ASP.NET Core SignalR - Quick Reference

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: signalr for comprehensive documentation.

Hub Setup

// Program.cs builder.Services.AddSignalR(); app.MapHub<ChatHub>("/hubs/chat");

Strongly-Typed Hub

public interface IChatClient { Task ReceiveMessage(string user, string message); Task UserJoined(string user); Task UserLeft(string user); }

public class ChatHub : Hub<IChatClient> { public async Task SendMessage(string user, string message) { await Clients.All.ReceiveMessage(user, message); }

public async Task JoinRoom(string room)
{
    await Groups.AddToGroupAsync(Context.ConnectionId, room);
    await Clients.Group(room).UserJoined(Context.User?.Identity?.Name ?? "Anonymous");
}

public async Task LeaveRoom(string room)
{
    await Groups.RemoveFromGroupAsync(Context.ConnectionId, room);
    await Clients.Group(room).UserLeft(Context.User?.Identity?.Name ?? "Anonymous");
}

public override async Task OnConnectedAsync()
{
    await base.OnConnectedAsync();
}

public override async Task OnDisconnectedAsync(Exception? exception)
{
    await base.OnDisconnectedAsync(exception);
}

}

Sending from Outside Hub

public class NotificationService { private readonly IHubContext<ChatHub, IChatClient> _hubContext;

public NotificationService(IHubContext<ChatHub, IChatClient> hubContext)
    => _hubContext = hubContext;

public async Task NotifyUser(string userId, string message)
{
    await _hubContext.Clients.User(userId).ReceiveMessage("System", message);
}

public async Task NotifyGroup(string group, string message)
{
    await _hubContext.Clients.Group(group).ReceiveMessage("System", message);
}

}

Streaming

public class StreamHub : Hub { // Server to client streaming public async IAsyncEnumerable<int> Counter(int count, int delay, [EnumeratorCancellation] CancellationToken cancellationToken) { for (var i = 0; i < count; i++) { cancellationToken.ThrowIfCancellationRequested(); yield return i; await Task.Delay(delay, cancellationToken); } }

// Client to server streaming
public async Task UploadStream(IAsyncEnumerable<string> stream)
{
    await foreach (var item in stream)
    {
        // Process each item
    }
}

}

Authentication

// Require auth on hub [Authorize] public class SecureHub : Hub<IChatClient> { [Authorize(Roles = "Admin")] public async Task AdminAction() { } }

// Access user info public class ChatHub : Hub { public async Task Send(string message) { var userId = Context.UserIdentifier; // From ClaimTypes.NameIdentifier var name = Context.User?.Identity?.Name; } }

Anti-Patterns

Anti-Pattern Why It's Bad Correct Approach

Storing state in Hub Hub is transient Use external storage or groups

Not using strongly-typed hubs No compile-time safety Use Hub<TClient>

Long-running hub methods Blocks connection Use background services

Not handling disconnection Resource leaks Override OnDisconnectedAsync

Quick Troubleshooting

Issue Likely Cause Solution

Connection refused Hub not mapped Check app.MapHub<T>()

Auth not working Token not sent Pass token in query string for WebSocket

Messages not received Wrong client method name Match interface method names exactly

Scaling issues No backplane Use Redis or Azure SignalR