Exception parsing json with System.Text.Json.Serialization

Question

My sample code is very simple:

using System.Text.Json.Serialization;
using Newtonsoft.Json;

public class C {
  public C(string PracticeName) { this.PracticeName = PracticeName; }
  public string PracticeName;
}

var x = new C("1");
var json = JsonConvert.SerializeObject(x); // returns "{\"PracticeName\":\"1\"}"

var x1 = JsonConvert.DeserializeObject<C>(json); // correctly builds a C

var x2 = System.Text.Json.Serialization.JsonSerializer.Parse<C>(json);

the last line raises:

Exception thrown: 'System.NullReferenceException' in System.Text.Json.dll Object reference not set to an instance of an object.

What am I doing wrong ?

(Note this is on latest .NET Core 3 preview 5 with latest System.Text.Json 4.6.0-preview6.19259.10)

Adding a parameterless constructor prevents the exception however I don't want/need a parameterless constructor and Json.Net parses fine without it.

Is there a way to make System.Text.Json parse using the given constructor like Json.Net does ?

Answer 1

In it's current state, JSON Support in .NET Core 3.0 is still not finished, and it seems only a parameterless constructor is supported. It might be, that that feature will be added in future.

One workaround option would be to make a parameterless constructor for your serialized model, when you want to use the new Json API from the .net framework. Probably we shouldn't use constructors for plain datatransfer objects at all, hence I see it as option, not as a workaround.

If you search for a way, on how to migrate from an older version to .net core 3.0, or use Newtonsoft.Json anyway, this is documented here:

MVC:

Install Microsoft.AspNetCore.Mvc.NewtonsoftJson package, and register it to your services:

services.AddMvc().AddNewtonsoftJson();

SignalR:

Install Microsoft.AspNetCore.SignalR.Protocols.NewtonsoftJson package

//Client
new HubConnectionBuilder()
.WithUrl("/chatHub")
.AddNewtonsoftJsonProtocol(...)
.Build();

//Server
services.AddSignalR().AddNewtonsoftJsonProtocol(...);

That way you should* be able to use Json.NET Features in .Net Core 3.0

_{*I don't have installed it, so I can not test it}

Answer 2

PracticeName needs to be a property, not a field. Try that w/ the parameterless constructor.

I developed a quick console program class. The class C1 is converted via the Newtonsoft.Json package. The class C2 is parsed via the System.Text.Json package.

using Newtonsoft.Json;

namespace TestJsonParse
{
    class Program
    {
        static void Main(string[] args)
        {
            var c1 = new C1("1");
            var json1 = JsonConvert.SerializeObject(c1); // returns "{\"PracticeName\":\"1\"}"
            var x1 = JsonConvert.DeserializeObject<C1>(json1); // correctly builds a C1

            var c2 = new C2();
            string json2 = "{\"PracticeName\":\"1\"}";
            var x2 = System.Text.Json.Serialization.JsonSerializer.Parse<C2>(json2); // correctly builds a C2
        }

        class C1
        {
            public C1(string PracticeName) { this.PracticeName = PracticeName; }
            public string PracticeName;
        }

        class C2
        {
            public C2() { }
            public string PracticeName { get; set; }
        }
    }
}