JsonSerializer support for immutable classes and structs.

[johanneshoehn]

A common pattern is to make data objects immutable for many different reasons.
For example Point:

public class Point
{
    int X { get; }

    int Y { get; }

    public Point(int x, int y) => (X, Y) = (x, y);

    public void Deconstruct(out int x, out int y) => (x, y) = (X, Y);
}

It would be very helpful if JsonSerializer supported immutable classes/structs like that, especially since Newtonsoft Json.NET supports deserialization through the constructor.

However there are a few issues surrounding immutable types:

• Constructor parameter names are usually camelCase while Properties are PascalCase. One way to solve this is by setting JsonSerializerOptions.PropertyNameCaseInsensitive in to true. Another way is to use the Deconstruct-pattern introduced in C# 7.0, instead of Properties for serialization.

• There may be several constructors (and Deconstruct methods). So it's not always clear which to use. Newtonsoft Json.NET uses a JsonConstructorAttribute.htm, which has the disadvantage that the data objects need to know about serialization.

• Constructor parameters, properties and Deconstruct method parameters might not match up, possibly leading to confusing situations where deserializing a previously serialized object does not work.

• Immutable structs always still have a default parameterless constructor.

[ahsonkhan]

This is a known limitation of the System.Text.Json serializer for v1. We plan to support this in the future.

[steveharter]

You can write a custom converter for this.

[Gnbrkm41]

I totally was going to write an issue about this, glad I noticed this before that 🙃

I personally thought that writing a private setter on otherwise a readonly property purely because of JSON (de)serialization is kind of hacky. I know that I could write a custom converter (when it's implemented) or use a constructor (which I remember will be supported soon? I could be wrong) but... that's effort ☺️

[steveharter]

For the 3.0 release, there is no planned additional support for calling a non-default constructor during deserialization. That would have to be done by a custom converter.

[Gnbrkm41]

That's fine, as long as it's in the plan in some way... One step at a time 😁

[khellang]

There's a fair bit of overlap with this issue and https://github.com/dotnet/corefx/issues/38569.

[Gnbrkm41]

@khellang, it probably overlaps because this issue is dotnet/corefx#38569 😄 I guess you meant 41973?

[khellang]

LOL, no, I meant https://github.com/dotnet/corefx/issues/40399 😂

[Mike-E-angelo]

😅

[danstur]

@ahsonkhan Is there work currently being done on this issue? If not, is this something where a pull request would be considered?

[git-net]

seems would support Deserialize anonymous object?

[layomia]

From @endeffects in dotnet/corefx#40399:

Please provide support for custom constructors and allow to specify the constructor with an attribute.

[ahsonkhan]

From @EdiWang in https://github.com/dotnet/corefx/issues/41102:

The following code throws exception.

static void Main(string[] args)
{
    var obj = new Response(true, "Hi") { ResponseCode = 100 };
    var json = System.Text.Json.JsonSerializer.Serialize(obj);

    // System.NotSupportedException: 'Deserialization of reference types without parameterless constructor is not supported.
    var response = System.Text.Json.JsonSerializer.Deserialize<Response>(json);

    Console.WriteLine(response.Message);
}

public class Response
{
    public bool IsSuccess { get; set; }

    public string Message { get; set; }

    public int ResponseCode { get; set; }

    public Response(bool isSuccess = false, string message = "")
    {
        IsSuccess = isSuccess;
        Message = message;
    }
}

Response class is considered not having a parameterless constructor, but actually it does use default values for parameters, that can be used just like a parameterless constructor, like this:

var test = new Response();

Could the new JSON API check if the constructor parameters have default values before consider it not paremeterless?

Related Issues:
#29490

PR:
dotnet/corefx#38061