How can you handle user connection and disconnection events in SignalR?

Question

Ravi Vishwakarma · Answer

In SignalR, you can handle user connection and disconnection events by
overriding the virtual methods OnConnectedAsync and
OnDisconnectedAsync in your Hub class.
Basic Implementation
public class ChatHub : Hub
{
    // Called when a new client connects
    public override async Task OnConnectedAsync()
    {
        string connectionId = Context.ConnectionId;
        string userId = Context.UserIdentifier;
        Console.WriteLine($"User connected. UserId: {userId}, ConnectionId: {connectionId}");
        // Optionally store user-connection mapping
        // Notify others or update UI
        await base.OnConnectedAsync();
    }
    // Called when a client disconnects
    public override async Task OnDisconnectedAsync(Exception exception)
    {
        string connectionId = Context.ConnectionId;
        string userId = Context.UserIdentifier;
        Console.WriteLine($"User disconnected. UserId: {userId}, ConnectionId: {connectionId}");
        // Clean up user-connection mapping
        // Notify others or update UI
        await base.OnDisconnectedAsync(exception);
    }
}
Notes
1. User Identification
Context.ConnectionId: Unique for each connection.Context.UserIdentifier: Set automatically if authentication is configured.Context.User: Full ClaimsPrincipal for the user.
To customize UserIdentifier, use a custom IUserIdProvider:
public class CustomUserIdProvider : IUserIdProvider
{
    public string GetUserId(HubConnectionContext connection)
    {
        // Return a custom ID, e.g., username from claims
        return connection.User?.FindFirst("sub")?.Value;
    }
}
Register it:
services.AddSingletonandlt;IUserIdProvider, CustomUserIdProviderandgt;();
2. Common Use Cases
Tracking online/offline statusManaging user groupsCleaning up resources or subscriptionsBroadcasting status changes (e.g., “User X has joined”)
Example: Notifying Others
public override async Task OnConnectedAsync()
{
    await Clients.Others.SendAsync("UserConnected", Context.UserIdentifier);
    await base.OnConnectedAsync();
}
public override async Task OnDisconnectedAsync(Exception exception)
{
    await Clients.Others.SendAsync("UserDisconnected", Context.UserIdentifier);
    await base.OnDisconnectedAsync(exception);
}

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How can you handle user connection and disconnection events in SignalR?

Ravi Vishwakarma

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1 Answers

Basic Implementation

Notes

1. User Identification

2. Common Use Cases

Example: Notifying Others

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