System.Text.Json.JsonException: The JSON value could not be converted

Question

I'm using Ubuntu and dotnet 3.1, running vscode's c# extension.

I need to create a List from a JSON file, my controller will do some calculations with this model List that I will pass to it

So, here is my code and the error I'm getting.

First, I thought my error was because at model my attributes were char and C#, for what I saw, cannot interpret double-quotes for char, it should be single quotes. Before losing time removing it, I just changed my type declarations to strings and it's the same error.

Can someone help me?

ElevadorModel

using System.Collections.Generic;

namespace Bla
{
    public class ElevadorModel
    {
        public int andar { get; set; }
        public string elevador { get; set; }
        public string turno { get; set; }
    }
}

Program.cs:

class Program
{
    static void Main(string[] args)
    {
        var path = "../input.json";

        string jsonString;
        
        ElevadorModel elevadoresModel = new ElevadorModel();

        jsonString = File.ReadAllText(path); //GetType().Name = String

        Console.WriteLine(jsonString); //WORKS           

        elevadoresModel = JsonSerializer.Deserialize<ElevadorModel>(jsonString);
    }

JSON:

You haven't shown us a snippet of the JSON that reproduced the problem. — David L, Jan 27 '20 at 23:06

score 21 · Accepted Answer · answered Jan 27 '20 at 23:11

21

Your input json has an array as the base token, whereas you're expecting an object. You need to change your deserialization to an array of objects.

var elevadoresModels = JsonSerializer.Deserialize<List<ElevadorModel>>(jsonString);
elevadoresModel = elavoresModels.First();

answered Jan 27 '20 at 23:11

David L

32,885
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hi, i'm new to c#. This is the correct way of initialize my list of objects now, right? List elevadoresModel = new List(); – nanquim Jan 27 '20 at 23:19
2

Correct, list instantiation is done via `new List()`. That said, in this case you do not need to initialize the list beforehand. The deserializer will create the collection for you. In my example, `var` is just compiler inference for `List`. You can use either interchangeably in this case. – David L Jan 27 '20 at 23:19

score 6 · Answer 2 · answered Jan 27 '20 at 23:11

6

Your input JSON is an array of models, however you're trying to deserialize it to a single model.

var models = JsonSerializer.Deserialize<List<ElevadorModel>>(jsonString);

answered Jan 27 '20 at 23:11

Evan Trimboli

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6
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66

score 3 · Answer 3 · answered Jun 12 '20 at 22:10

This is also a problem in Blazor-Client side. For those calling a single object
e.g ClassName = await Http.GetFromJsonAsync<ClassName>($"api/ClassName/{id}");

This will fail to Deserialize. Using the same System.Text.Json it can be done by:

List<ClassName> ListName = await Http.GetFromJsonAsync<List<ClassName>>($"api/ClassName/{id}");

You can use an array or a list. For some reason System.Text.Json, does not give errors and it is successfully able Deserialize.

To access your object, knowing that it is a single object use:

ListName[0].Property

In your case the latter solution is fine but with the path as the input.

Isn't that essentially what the other answers stated, however, you don't clearly mention that the root cause was deserializing to a single object instead of a collection — BenCamps, Jun 13 '20 at 00:53

score 2 · Answer 4 · answered Oct 12 '22 at 17:35

In my case, I was pulling the JSON data to deserialize out of an HTTP response body. It looked like this:

var resp = await _client.GetAsync($"{endpoint}");    
var respBody = await resp.Content.ReadAsStringAsync();
var listOfInstances = JsonSerializer.Deserialize<List<modelType>>(respBody);

And the error would show up. Upon further investigation, I found the respBody string had the JSON base object (an array) wrapped in double quotes...something like this:

"[{\"prop\":\"value\"},...]"

So I added

respBody = respBody.Trim('\"');

And the error changed! Now it was pointing to an invalid character '\'. I changed that line to include

respBody = respBody.Trim('\"').Replace("\\", "");

and it began to deserialize perfectly.

For reference:

var resp = await _client.GetAsync($"{endpoint}");    
var respBody = await resp.Content.ReadAsStringAsync();
respBody = respBody.Trim('\"').Replace("\\", "");
var listOfInstances = JsonSerializer.Deserialize<List<modelType>>(respBody);

System.Text.Json.JsonException: The JSON value could not be converted

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