JsonSerializer.Deserialize fails

Question

Consider the code...

using System;
using System.Text.Json;

public class Program
{
    public static void Main()
    {
        int id = 9;
        string str = "{\"id\": " + id + "}";
        var u = JsonSerializer.Deserialize<User>(str);
        Console.WriteLine($"User ID: {u.Id}, Correct: {id == u.Id}");  // always 0/init/default value
    }
}


public class User {
    public int Id { get; set; }
}

Why isn't the data being deserialized properly into the User object? I also verified the behavior via DotNetFiddle in case it was an issue local to my system. No exception is thrown.

My actual implementation is read from an [ApiController]'s [HttpPost] action after I return Created("user", newUser). It is called in my MVC/Razor project via _httpClient.PostAsync. I verified the values are correct when Created is returned to the PostAsync call, but no matter what, the value parsed from the response body contains only default values (the actual ID is a Guid).

I initially thought it might have been an UTF8 related issue, as that is the encoding for the StringContent I post to the ApiController. UTF8 deserialization is referenced here, but I had trouble getting from the IO.Stream of the HttpContent to a ReadOnlySpan or Utf8JsonReader.

I found this project while searching, which makes me think it should work as I expected.

Answer 1

Your problem is that System.Text.Json is case-sensitive by default, so "id": 9 (all lowercase) is not mapped to the Id property. From the docs:

Case-insensitive property matching

By default, deserialization looks for case-sensitive property name matches between JSON and the target object properties. To change that behavior, set JsonSerializerOptions.PropertyNameCaseInsensitive to true:

Note: The web default is case-insensitive.

var options = new JsonSerializerOptions
{
   PropertyNameCaseInsensitive = true,
};
var weatherForecast = JsonSerializer.Deserialize<WeatherForecast>(jsonString, options);

So you need to do that also:

var u = JsonSerializer.Deserialize<User>(str, new JsonSerializerOptions { PropertyNameCaseInsensitive = true });

Demo fiddle #1 here.

(If the difference is entirely due to camel casing and not more general differences in case, you can instead configure the serializer to use camel case as shown in this answer by t.j..)

You can configure the option on startup in ASP.NET Core 3.0 as shown in How to set json serializer settings in asp.net core 3?:

services.AddControllers().AddJsonOptions(options =>
{
    options.JsonSerializerOptions.PropertyNameCaseInsensitive = true;
});

Alternatively you could apply [JsonPropertyName("id")] to your model:

public class User {
    [JsonPropertyName("id")]
    public int Id { get; set; }
}

Demo fiddle #2 here.

Answer 2

Thanks to mr5 who suggested it was a casing issue via chat.

Changing the string to use TitleCase ("Id") solves the issue.

I was in the process of submitting a ticket, and one of the possibly related issues comments lead me to another issue, which lead to the documentation, which has a solution

var options = new JsonSerializerOptions();
options.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;

Using the options, solves the problem...

string str = "{\"id\": " + id + "}";
var options = new JsonSerializerOptions();
options.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
var u = JsonSerializer.Deserialize<User>(str, options);

Leaving this all up in case it helps someone else.

Answer 3

You can also read this microsoft documentation JSON Serialization, for the configuration you can use the following:

JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
var serialized = JsonSerializer.Serialize(data, _jsonOptions);
var deserialized = JsonSerializer.Deserialize<TEntity>(serialized , _jsonOptions);

JsonSerializerDefaults.Web it is a predefined enum setting like "camelCase value", "case-insensitive property names", etc, check this JsonSerializerDefaults.? , if you want to customize you would check the documentation again here

Answer 4

In ConfigureServices at Startup.cs

services.AddControllers()
        .AddJsonOptions(o => {
            o.JsonSerializerOptions.PropertyNamingPolicy=JsonNamingPolicy.CamelCase;
            o.PropertyNameCasInsensitive=true
        });