System.Text.Json POCOs with constructor deserialization doesn't support metadata reads ($ref)

[Kiritsu]

Description

I'm trying to deserialize some JSON that references other objects but it fails to do so.

I have the following configuration (only for the ReferenceHandler)

internal static readonly JsonSerializerOptions DefaultOptions = new()
{
    WriteIndented = true,
    ReferenceHandler = ReferenceHandler.Preserve
};

But when trying to deserialize the JSON, it fails with the following exception:

System.NotSupportedException : Reference metadata is not supported when deserializing constructor parameters. See type 'NTMA.Domain.Synoptic'. The unsupported member type is located on type 'NTMA.Domain.Component'. Path: $.Synoptics.$values[0].Components.$ref | LineNumber: 47 | BytePositionInLine: 21.
  ----> System.NotSupportedException : Reference metadata is not supported when deserializing constructor parameters. See type 'NTMA.Domain.Synoptic'.

I assume this issue is caused for the same reason as #66604 which attempted to be fixed in #67183. I had the issue with other objects with the field $id causing the same exception, so I updated to 7.0.0-preview.6.22324.4 which fixed the issue.

Below is a working reproduction code.

Reproduction Steps

The following code breaks:

// See https://aka.ms/new-console-template for more information

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

var exampleA = new ExampleA("Xyz")
{
    Bs = new HashSet<ExampleB>()
};

exampleA.Bs.Add(new ExampleB("Abc")
{
    A = exampleA
});

var options = new JsonSerializerOptions
{
    ReferenceHandler = ReferenceHandler.Preserve
};

var json = JsonSerializer.Serialize(exampleA, options);
exampleA = JsonSerializer.Deserialize<ExampleA>(json, options);

Console.WriteLine("Done.");

public class ExampleA
{
    public ISet<ExampleB> Bs { get; set; }
    
    public string Test { get; set; }

    public ExampleA(string test)
    {
        Test = test;
    }
}

public class ExampleB
{
    public ExampleA A { get; set; }

    public string Test { get; set; }

    public ExampleB(string test)
    {
        Test = test;
    }
}

The following code works:

// See https://aka.ms/new-console-template for more information

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

var exampleA = new ExampleA
{
    Bs = new HashSet<ExampleB>(),
    Test = "Xyz"
};

exampleA.Bs.Add(new ExampleB
{
    A = exampleA,
    Test = "Abc"
});

var options = new JsonSerializerOptions
{
    ReferenceHandler = ReferenceHandler.Preserve
};

var json = JsonSerializer.Serialize(exampleA, options);
exampleA = JsonSerializer.Deserialize<ExampleA>(json, options);

Console.WriteLine("Done.");

public class ExampleA
{
    public ISet<ExampleB> Bs { get; set; }
    
    public string Test { get; set; }
}

public class ExampleB
{
    public ExampleA A { get; set; }

    public string Test { get; set; }
}

Expected behavior

Deserialization happens properly even when there's a constructor.

Actual behavior

Deserialization can't happen because it says it's not supported even though it should be?

Regression?

No response

Known Workarounds

The only workaround in my case is to remove the constructors of my classes and use init-only properties but I'd like to not do this.

Configuration

.NET 6 (6.0.302)
Windows 11 (Version 21H2 (OS Build 22000.795))
x64

Other information

No response

[ghost]

Tagging subscribers to this area: @dotnet/area-system-text-json, @gregsdennis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Description

I'm trying to deserialize some JSON that references other objects but it fails to do so.

I have the following configuration (only for the ReferenceHandler)

internal static readonly JsonSerializerOptions DefaultOptions = new()
{
    WriteIndented = true,
    ReferenceHandler = ReferenceHandler.Preserve
};

But when trying to deserialize the JSON, it fails with the following exception:

System.NotSupportedException : Reference metadata is not supported when deserializing constructor parameters. See type 'NTMA.Domain.Synoptic'. The unsupported member type is located on type 'NTMA.Domain.Component'. Path: $.Synoptics.$values[0].Components.$ref | LineNumber: 47 | BytePositionInLine: 21.
  ----> System.NotSupportedException : Reference metadata is not supported when deserializing constructor parameters. See type 'NTMA.Domain.Synoptic'.

I assume this issue is caused for the same reason as #66604 which attempted to be fixed in #67183. I had the issue with other objects with the field $id causing the same exception, so I updated to 7.0.0-preview.6.22324.4 which fixed the issue.

Below is a working reproduction code.

Reproduction Steps

The following code breaks:

// See https://aka.ms/new-console-template for more information

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

var exampleA = new ExampleA("Xyz")
{
    Bs = new HashSet<ExampleB>()
};

exampleA.Bs.Add(new ExampleB("Abc")
{
    A = exampleA
});

var options = new JsonSerializerOptions
{
    ReferenceHandler = ReferenceHandler.Preserve
};

var json = JsonSerializer.Serialize(exampleA, options);
exampleA = JsonSerializer.Deserialize<ExampleA>(json, options);

Console.WriteLine("Done.");

public class ExampleA
{
    public ISet<ExampleB> Bs { get; set; }
    
    public string Test { get; set; }

    public ExampleA(string test)
    {
        Test = test;
    }
}

public class ExampleB
{
    public ExampleA A { get; set; }

    public string Test { get; set; }

    public ExampleB(string test)
    {
        Test = test;
    }
}

The following code works:

// See https://aka.ms/new-console-template for more information

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

var exampleA = new ExampleA
{
    Bs = new HashSet<ExampleB>(),
    Test = "Xyz"
};

exampleA.Bs.Add(new ExampleB
{
    A = exampleA,
    Test = "Abc"
});

var options = new JsonSerializerOptions
{
    ReferenceHandler = ReferenceHandler.Preserve
};

var json = JsonSerializer.Serialize(exampleA, options);
exampleA = JsonSerializer.Deserialize<ExampleA>(json, options);

Console.WriteLine("Done.");

public class ExampleA
{
    public ISet<ExampleB> Bs { get; set; }
    
    public string Test { get; set; }
}

public class ExampleB
{
    public ExampleA A { get; set; }

    public string Test { get; set; }
}

Expected behavior

Deserialization happens properly even when there's a constructor.

Actual behavior

Deserialization can't happen because it says it's not supported even though it should be?

Regression?

No response

Known Workarounds

The only workaround in my case is to remove the constructors of my classes and use init-only properties but I'd like to not do this.

Configuration

.NET 6 (6.0.302)
Windows 11 (Version 21H2 (OS Build 22000.795))
x64

Other information

No response

Author:	Kiritsu
Assignees:	-
Labels:	area-System.Text.Json
Milestone:	-

[eiriktsarpalis]

This is a known limitation. Ultimately, it is not possible to implement correctly out of bootstrapping concerns: constructor parameters must be deserialized before the current value can be instantiated. As such, it becomes impossible to deserialize values like the following:

JsonSerializer.Deserialize<Person>("""{ "$id" : "1", "parent" : { "$ref" : "1"} }""");

public record Person(Person? parent);

Even though not all reference preservation scenaria contain cycles, it arguably is the raison d'etre of the feature. While we could try to make the feature smarter and only fail if cycles are detected, this would require substantially more state to track correctly and it would impact performance. Consequently, explicitly not supporting the feature at all for constructor deserialization is the right apprioach in my view. As you mention, the workaround is to simply refactor constructor parameters to be init or required properties.