Sending a list of different objects via SignalR

Question

What is the best way to send a list of different objects via SignalR to the clients?

When i use a list of objects with the same base, the client only receive properties of the base class:

class Update { }
class UpdateA { public string PropertyA {get; set;}}
class UpdateB { public string PropertyB {get; set;}}
...
IEnumerable<Update> updates = GetUpdates();
await Clients.Caller.SendAsync("update", updates);

When I use pre serialized data in form of strings, the serialzier escapte the json data as strings.

IEnumerable<string> updates = GetUpdates();
await Clients.Caller.SendAsync("update", updates);
// data looks like: [ "{...}", "{...}" ]

Do I have to write my own serialzier? How does it work for SignalR in dotnet core?

Why not directly use a complex class and inlcude all the objects , and at last send to clients? — Nan Yu
– Nan Yu, Commented Feb 11, 2020 at 7:38

fharreau · Accepted Answer · 2020-11-25 16:36:33Z

8

I know this is an old question but here is an other solution that does not requires to write your own CustomConverter and relies on the NewtonSoft solution instead:

First you need to install this package: Microsoft.AspNetCore.SignalR.Protocols.NewtonsoftJson

On the server side, you need to add the following calls in you Startup.cs class:

services.AddSignalR()
    .AddNewtonsoftJsonProtocol(opts => 
        opts.PayloadSerializerSettings.TypeNameHandling = TypeNameHandling.Auto);

if your client is a C# client, you have to add the same configuration when building the connection:

new HubConnectionBuilder()
    .WithUrl("/yourhub")
    .AddNewtonsoftJsonProtocol(opts => 
        opts.PayloadSerializerSettings.TypeNameHandling = TypeNameHandling.Auto)
    .Build();

If your client is not a C# client, I guess you will have to manage the deserialization yourself. NewtonSoft.Json just adds a $type property to the generated JSON containing the fully qualified type name (for example: Your.Namespace.ClassName) so you can use it to select the according type to deserialize with.

source: MSDN

edited Nov 25, 2020 at 16:36

answered Nov 25, 2020 at 16:31

fharreau

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2 Comments

Dipon Roy Over a year ago

after trying so many things finally its working. Thank you

collinsmarra Dec 17, 2024 at 16:41

This was really helpful. Thank you

Fei Han · Accepted Answer · 2020-02-11 08:43:50Z

send a list of different objects with the same base via SignalR to the clients

You can try to achieve the requirement by creating and using a custom converter, like below.

In custom converter MyConverterWithTypeDiscriminator

public override void Write(Utf8JsonWriter writer, Update value, JsonSerializerOptions options)
{
    writer.WriteStartObject();

    if (value is UpdateA updateA)
    {
        writer.WriteNumber("TypeDiscriminator", (int)TypeDiscriminator.UpdateA);
        writer.WriteString("PropertyA", updateA.PropertyA);
    }
    else if (value is UpdateB updateB)
    {
        writer.WriteNumber("TypeDiscriminator", (int)TypeDiscriminator.UpdateB);
        writer.WriteString("PropertyB", updateB.PropertyB);
    }

    writer.WriteString("UpdatedAt", value.UpdatedAt);

    writer.WriteEndObject();
}

Base class and derived classes

public class Update
{
    public DateTime UpdatedAt { get; set; }
}

public class UpdateA : Update
{
    public string PropertyA { get; set; }
}

public class UpdateB : Update
{
    public string PropertyB { get; set; }
}

In Startup.cs

services.AddSignalR().AddJsonProtocol(options => options.PayloadSerializerOptions.Converters.Add(new MyConverterWithTypeDiscriminator()));

In Hub method

var updates = new List<Update>
{
    new UpdateA
    {
        UpdatedAt=DateTime.Now.AddDays(-1),
        PropertyA = "A"
    },
    new UpdateB
    {
        UpdatedAt=DateTime.Now.AddDays(-1),
        PropertyB = "B"
    }
};

await Clients.All.SendAsync("ReceiveUpdate", updates);

Test Result

Note: to implement your custom converter, please check this link: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to#support-polymorphic-deserialization

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