SignalR: Verify union types work as hub method parameters and return values

[DeagleGross]

Verify that C# union types work correctly in SignalR hub methods and add test coverage

Expected behavior

union ChatMessage(TextMessage, ImageMessage, SystemNotification);

public class ChatHub : Hub
{
    // Union as parameter — client sends JSON, STJ deserializes to union
    public async Task SendMessage(ChatMessage message)
    {
        await Clients.All.SendAsync("ReceiveMessage", message);
    }

    // Union as return value — STJ serializes the active case
    public ChatMessage GetLastMessage()
    {
        return new ChatMessage(new TextMessage { Text = "Hello" });
    }
}

Areas to investigate

1. JSON Hub Protocol (JsonHubProtocol.cs):

   • Uses IInvocationBinder.GetParameterTypes(methodName) to resolve parameter types, then calls JsonSerializer.Deserialize(ref reader, paramType) with the exact parameter type
   • Since paramType will be the union type, and STJ handles union deserialization, this should work without code changes
   • Verify with both unambiguous unions (union(int, string)) and ambiguous unions with custom classifiers (union Pet(Cat, Dog))

2. Return value serialization:

   • Hub return values are serialized via JsonSerializer.Serialize(writer, value, type) where type comes from the return type
   • Verify the union case value is serialized correctly (not the union wrapper)

3. Streaming:

   • Verify union types work in IAsyncEnumerable<UnionType> streams and ChannelReader<UnionType>

4. Newtonsoft.Json protocol (NewtonsoftJsonHubProtocol.cs):

   • Union support will NOT work with Newtonsoft.Json unless Newtonsoft adds union support
   • Verify graceful error handling — what happens if a union type is used with the Newtonsoft protocol?

[github-actions]

Triage Summary

Area: area-signalr (already labeled — issue is about SignalR hub method parameter/return value serialization with C# union types)
Type: feature-request — This is a verification + test coverage task for new C# language feature (union types) support in SignalR hub methods; no existing functionality is broken.

Potential Duplicates

• None found

Notes

• Issue is filed by a repo member and already milestoned to .NET 11 Planning.
• The body focuses on investigation areas: JsonHubProtocol, return value serialization, streaming, and Newtonsoft.Json compatibility (which is expected not to work).
• No regression indicators — this is forward-looking feature validation work.

Generated by Issue Triage Agent for dotnet/aspnetcore for issue #66547 · ● 143K · ◷