Why System.Text.Json requires that class explicitly have parameterless constructor

[neman]

I have a class containing one constructor with some parameters.
Why do I also have to explicitly declare default constructor?

I'm already using both constructors throughout the code, and System.Text.Json does not serialize if I do not explicitly write default constructor. Newtonsoft json did not required this intervention. Now I have to revert back to old code.

Example of code which does not work

string test = "{\"First\": {\"Name\": \"Nemanja\"}}";
var value = JsonSerializer.Deserialize<Dictionary<string, Person>>(test);

public class Person
{
   public string Name { get; set; }
   public Person(int x)   { }
}

This code throws exception. It works when I add parameterless constructor

[khellang]

The serializer needs a constructor to call in order to create the Person object. Does the code you showed even work with Newtonsoft.Json? I thought it matched constructor argument names with JSON property names? Anyway, the feature you're talking about is probably already covered by either https://github.com/dotnet/corefx/issues/40399 or https://github.com/dotnet/corefx/issues/38569.

[neman]

The same code works well with Newtonsoft.Json

using System;
using System.Collections.Generic;

namespace ConsoleApp
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            string test = "{\"First\": {\"Name\": \"Nemanja\"}}";
            var worked = Newtonsoft.Json.JsonConvert.DeserializeObject<Dictionary<string, Person>>(test);
            var failed = System.Text.Json.JsonSerializer.Deserialize<Dictionary<string, Person>>(test);
        }

        private class Person
        {
            public string Name { get; set; }

            public Person(int x) { }
        }
    }
}

There is also problem with converting boolean values to object with new Json.
I regret the day I started to migrate code from Newtonsoft.Json, who now what else will break my code. I'll stick with Newtonsoft.Json for now, although it's less performant, at least it works. I do not need premature optimization at this moment.

[khellang]

Hmm. What does Newtonsoft pass as the x argument in that Person constructor? Or does it just not call a constructor at all?

[neman]

It passes default for int, 0

[ericstj]

Related https://github.com/dotnet/corefx/issues/40399. As a simplification v1 of System.Text.Json only supports parameterless constructors.