbed9d>:0 
  at UnityEngine.JsonUtility.FromJson[T] (System.String json) [0x00001] in <5070e0347dee4c9faba7201166fbed9d>:0 
  at DataService+<createPatient>d__8.MoveNext () [0x0021b] in C:\Users\diete\Documents\Stage\bedrijf_CLEAN-CLONE\VRStrokeRehabilitation\Unity\UnityProject\Assets\Scripts\DataService.cs:87 
UnityEngine.Debug:Log(Object)
<createPatient>d__8:MoveNext() (at Assets/Scripts/DataService.cs:90)

Does anyone know why this isn't working?

The exception is not very clear but i think it could be because the `PatientBackend ` does not have an empty constructor — Connor Stoop, Apr 16 '21 at 12:07
No that doesn't seem to be the issue. I just made an empty constructor and it didn't solve it. — Dieter Claessens, Apr 16 '21 at 12:11

score 1 · Accepted Answer · answered Apr 16 '21 at 12:14

1

PatientBackend should not be a MonoBehaviour. The JsonUtility can only be used for data classes. MonoBehaviours are components which need to be attached to gameobjects. You can't simply "create" an instance, that's why it failed.

see Answers.unity

answered Apr 16 '21 at 12:14

Connor Stoop

1,574
1
12
26

score 1 · Answer 2 · answered Apr 16 '21 at 14:46

You can not create instances of MonoBehaviour via the JsonUtility. A MonoBehaviour only makes sense if it is attached to a GameObject.

It is also not allowed that a MonoBehaviour implements a constructor.

Either do not make it a MonoBehaviour at all. And rather use

[System.Serializable]
public class PatientBackend
{
    public int subjectid;
    public string name;
    public string first_name;
    public string date_of_birth;
    public int age;
    
    public PatientBackend(string name, string first_name, string date_of_birth)
    {
        this.name = name;
        this.first_name = first_name;
        this.date_of_birth = date_of_birth;
    }
}

and

var patientBackend = JsonUtility.FromJson<PatientBackend>(jsonString);

Or if you really need this as a MonoBehaviour instead rather use JsonUtility.FromJsonOverwrite in order to only overwrite the fields of an existing instance.

public class PatientBackend : MonoBehaviour
{
    public int subjectid;
    public string name;
    public string first_name;
    public string date_of_birth;
    public int age;
}

and

JsonUtility.FromJsonOverwrite(jsonString, existingpatientBackend);

Thanks and I did as Connor Stoop already said. I just made it without making patientBackend a monobehaviour and it worked. — Dieter Claessens, Apr 16 '21 at 15:10

JsonUtility.FromJson keeps throwing ArgumentException

2 Answers2