✅ Complicated error with C# SignalR client

Verdur · 2024-01-04T23:29:10.141Z

Hi, I'm trying to receive base class in signalr using `connection.on` function. My base class has inherits and they all may be sent to the client. Problem is in `connection.on ` method. It converts whatever JSON serialized data comes to it straight to the `T` type, ignoring `T`'s dervied types. I have tested it by wrapping my `Foo` class in wrapper class `Bar` and passing it to the method: ```c# connection.On (...) ``` This way, if I store `Baz: Foo` type in `Bar` and pass it to client, it will recieve `Bar` with `foo` field having `Baz` type. Is there a way to make `connection.on ` convert incoming data not just to `T`, but to it's derivative too without using wrapper classes?

D

Denis•1/5/24, 12:05 AM

Yes it is. https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/polymorphism?pivots=dotnet-8-0

How to serialize properties of derived classes with System.Text.Jso...

Learn how to serialize polymorphic objects while serializing to and deserializing from JSON in .NET.

D

Denis•1/5/24, 12:05 AM

What you need is a type discriminator

D

Denis•1/5/24, 12:06 AM

Alternatively, you could write a custom json converter, but that can get ugly very fast, if the only thing you need it for is type discrimination

V

VerdurOP•1/5/24, 12:57 AM

I have

JsonDerivedType

JsonDerivedType

JsonDerivedType

JsonDerivedType attribute on my base class, but

connection.on

connection.on

still refuses to convert data to the derived classes, it just stops converting at the type you passed in generic

D

Denis•1/5/24, 7:51 AM

And when serializing, each type inheriting your base has a $type json element?

DDenis Yes it is. https://learn.microsoft.com/en-us/dotnet/standard/serialization/syste...

D

Denis•1/5/24, 7:52 AM

I've recently faced the same issue and this helped me solve it

DDenis And when serializing, each type inheriting your base has a $type json element?

V

VerdurOP•1/5/24, 9:37 AM

Just checked it and looks like problem is in serializer indeed. It doesn't provide $type element if I'm serailizing base class, but if I serialize wrapper class that contains base class it does provide $type element

D

Denis•1/5/24, 11:01 AM

Consider the following hierarchy:

public interface IPet
{
  string Name { get; }
}

public class Dog : IPet
{
  public required string Name { get; init; }
  public bool CanFetch { get; set; } = true;
}

public class Cat : IPet
{
  public required string Name { get; init; }
  public bool CanMeow { get; set; } = true;
}

public interface IPet
{
  string Name { get; }
}

public class Dog : IPet
{
  public required string Name { get; init; }
  public bool CanFetch { get; set; } = true;
}

public class Cat : IPet
{
  public required string Name { get; init; }
  public bool CanMeow { get; set; } = true;
}

public interface IPet
{
  string Name { get; }
}

public class Dog : IPet
{
  public required string Name { get; init; }
  public bool CanFetch { get; set; } = true;
}

public class Cat : IPet
{
  public required string Name { get; init; }
  public bool CanMeow { get; set; } = true;
}

public interface IPet
{
  string Name { get; }
}

public class Dog : IPet
{
  public required string Name { get; init; }
  public bool CanFetch { get; set; } = true;
}

public class Cat : IPet
{
  public required string Name { get; init; }
  public bool CanMeow { get; set; } = true;
}

And the following collection:

var pets = new List<IPet>
{
  new Dog { Name = "Snoopy", CanFetch = false },
  new Cat { Name = "Garfield", CanMeow = true }
};

var pets = new List<IPet>
{
  new Dog { Name = "Snoopy", CanFetch = false },
  new Cat { Name = "Garfield", CanMeow = true }
};

var pets = new List<IPet>
{
  new Dog { Name = "Snoopy", CanFetch = false },
  new Cat { Name = "Garfield", CanMeow = true }
};

var pets = new List<IPet>
{
  new Dog { Name = "Snoopy", CanFetch = false },
  new Cat { Name = "Garfield", CanMeow = true }
};

Serializing the

pets

pets

pets

pets as is will produce this JSON:

[
  {
    "Name": "Snoopy"
  },
  {
    "Name": "Garfield"
  }
]

[
  {
    "Name": "Snoopy"
  },
  {
    "Name": "Garfield"
  }
]

[
  {
    "Name": "Snoopy"
  },
  {
    "Name": "Garfield"
  }
]

[
  {
    "Name": "Snoopy"
  },
  {
    "Name": "Garfield"
  }
]

Decorating the

IPet

IPet

IPet

IPet interface with:

[JsonPolymorphic]
[JsonDerivedType(typeof(Dog), nameof(Dog))]
[JsonDerivedType(typeof(Cat), nameof(Cat))]

[JsonPolymorphic]
[JsonDerivedType(typeof(Dog), nameof(Dog))]
[JsonDerivedType(typeof(Cat), nameof(Cat))]

[JsonPolymorphic]
[JsonDerivedType(typeof(Dog), nameof(Dog))]
[JsonDerivedType(typeof(Cat), nameof(Cat))]

[JsonPolymorphic]
[JsonDerivedType(typeof(Dog), nameof(Dog))]
[JsonDerivedType(typeof(Cat), nameof(Cat))]

....and serializing again shall produce the following result:

[
  {
    "$type": "Dog",
    "Name": "Snoopy",
    "CanFetch": false
  },
  {
    "$type": "Cat",
    "Name": "Garfield",
    "CanMeow": true
  }
]

[
  {
    "$type": "Dog",
    "Name": "Snoopy",
    "CanFetch": false
  },
  {
    "$type": "Cat",
    "Name": "Garfield",
    "CanMeow": true
  }
]

[
  {
    "$type": "Dog",
    "Name": "Snoopy",
    "CanFetch": false
  },
  {
    "$type": "Cat",
    "Name": "Garfield",
    "CanMeow": true
  }
]

[
  {
    "$type": "Dog",
    "Name": "Snoopy",
    "CanFetch": false
  },
  {
    "$type": "Cat",
    "Name": "Garfield",
    "CanMeow": true
  }
]

D

Denis•1/5/24, 11:04 AM

Are you using

System.Text.Json

System.Text.Json

System.Text.Json

System.Text.Json or something else?

D

Denis•1/5/24, 11:08 AM

I forgot to provide the code for serializing

var json = JsonSerializer.Serialize(pets);

var json = JsonSerializer.Serialize(pets);

var json = JsonSerializer.Serialize(pets);

var json = JsonSerializer.Serialize(pets);

V

VerdurOP•1/5/24, 12:09 PM

Yes, I'm using

System.Text.Json

System.Text.Json

System.Text.Json

System.Text.Json. There is no problem with manual serialization, but when I pass a value to signalr's

SendAsync

SendAsync

SendAsync

SendAsync function it doesn't serialize it's value with

$type

$type

property

V

VerdurOP•1/5/24, 12:13 PM

Here's simple example to reproduce this bug:
Models:

c#
[JsonDerivedType(typeof(Bar), nameof(Bar))]
public class Foo
{
    public int IntField { get; set; }
}

public class Bar: Foo
{
    public string StringField { get; set; } = null!;
}

public class FooWrapper
{
    public Foo Foo { get; set; } = null!;
}

c#
[JsonDerivedType(typeof(Bar), nameof(Bar))]
public class Foo
{
    public int IntField { get; set; }
}

public class Bar: Foo
{
    public string StringField { get; set; } = null!;
}

public class FooWrapper
{
    public Foo Foo { get; set; } = null!;
}

c#
[JsonDerivedType(typeof(Bar), nameof(Bar))]
public class Foo
{
    public int IntField { get; set; }
}

public class Bar: Foo
{
    public string StringField { get; set; } = null!;
}

public class FooWrapper
{
    public Foo Foo { get; set; } = null!;
}

c#
[JsonDerivedType(typeof(Bar), nameof(Bar))]
public class Foo
{
    public int IntField { get; set; }
}

public class Bar: Foo
{
    public string StringField { get; set; } = null!;
}

public class FooWrapper
{
    public Foo Foo { get; set; } = null!;
}

Hub:

c#
public class MyHub : Hub
{
    public void SendFoo()
    {
        Foo foo = new Bar { IntField = 1, StringField = "hello" };

        var wrapper = new FooWrapper { Foo = foo };
        Clients.All.SendAsync("ReceiveFoo", wrapper);
        Clients.All.SendAsync("ReceiveFoo", foo);
    }
}

c#
public class MyHub : Hub
{
    public void SendFoo()
    {
        Foo foo = new Bar { IntField = 1, StringField = "hello" };

        var wrapper = new FooWrapper { Foo = foo };
        Clients.All.SendAsync("ReceiveFoo", wrapper);
        Clients.All.SendAsync("ReceiveFoo", foo);
    }
}

c#
public class MyHub : Hub
{
    public void SendFoo()
    {
        Foo foo = new Bar { IntField = 1, StringField = "hello" };

        var wrapper = new FooWrapper { Foo = foo };
        Clients.All.SendAsync("ReceiveFoo", wrapper);
        Clients.All.SendAsync("ReceiveFoo", foo);
    }
}

c#
public class MyHub : Hub
{
    public void SendFoo()
    {
        Foo foo = new Bar { IntField = 1, StringField = "hello" };

        var wrapper = new FooWrapper { Foo = foo };
        Clients.All.SendAsync("ReceiveFoo", wrapper);
        Clients.All.SendAsync("ReceiveFoo", foo);
    }
}

When I'm sending

foo

foo

, json serializator doesn't include

$type

$type

property to json.
When I'm sending

wrapper

wrapper

wrapper

wrapper, json serializator includes

$type

$type

property to json.

V

VerdurOP•1/5/24, 12:14 PM

I've checked it via WIreshark

V

VerdurOP•1/5/24, 12:23 PM

if I serialize and deserialize manually, then derived types are resolved properly

D

Denis•1/5/24, 12:46 PM

seems like a bug in signalr

https://github.com/dotnet/aspnetcore/issues/52342

GitHub

The JSON returned by SignalR does not contain the $type field · Iss...

Is there an existing issue for this? I have searched the existing issues Describe the bug When serializing descendant types, for methods that take a single (not a collection) parameter of the base ...

V

VerdurOP•1/5/24, 1:04 PM

Oh, thank you for sending a link, I was thinking about creating an issue myself

D

Denis•1/5/24, 1:09 PM

this kind of an issue is very unfortunate... Is it possible for you to ser/deser your self and send JSON directly via SignalR?

V

VerdurOP•1/5/24, 1:13 PM

Yes, I can do that or use wrapper type for now.

V

VerdurOP•1/5/24, 1:14 PM

Thank you for help, I think this problem is now resolved

D

Denis•1/5/24, 1:22 PM

$close

DDenis $close

M

MODiX•1/5/24, 1:22 PM

Use the </close:1007753098107813888> command to mark a forum thread as answered